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Multifamily Passive House Ventilation Design Part 1: Unitized or Centralized HRV/ERV?

*click here to read Part 2 of this blog

Project teams pursuing Passive House frequently ask, “Where do we locate the HRV/ERV?” The answer is complex when the Passive House concept is scaled to a multifamily program.  While there are two primary arrangements for HRV/ERV systems, the trade-off is dynamic and needs to be carefully considered as multifamily Passive House projects begin to scale. A low volume HRV/ERV unit ventilating an individual apartment is a unitized HRV/ERV. High volume HRV/ERV units ventilating multiple apartments and often servicing several floors, is referred to as centralized HRV/ERV.

As Passive House consultants we can attempt to address the system arrangement question with building science; however, in New York City rentable floor space is very valuable, so considering the floor area trade-off is of particular interest to project teams. When a unitized HRV/ERV system cannot be located in a drop-ceiling due to low floor-to-floor height, it is placed in a dedicated mechanical closet. This closet is typically no smaller than 10 ft2 and includes the necessary ductwork connections to the HRV/ERV unit. The alternative solution is to increase the floor-to-floor height to accommodate the HRV/ERV unit and horizontal duct runs in the ceiling. Centralized HRV/ERV systems, however, allow short horizontal duct runs but require floor space to accommodate vertical shafts. With supply and exhaust ducts coupled together the required floor area is about 8-12 ft2. As a result, centralized HRV/ERV systems may actually require more floor area than a unitized system.

Example: In the case of Cornell Tech, vertical supply and exhaust duct work for the centralized HRV/ERV system required 222.5 ft2 per floor, or 13 ft2 per apartment (see image 1 below). Unitized HRV/ERV mechanical closets would have required an estimated 170 ft2 per floor, or 10 ft2 per unit (image 2 on right).

Comparison images HRV/ERV

Image 1 & 2:  These images compare the amount of floor area required for centralized and unitized HRV/ERV systems. Image 1 on the left, shows the 12ft2 floor area required for vertical shafts servicing the centralized ERV at Cornell Tech. Image 2 on the right is hypothetical, showing the typical location and 10ft2 floor area required for a unitized HRV/ERV mechanical closet.

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Over Pressure (Part One)

Steam pressure gets a disproportionate amount of attention. That’s partially due to the common, but not necessarily true idea that higher pressure equals more fuel use. Remember, it’s not the steam’s pressure that heats the building; it’s the steam’s heat energy. In fact, you can heat a building with 0 psig steam. You can even heat a building with a boiler that’s too small and never builds positive pressure. You can’t do it well, but you can do it.

System Operation

Thanks to the law of conservation of energy, we know that energy cannot be created or destroyed — it can only be altered from one form to another. In a steam heating system, the flow of energy goes like this:

  1. The boiler transfers Btus from the fuel to the steam (energy input).
  2. The steam transfers those Btus to the rooms.
  3. The rooms transfer those Btus to the outdoors (heat loss, aka the load).
image of radiator

Too much heat at any pressure

It’s important to keep this energy flow in mind because they are linked and self-equalizing. If the energy input exceeds the heat loss, the building temperature will increase, which, in turn, increases the heat loss. And, a building’s heat loss depends on the temperature difference between inside and outside and the amount of air transfer occurring. So, the best way to keep the heat loss down is to keep the indoor temperatures as low as possible, and keep the windows closed. Furthermore, in an apartment building, the coldest room drives the load in any steam-heated building and the Super needs to send enough heat around to satisfy the hardest-to-heat apartment.

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Recovering from Heat Recovery Woes

IECC Image

The International Energy Conservation Code (IECC) has a number of requirements involving energy recovery on ventilation systems. Requirements vary based on climate zone, building type and size, equipment capacity, and equipment operating hours. As a result, many new construction projects must now incorporate energy recovery considerations into their design.

An energy recovery unit (ERU) equipped with a heat wheel can be a great way to satisfy these energy recovery requirements. The ERU can be a roof-mounted air handling unit, or can be an air handling unit located inside a mechanical room with outdoor air and exhaust streams ducted in. The heat wheel is positioned so that half of the wheel sits in the exhaust air duct and the other half sits in the outdoor air intake duct. During cold weather, the wheel spins, transferring heat from the exhaust stream to the outdoor air intake stream. During hot weather, the wheel transfers heat from the outdoor air intake stream to the exhaust stream. In both cases the heat exchange enables the building to take advantage of the more comfortable conditions of the exhaust air, while still allowing fresh air to enter the building. During extreme weather conditions, heat wheels can save energy on space conditioning while still allowing for healthy indoor air quality.

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When the Rubber Meets the Road

 

As the Passive House standard continues to make waves across New York City and the U.S., an entirely new design process has evolved to respond to the challenges of higher insulation levels, balanced mechanical ventilation, and perhaps the most difficult hurdle – an air tightness level that most would think is impossible. For the recently certified Cornell Tech building on Roosevelt Island, the tallest Passive House in the world, a several year-long coordinated effort was required to achieve such a feat. So what is the requirement, how is it measured, and what are the strategies and considerations required to achieve it?

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It’s all in the Details: Designing for Passive House & Accessibility Compliance

The number of multifamily residential projects targeting Passive House certification has been rising steadily over the past several years, bringing along many exciting challenges. This has been especially prevalent in New York City, where increasingly stringent energy standards and a desire for innovation have made designing to Passive House standards an attractive goal. As the number of these projects passing through our office continues to grow, we have discovered some important overlaps with one of our other consulting services – Accessibility Compliance.

In the United States, multifamily new construction projects consisting of four or more dwelling units are subject to the Fair Housing Act, as well as state, city, and local accessibility laws and codes. For the purposes of this blog we will focus on projects in NYC, although the majority of newly constructed residential projects across the country will be subject to some variation of the criteria discussed below, for both Passive House and Accessibility standards. With this in mind, we have chosen a couple of common problem areas that require particularly close attention. (more…)

Transformers: Problems in Disguise

Sometimes a significant source of energy inefficiency in a building can be hiding in a place difficult to detect. In some buildings, a single transformer can have a substantial impact on electrical consumption.

Image of currents flowing through a transformer

click to enlarge

Some Background

Transformers are responsible for stepping the incoming voltage to a building up or down depending on the design, intended use, or connected equipment.  A standard electrical socket in a US home or office will deliver 110-120 volts AC. Some appliances require 240 V instead. Large mechanical equipment, such as the air handling units, distribution pumps and chillers found in commercial or multifamily buildings may require 460 V. In buildings where the incoming voltage from the utility does not match the voltage required by connected equipment, a transformer is used to deliver the necessary voltage.  The voltage entering the transformer is called the primary voltage and the voltage delivered by the transformer to the facility’s equipment is called the secondary voltage.

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Technically Speaking: Not All Insulation is Graded Equally

About a year ago, I worked along with other HERS raters and the North American Insulation Manufacturers Association (NAIMA, a.k.a. Insulation Institute) to conduct a study on the importance of insulation installation quality and grading.

RESNET, the nation’s leading home energy efficiency network and the governing body of the Home Energy Rating System (HERS® Index) established standards for grading insulation installation.

The grading is as follows:

Grade I— the best and nearly perfect install which includes almost no gaps or compression… what some would call “G.O.A.T.”
Grade II—allows for up to 2% of missing insulation (gaps) and up to 10% compression over the insulation surface area… what some would call “mad decent”.
Grade III—insulation gaps exceed 2% and compression exceeds 10%… anything worse and the insulated surface area is considered un-insulated.

RESNET Insulation Diagram

Source: RESNET Mortgage Industry National HERS Standards

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Why the Whole Building Approach Matters

At Steven Winter Associates, Inc., we support the whole building approach to design and construction by doing our best to ensure that projects meet sustainability, energy efficiency, and accessibility requirements, among other design strategies and goals. From our perspective, accessibility compliance is a key factor in determining whether a project is truly sustainable and efficient.

The Whole Building Approach to Design (from the Whole Building Design Guide, “Design Objectives”)

As an example, I was recently contacted by a New York City-based housing developer. They received a letter from an attorney stating that three of their recently constructed projects in New York City were “tested” and found to be noncompliant with the accessible design and construction requirements of the Fair Housing Amendments Act and the New York City Building Code. SWA toured the buildings and confirmed that the allegations were in fact true. We identified issues such as excessive cross slopes along the concrete entrance walk, the presence of steps between dwelling units and their associated terraces, the lack of properly sized kitchens and bathrooms, the lack of compliant clear width provided by all user passage doors, etc. It quickly became apparent to us and to the developer that the cost of the remediation required to bring the projects into full compliance would be astronomical.

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Five Year Solar Performance on Connecticut Home

Written by Gayathri Vijayakumar, VP – Senior Building Systems Engineer

Over the last 10 years, we’ve seen great strides in the solar PV market in the United States. Between the federal tax credit and utility-sponsored incentives, the price to install PV systems came within reach of many homeowners. For others, eager to make a positive impact on the environment, power purchase agreements with solar companies and no up-front costs made it possible to utilize their roofs to generate electricity.

While the calculated cost-effectiveness of solar panels relies on the future price of electricity (which we can’t predict), we can confirm that they do deliver energy. In a very scientific study of exactly one home, owned by a SWA engineer, five years of generation data is available. Sure, it’s not the pretty Tesla roof, but these panels were installed back in November 2011. At 4.14 kW, with no shading and great Southern exposure, the panels were estimated to generate 5,400 kWh/year of electricity in New Haven, Connecticut (Climate Zone 5). The panels have exceeded expectations, generating on average, 6,200 kWh/year, which is roughly 70-80% of the electricity required by the 2,500 ft2 gas-heated home and its 4 occupants.

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2016 New York Energy Code Blower Door Testing – How Does it Measure Up?

Written by Sunitha Sarveswaran, Energy Engineer

Welcome to part three of the air sealing blog post series! In previous posts, we have reviewed the substantive changes in 2016 New York Residential and Commercial Energy Code, focusing specifically on the new blower door testing requirements. In this blog post, we’ll examine how these requirements stack up in comparison to green building certifications that we are already familiar with: LEED for Homes, LEED BD+C, ENERGY STAR® Certified Homes, ENERGY STAR® Multifamily High-Rise (ES MFHR) and Passive House (PH).

To make this easier to digest, we’ve divided this comparison into two parts – compartmentalization and building envelope. If you need a refresher on the difference between these two types of blower door tests, we recommend referring to the article “Testing Air Leakage in Multifamily Buildings” by SWA alumnus Sean Maxwell.

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Oh, the Weather Inside is Frightful!

Winter in the City

Wintertime in New York City: cold wind whips down the avenue and seems to follow you as you leave the frozen street and enter your building. The cold gust pulls the heat out of the lobby and even seems to follow you as you make your way up the building, whistling through the elevator shaft as it goes. The colder it gets outside, the worse it gets inside. Can’t somebody please make it stop? Is it too much to ask to be comfortable in your own lobby?

No, it is not too much to ask, and yes, we can help. It is 2016 and we have the technologies and expertise to better manage this all-too-common problem, but first we must examine what forces lay at the heart of the issue.

multifamily_ventilation_winter

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Project Spotlight: 1115 H Street – Transforming the Neighborhood with LEED Platinum

1115hstreet_front_elevation

Front elevation of the building

The newly constructed five-story mixed-use building located at 1115 H Street, NE is raising the bar with a LEED for Homes Platinum certification in the works. Offering 16 high-performance condominiums with an array of sustainable practices, including environmentally preferable products, and water- and energy-conserving fixtures and appliances, the project is contributing to the rapid revitalization of the H Street Corridor neighborhood. Steven Winter Associates, Inc. supported the energy and green building goals for the project, including LEED certification.

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2016 New York Energy Codes: Commercial Edition

By Sunitha Sarveswaran, Energy Engineer

Multifmily Buildings

Multifamily buildings greater than three stories follow the commercial section

It has now officially been over one month since the 2016 NYS energy code went into effect. In a recent blog post, we covered some of the significant changes for residential buildings in New York. In this post, we will explore the substantive changes made in the commercial code section, particularly with respect to envelope and air barrier requirements.

As a reminder, in this post, we are referring to retail, commercial, or larger than three-story R-2, R-3, or R-4 buildings. New York buildings can choose between one of two compliance pathways: ASHRAE 90.1 2013 or IECC 2015, by applying the appropriate state and city amendments. Prescriptive as well as performance options are available, depending on the chosen pathway. (more…)

SWA Helps Implement STEP, the Sustainable Technical Education Program

karla_butterfield

Written by Karla Butterfield, Senior Sustainability Consultant 

In a new and exciting opportunity, we’re partnering with Energize CT, the Connecticut Technical High School System, The Connecticut Light and Power Company dba Eversource, The United Illuminating Company, and The Connecticut Business & Industry Association (CBIA) Education and Workforce Partnership to help implement Green STEP (Sustainability Technical Education Program). This program will train CT technical high school students in a construction career track in energy, water, and resource efficiency.

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How Effective is that Range Hood?

Next time you are cooking, take a look at your kitchen hood. You are likely cooking on the front two burners, but your kitchen hood is not likely to extend fully over these burners. For typical exhaust fans, they do a good job of exhausting steam, contaminants, etc. from directly below them, but don’t necessarily pull all fumes that are outside the perimeter of the fan enclosure.  According to Lawrence Berkeley National Laboratory (LBNL) the capture efficiency of standard hoods is typically in the range of 30-40% on front burners and can be as high as 90% on back burners. To demonstrate this, I boiled some water in a tea pot on my stove. Once steam was coming out, I pulled out an infrared camera and started to take images. Wait…you don’t have an IR camera just sitting around your home? You are missing out on hours and hours of fun with the kids. They are great for science projects.

Back to my point. I have an LG over-the-range microwave with extenda™ vent. This allows the vent area to extend out an additional ~6”. When the microwave hood (exhausted to outside) was operating on turbo mode (just over 300 cfm exhaust) and without the vent extension slid out, the majority of steam from the tea pot on the front burner was passing by the vent and going up the front of the microwave (as evidenced by moisture build up on the microwave door). And yes, I realize that I turned the spout of the tea pot outwards to more dramatically show the point I am trying to make. When the slide out vent was pulled out, the amount of steam capture increased dramatically, but there was still some moisture build up on the front edge of the vent slide out.  Obviously, this is not a scientific study; it is just anecdotal evidence to further the discussion on the need to consider capture efficiency in the design of kitchen range hoods.

Infared_Collage

Figure 1. (Left) IR image of steam from a tea pot bypassing vent hood without hood extension slide out. (Center) Picture of range and hood setup with hood extension slide out. (Right) IR image of steam from a tea pot mostly being captured by vent hood with hood extension slide out.

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Air-Source Heat Pumps in Cold Climates (Part II)

A few months ago I wrote about air-source heat pumps (ASHPs) in cold climates, and I promised more info on how to select the right systems and get the best performance. Below are some things we’ve learned from our work with ASHPs in the Northeast; much of this is based on the results from a study supported by the DOE Building America program. To be clear, we’re talking about inverter-driven (variable-speed) heat pumps in residential applications during heating season. Cooling is certainly important also, but we’ve been more focused on the heating performance, especially at lower temperatures. (more…)

Popular Multifamily Retrofits, Pt III

SWA_MF_energy_storage_systems

In the first two entries of this series (Part One | Part Two), we explored advanced controls for electrically heated buildings; combined heat and power systems; upgraded atmospheric boilers and ventilation systems. For the final installment of SWA’s Favorite Multifamily Retrofits, we’ll examine the ins-and-outs of stand-alone energy storage. (more…)

Popular Multifamily Retrofits, Part II

SWA_PWpopMFRx2

In our first entry of this three-part series, we described advanced controls for electrically heated buildings, combined heat and power systems, and upgraded atmospheric boilers. This time around, we’ll examine the ins-and-outs of exhaust ventilation in multifamily buildings. (more…)

Hotels, Motels, Reining Emissions In

I’ll save the long-winded introduction and get straight to the facts. Based on New York City’s publicly available Local Law 84 (LL84) benchmarking data for 2015, hotels emit 32% more greenhouse gas (GHG) per square foot than the average for all buildings. I also want to qualify this by making a few statements about the data:

  1. There are 13,973 buildings on the Department of Finance list; of which 2,353 did not comply with LL84 or are not required to comply.
  2. We removed the outliers. Weather-normalized source energy use intensity (EUI) over 550 and under 100 (kBtu/ft2) typically indicates erroneous data. Most likely either the building’s benchmarking activities or report filed with NYC were completed incorrectly.
  3. A significant portion of the list comprises the buildings with erroneous data: 4950. Seems a little crazy, no? Leaving us with a good topic for another day….
  4. For clarity, that means we analyzed the remaining 6,654 buildings.
2016 Emissions Map

Click to View Interactive NYC GHG Emissions Map – via CityLab. Map credit: Jill Hubley

The good news – for the sake of this post – is that the hotel market had one of the higher rates of correctly reported compliance data. Out of 187 buildings, 143 reported with numbers that were in a normal range. The average for the sector however, reflects EUI and GHG emissions per square foot that are much higher than other similar building types. Multifamily buildings, for example, have an average of 42% lower GHG emissions/ft2 than hotels (see table below). (more…)

The POWER of Partnership!

PowerDownDC logoHoriz (4)

In partnership with the District of Columbia’s Department of Energy & Environment (DOEE) and the Institute for Better Communities (IFBC), SWA is implementing DC’s first multifamily housing energy and water challenge.

What is the POWER DOWN DC Challenge?

POWER DOWN DC is a 4 month building-to-building, education focused competition in Washington, DC with a goal of empowering  building residents and staff to change behavior and reduce overall energy and water usage. Residents compete as a building team against  other apartment buildings to hit a reduction target and strive to make the greatest overall  reduction. 

Banner

Driving Savings through Friendly Competition

The basic concept is simple: bringing people together for friendly competition is more likely to encourage meaningful action than simply providing information about energy and water efficiency alone. By joining the competition, participants try to reduce their own energy and water use and help members of their apartment community  do the same. Residents will be encouraged to make a commitment to efficiency and take simple steps every day that collectively will have a big payoff. Actions like turning off lights, fixing a leak, and taking shorter showers, multiplied across dozens of apartment units will have quick results. In DC, residential buildings make up 20% of total energy use and 23% of total water use.  If all multi-family residents take action, we can save 83,000,000 kilowatt hours (KWH)  of energy, 96,000,000 gallons of water, and $31, 400,000 dollars annually. Small steps = big savings. 

Power Down DC

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Game Changers in Building Science

Thank you to everyone who stopped by our booth last week at Greenbuild 2015 in Washington, D.C.! By all accounts, this year’s event was a great success. In case you missed it, our fearless leader, Steven Winter, spoke at the GAF booth on Wednesday. As an architect who has been practicing building science for the past 50 years, he shared insights about some building science innovations that he thinks have been “game changers” and have intrigued him: they are changing the way we design, build and operate buildings.

Untitled

Here are the highlights:

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Heat Pumps Are Taking Over

Air-source heat pumps are a booming business. In the Northeast, manufacturers report that sales of residential systems have increased by 25-35% per year over the past 5-10 years. We’ve seen more and more systems being installed in existing homes (to provide cooling while offsetting oil or propane used for heating) and into new homes (often as the sole source of heating and cooling).

We’ve looked into these systems often, and from many perspectives. I’m planning a series of posts, but, for now, here are the answers to some basic questions we receive from clients.

First, the basics: What is an air-source heat pump (ASHP)?

It’s an air conditioner that can operate in reverse. During the summer, it moves heat from indoors to outdoors. In the winter, it moves heat from outdoors to indoors. We helped NEEP (the Northeast Energy Efficiency Partnerships) to put together a market assessment and strategy report on ASHPs. The early sections in this document (see p. 12) outline the different terms and types of heat pumps (ducted/ductless, split/packaged, mini-split, multi-split, central, etc.) Unfortunately, different people can use the same term to mean different things, but hopefully the NEEP Northeast/Mid-Atlantic Air-Source Heat Pump Strategies Report can help clarify things.

Indoor section of heat pump.

 

Outdoor section of a heat pump.

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When Did Building Science Become Energy Efficiency?

If nothing else, people are adaptable. While something might be an annoyance at first, we often figure out a way to manage it and move on. Unfortunately, we all too often do this when it comes to our greatest life investment…our homes. Whether an existing or new home, we almost always are not comfortable in our home or at least portions of our home. One, several, or even the entire home may never be at desirable conditions, but we learn to cope with it by putting on layers of clothing or adding small electric heaters to cold spaces, or supplemental fans in hot ones. So we are not comfortable as we allow our conditioned air to easily escape our homes and our utility bills continue to be high. The simple question is…why?

Mike Trolle

“People have all sorts of misconceptions about the sacrifices that they feel they have to make in high performance homes and it is completely untrue. It is exactly the opposite. The even temperatures, the lack of drafts, the feeling of warmth, comfort, and right levels of humidity and fresh air…they are unrivaled. Comfort is something you have never experienced properly in a home until you have a high performance home.” – Michael Trolle, BPC Green Builders
(Source: CT Zero Energy Challenge 2012)

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Energy and Water Use Study in DC Multifamily Buildings

Do you live in DC? Do you own, manage or reside in a multifamily building? If so, we would love to get your feedback!

The District of Columbia’s Department of Energy and Environment (DOEE) has engaged Steven Winter Associates to gain feedback from multifamily owners, managers and residents about their energy and water usage. To start off, we’re conducting brief surveys (10-minutes max), and hope this effort will have positive outcomes for future multifamily projects in DC by raising awareness of green/energy efficiency initiatives.

PURPOSE:
Survey responses will inform the potential development of a voluntary energy and water conservation program tailored exclusively for the multifamily rental sector in the District. This program will include a customized toolkit to engage residents and building managers in improving energy and water efficiency. It will also encourage participation in a peer-to-peer energy and water reduction competition.

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Can A House Be Too Tight?

 

The Importance of Mechanical Ventilation

During most presentations we give about air sealing and infiltration, like clockwork someone will ask, “but doesn’t the house need to breathe, aren’t we making buildings too tight?” This is a popular green building myth, but  people need to breathe, walls don’t. In fact buildings perform best when they’re air tight and we can temper, filter and regulate the amount of fresh air.

We know the symptoms of poor ventilation – odors, humidity issues, condensation on windows, high levels of chemical off-gassing and even elevated carbon monoxide levels. Some of these effects are immediately apparent to occupants (odors, window condensation) while others may be imperceptible (carbon monoxide). Indoor air quality is a comfort, health and safety concern. However, these problems aren’t necessarily symptoms of tight buildings and can occur in all types of construction, old and new, tight and leaky.

Natural Ventilation Doesn’t Work Anymore

In the past buildings were ventilated with outside air naturally when the wind blew and/or it was cold. If this natural ventilation (or what building professionals call air infiltration) ever worked it doesn’t anymore.

red barn image

“Did you grow up in a barn?” Most of us learned as children the importance of keeping outside air out during heating and cooling seasons. However natural ventilation through building cracks brings unintended moisture and temperature differences that can cause condensation.

 

Old buildings had no insulation or air sealing, so structural failures caused by condensation within a wall assembly rarely occurred. Building codes now require insulation and air sealing which helps lower our energy bills and keep us comfortable inside. But when infiltration happens in a wall full of insulation, condensation can occur on the cool side of the wall assembly, which over time can rot the framing and cause structural issues. This is why it’s critical to prevent air leaks and better understand the thermal boundary.

Americans spend more time in our homes than ever, almost 15 hours per day by some estimates, and humans give off a lot of moisture. While home we tend to keep the windows closed. We’re also seeing increasing amounts of Volatile Organic Compounds (VOCs) emitted from our paints, furniture and household products that are made with chemical compounds that we know little about. For example, solid-wood furniture does not offgass, but plywood, particle board and foam sure do. How much solid wood furniture do you have in your house? Taken together this means there is more moisture, odors and pollutants added to our homes each day than was the case 30 years ago. The EPA estimates indoor pollutants to be 2 to 5 times higher inside homes than outside.Because of all these indoor pollutants, we clearly need to bring fresh outdoor air into the house.

However, the unintentional natural ventilation air our buildings do get rarely comes directly from outside. In the best-case scenario it creeps in through the various cracks in the exterior walls and windows, but most often comes from the least desirable locations shown in the image below: crawlspaces, garages and attics. Leakage from those locations is certainly not “fresh” air. Do you want to breathe in hot dusty attic air, or damp air from your crawlspace? You just might be.

Image of infiltration

Natural ventilation is forced through infiltration points which are most often from the unhealthiest locations in homes

Moreover, unintentional natural ventilation (infiltration) is unreliable and poorly distributed. Infiltration is primarily driven by wind speed and the temperature difference between outdoors and indoors. These weather variables vary day-by-day and season-to-season. For instance, the chart below shows the average conditions for Lancaster, PA. Note the weather fluctuations throughout the year:

  • During summer wind speeds are almost 50% lower
  • The temperature difference is 6-8 times greater during winter

lancaster-weather-conditions chart

These erratic conditions cause the building to be over-ventilated half the time and under-ventilated the other half. Also, infiltration is poorly distributed throughout the house. A room with a couple exterior walls and leaky windows will get far more outside air than an interior kitchen or bathroom. Wind and temperature differences drive ‘natural ventilation’ in the form of infiltration in homes. However these factors are highly variable and unreliable.

To summarize the need for mechanical ventilation:

  • There are more pollutants in our homes than ever, requiring more ventilation air
  • Homes are better insulated and air sealed than they used to be
  • Much of the infiltration that does occur comes from undesirable locations
  • Even the portion of infiltration that can be considered “fresh air” varies sporadically based on weather conditions
  • Having air leaks in an insulated wall, attic or floor assembly can cause condensation and create structural failures.

For all these reasons, relying on air leaks as natural ventilation no longer works. It doesn’t work for normal homes, and it especially doesn’t work for insulated or tight homes.

Build It Tight, Ventilate It Right

The better approach is to provide controlled mechanical ventilation by providing enough air to meet ASHRAE 62.2 and air seal the house to prevent moisture issues, high energy bills, and air from the attic and crawlspace or basement from polluting our indoor air.  As the mantra goes, “build it tight, ventilate it right!”

A well-designed ventilation system brings several advantages.

  • It allows control over exactly how much fresh air is delivered and when.
  • You can adjust the amount of ventilation air if the occupancy changes (e.g. kids go off to college) or shut it down altogether while on vacation, or when windows are open.
  • It delivers a consistent amount of air year-round, no matter what the weather conditions.
  • It draws air directly from outside, so the air is guaranteed to be fresh.

The main disadvantage to mechanical ventilation is the cost to run the fan. There are many different types of systems, with widely varying costs. As the following case studies shows, this additional cost can be more than offset by the savings in reducing the uncontrolled infiltration.

Mechanical Ventilation Case Study

Consider the following single family detached home renovation project in Lancaster, Pennsylvania. Before renovation, the house had no mechanical ventilation, and much of the infiltration air came from the attic and basement, providing dirty air to the house. The house was leaky enough to meet ASHRAE 62.2 levels for natural ventilation. But with an infiltration rate of 1.1 air changes per hour, the house was replacing all its indoor air every hour, leading to huge heating bills.

During the renovation air sealing brought the infiltration down by 70% and mechanical ventilation was added to deliver the recommended ventilation rate, which in this case was 0.20 ACHn.

Looking at the annual utility bills, in the original house it cost almost $600 per year to heat the infiltration air. After air sealing this was cut to $217. Heating the ventilation air cost $174, and running the fan cost an additional $14 per year. Not only is the house now less drafty and more comfortable, the indoor air quality is substantially better AND the homeowner is saving $194 per year.

Not every case follows this same savings ratio. If the original house was  tighter to begin with there may not have been any theoretical savings. If the mechanical ventilation system were more efficient, there could be more savings.

But remember that mechanical ventilation puts the control in the hands of the occupant, not mother nature. If there seems to be too much ventilation, the occupant can dial it back. If there are indoor air concerns the occupant can increase the rate.

Designing an Effective Mechanical Ventilation System

There are several strategies for designing a good mechanical ventilation system, and there isn’t a one-size fits all approach for homes, multifamily buildings and commercial spaces. It’s important to keep occupants in mind and install the proper controls to make the system work for them. Everyday Green has helped MEPs and HVAC contractors select and size mechanical ventilation systems for all budgets and size buildings, homes and unit spaces. But one thing is clear: relying on air leaks to provide fresh air is no longer an effective strategy. Contact us today with your mechanical ventilation questions.

Andrea Foss

 

By Andrea Foss, Director,  Mid-Atlantic Sustainability Services