Critical to the make-up of a high-performance building, is a well-designed, carefully construction building enclosure. Thanks to advancements in building-science knowledge, building materials, and construction best practices, achieving a well-insulated, air-tight building envelope can be possible if executed correctly.
On this episode of Buildings and Beyond, we talk with SWA Senior VP and building enclosures expert, Bill Zoeller. Bill shares some strategies that professionals should consider when designing and constructing building enclosures and high-performance wall assemblies.
Episode Guests: Bill Zoeller
Bill Zoeller is a Registered Architect and Senior Vice President with Steven Winter Associates. He has 33 years of experience in building design and construction, building science research, energy-efficiency, disaster-resistant construction, and building materials product development. Bill has specialized expertise in advanced and traditional materials; design to resist natural hazards; energy efficient building practices; and energy upgrades in historic buildings. He has participated in product development and marketing analysis work for major building material manufactures and has worked on hazards resistance research for HUD and FEMA. Bill leads SWA’s team of enclosure specialists which has over 50 years of combined experience in condition assessments, design consulting and construction administrative services, and has participated on projects ranging from historic museums with rare collection archives, to high-rise Passive House towers.
Episode Information and Resources
About Buildings and Beyond
Buildings and Beyond is a production of Steven Winter Associates. We provide energy, green building, and accessibility consulting services to improve the built environment. For more information, visit www.swinter.com.
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Kitchen Ventilation with Iain Walker
Guest: Dr. Iain Walker from Lawrence Berkeley National Laboratory
Kelly: 00:01 Welcome to buildings and beyond
Robb: 00:09 The podcast that explores how we can create a more sustainable built environment
Kelly: 00:14 By focusing on efficiency, accessibility and health.
Robb: 00:18 I’m Robb Aldrich.
Kelly: 00:19 And I’m Kelly Westby.
Robb: 00:21 This week I’m talking with Bill Zoeller who is a senior vice president here at Steven Winter Associates. He and I have worked together for almost almost 20 years now and it’s been observed by several people that Bill and I really don’t have many conversations. We have many more arguments, but I think we’re both very well behaved in this episode. We’re talking about high performance envelopes, specifically high performance walls. I mean designing building good envelopes has changed a lot over the past couple of decades. So we dig into this, the reasons for this a little bit and specifically talk about some high performance wall systems.
Robb: 01:03 Thanks for being here, Bill.
Bill: 01:05 Thanks Robb. My pleasure.
Robb: 01:07 So I talked to you about doing an session on envelopes, a podcast episode on envelopes, which is a huge topic, but the way envelopes are designed and built now is so drastically different and continues to change a lot with new materials, with new techniques, new details. I wanted to dig into that topic a little bit and we ended up deciding to talk about walls because that’s what came up first because every building has walls. So big picture, why are envelopes and enclosures so different now?
Bill: 01:48 Well, in order to look at why they’re different now, I think we need to look at the context of where they were and where they were is really the impetus for why they changed and where they were a bunch of years ago, let’s say 10, 15, 20 years ago. If someone was building your basic house pretty much anywhere in the country it was framed with two by four walls, add ore 13 bad insulation, jammed into the stud cavities without much care or Thought of how good or neat or quality the job was. Essentially no care at all to any sort of air sealing between the inside and the outside. So basically you had you know, a frame wall cavity with a little bit of insulation in it and a lot of air blowing through it. Well, it wasn’t very good. And when the energy codes changed to improve the performance,
Robb: 02:46 Or actually came into being in lots of places
Bill: 02:49 Or came into being in lots of places, they incorporated some attributes that caused some changes to occur. For instance if we have your basic r-13 wall with bad insulation and no air sealing capability, what you’ve got is a little bit of thermal insulation that’s stopping the flow of heat from inside to outside a little bit. But all the air is blowing around it. So even if I have installation in the cavities, the outside air is blowing through the insulation via infiltration and really bypassing the thermal installation altogether. Then, the building codes increased a little bit. And we ended up with two by six walls, r-19 or r-20 insulation in that same space. And we started to employ some air sealing measures. It became known that air infiltration into buildings was one of the largest causes of heat loss and comfort issues and so on in buildings. So the next reasonable response is, well, let’s tighten up the buildings.
Robb: 04:06 But there wasn’t yet a durability concern.
Bill: 04:12 The building science was not really a known quantity when we started to do that. You know, like anything else, when we start employing new measures and new aspects of anything, there’s always unintended consequences and we certainly found them. In this case, the unintended consequences have to do with the second law of thermal dynamics. And usually when I start this conversation, I start with the first law of thermodynamics, which is basically in a nutshell that energy can’t be created or destroyed. The reason I start with the first law is because I really want to talk about the second law, but if I do that, people always say, well, what the heck’s the first law? So we got that out of the way, right? And the second law of thermodynamics is really the main issue with high performance wall assemblies and why theyre causing problems and why they can cause problems. And that’s that it’s the law of entropy, which basically means that any system of energy in an isolated chamber, or setting, wants to morph towards basically chaos. So if I have heat on one side of the wall and cold on the other side of the wall, what the energy wants to do is basically equalize itself on either sides of the wall. So the heat’s going from hot to cold. So if I’m hot on the inside of the house, cold on the outside of the house, the heat wants to flow through that assembly to get to the outside. But in order for that to occur, I need to have energy. The energy wants to move from hot to cold. But the transfer of energy actually takes two forms of energy to occur In the case of these buildings, in the case with thsese walls. There’s mechanical energy and that’s the air movement, which aids drying and then there’s thermal energy which also aids drying. And if I am reducing the air flow and the energy flow through that wall system, I am decreasing the capability of that wall system to dry. That’s the new part.
Robb: 06:41 Okay. All right. But it can go both ways. I mean, heat flow and energy flow can cause drying. It can also bring moisture into building cavities. It goes both ways, but I think it’s like, you know, 20, 30, whatever, long time ago, buildings were solely heat that even if moisture got brought into the wall cavity, it dried out pretty quickly because there was just so much air flow and heat flow.
Bill: 07:07 That’s absolutely correct. Right. So now we’ve, we’ve diminished the capability of the drying, but we didn’t diminish the capability of the wetting. It’s still raining on the outside of the building. We still have our cladding dealing with the bulk water as the water flows down in the siding of the building, probably leaks a little bit into the walls because you know, walls aren’t perfect. And we’ve got the other issue of the air infiltration from the inside of the building carrying the relatively warmer moisture moister air into the wall cavity, finding something cold and condensation on it. So the air infiltration for the inside can cause condensation within the cavity. So now we’ve set up a little problem for ourselves. We’ve, we’ve increased the thermal insulation and the air tightness a little bit, but we haven’t really dealt with the moisture management part. We’ve taken the wetting and the drying and pushed it a little bit out of balance. And when we push it a little bit out of balance, we ended up with dampness inside our walls and we get mold and decay and all kinds of bad stuff.
Robb: 08:23 So to keep water out?
Bill: 08:25 Keep water out. Water is the enemy. Bulk water management is one thing.
Robb: 08:31 Siding, flashing, secondary drainage plains?
Bill: 08:36 Well, you know, water resistant barriers need to be excuse me, adequate. They’re never going to be perfect. So we have to understand that they’re not going to be perfect. But they have to be well detailed. They have to be proper materials. The water has to be able to go some place. We see a lot of designs for wall assemblies where they look pretty good if you look at a basic section. But if you look at the details, the water really has no place to go. You know, you’re kind of trapped. There’s no capillary breaks. There’s that kind of an item.
Robb: 09:10 So let me back up a little bit here. So my perhaps simplistic take is that most walls have some kind of siding. The siding should keep most of the water out. If water happens to get behind the siding, through some defect in construction, or just crazy horizontal rain or up flow rain or you know, then there should be some kind of house wrap or building paper or something that the water can’t get through. Which people often call it the drainage plane. I like to call the secondary drainage plane.
Bill: 09:51 Well in code speak that’s the weather resistant barrier. And if I’m cladding my house in shingles or clapboard or something else, all that material overlaps and it’s got gaps in joints and you’re going to get wind driven rain in there and there’s going to be pressure differentials on the lease side of the house or the windward side of the house, depending on what the weather’s doing, the water’s going to get back there. So it needs a place to, you know, to go to, whether it’s a secondary drainage plane or a weather resistant barrier, it’s all that’s all about bulk water management. That’s what happens in the outside of the building.
Robb: 10:29 Gotcha. And those come in the form of like in frame construction, which is, you know, very common at least in residential, house wrap building paper or like zip, I see more and more zip walls going up, which has the weather resistant barrier on the face of OSB. Are all those created equal, all those more or less the same function?
Bill: 10:56 Well, no. They’re all different. And you know, it depends on the overall assembly of, of the wall as to which is going to work the best. If you look at the old school stuff you know, good old 15 pound building felt, that was in essence the weather resistant barrier. And no way was it an air barrier. You can’t tape it, you can’t glue it, you know, like originally when the house wraps came out, they were lapped six inches according to the manufacturer and stapled. And it’s like, Hey, this is an air infiltration barrier, isn’t it great? Well, you know, if I take a one by one sample and test it in the laboratory, it’ll come out as a rated air infiltration barrier. If I put it in a house and I don’t bother doing anything with the seams and joints, it’s not an air barrier of anything. It’s just for leaks. It’s like a bucket with holes in it. Even though the plastic bucket is made out of material, it’s got holes in it. So it’s not going to do any good in terms of holding water.
Robb: 11:57 And this is, I think one of the newer things, because 10-20 years ago when people came out with tapes that tape all the seams of your house wrap of your building paper of your zip sheathing then, you know, nobody trusted that those tapes would hold up over decades. And you know, they would decompose, the adhesive would break down and they’d fall off and then you’d get leaks, air leaks, water leaks, et cetera. I mean, do you agree? I mean, that seems like very new.
Bill: 12:25 Well, those concerns were well founded. They didn’t work. You know, in terms of longterm durability, there was issues.
Robb: 12:35 Okay, but now?
Bill: 12:35 Well there’s different materials on the market. Tapes are a lot better Generally. Some are still better than others, but you can get tapes and systems that absolutely work to air seal your building on the outside. And, and truly that’s the place you should air seal your building on the exterior sheathing plane. You want to air seal where the air movement is, the air movements on the outside, not the inside. So that’s your first line of defense. That’s where it ought to go.
Robb: 13:04 Alright. So keep the water out. Flashing details, you know, more important now than they ever were.
Bill: 13:11 Yeah. Because the drying capabilities lower.
Robb: 13:13 Because there’s low drying, and getting that weather resistant barrier, seamless, really impervious, is the way to go. You got to do it in a high performance wall.
Bill: 13:27 You have to do it in a high performance wall. A little bit of a problem in that barrier can cause big problems in the, in the overall wall assembly.
Robb: 13:35 So r-value. Higher our values. How do you do that in the best way? And again, in a framed wall now?
Bill: 13:41 Well there’s different approaches. There’s obviously the standard traditional way, is to fill the cavity with fiberglass insulation. That’s what we used to do. That works up to a certain point. It still keeps a lot of the thermal bridges in the form of the wood studs, for example, in place. So where the wood studs are, I don’t have any installation at all. I have wood studs.
Robb: 14:02 R-3 or whatever. Less than five.
Bill: 14:06 So really now the more effective approaches utilize continuous exterior insulation. A layer of insulation that basically blankets the entire building around the exterior, covering all the thermal bridges. So that for as an example, I’ll have a and r-5 or r-6 or an r-7 continuous around the entire building envelope. That’ll be directly integrated with my air sealing. Then if I put cavity insulation in place the cavity insulation will actually be effective to its rated value or close to it, because the thermal bridges essentially been defeated.
Robb: 14:46 Okay. So continuous installation is the way to go, you think?
Bill: 14:50 It’s one of the better approaches. Again, there’s different approaches, but that absolutely is a preferred approach, I’ll call it.
Robb: 14:59 And the material? Foam is what I see mostly. Rigid foam?
Bill: 15:05 Rigid foam works. You know, if you use rigid foam, you could do your air barrier on this and the weather resistant barrier that we talked about before. The secondary drainage plane as we called it, on the surface of the foam. There’s other materials I could use like mineral wool, rigid mineral wool for continuous installation on the exterior, in which case I have to use the sheathing below it, underneath it, for the weather resistant barrier and my air ceiling barrier, because the mineral wool is not an air barrier. The cladding or siding needs to be installed on furring strips of some sort, because you can’t nail directly through the mineral wool because it’s a compressible material, but it’s drainable. So you know, if you’re going to do a rain screen, which basically puts a gap between the sheathing and the back of the cladding as a siding approach, then the mineral wool works just fine.
Robb: 16:08 Rainscreen is any gap? I mean, furring strips constitute a rain screen?
Bill: 16:12 Well, you could do a rain screen multiple different ways. Basically, it’s an air gap or a capillary break between the back of the sheathing and whatever the sheathing is attached to. So if I have a drainable insulation, like mineral wool, the back of the cladding, whether it’s clapboard or something else, will be spaced out at least typically a quarter of an inch from the face of the insulation, so that if water gets back there, it’s got a place to drain. It doesn’t cling to the back of the installations, there’s a little air movement up in there. And we talked about, you know, the second law of thermodynamics before, if I’ve got some air movement, I’ve got some higher drying capability. So I’m introducing some drying capability within that wall cavity.
Robb: 17:04 So the double wall systems, you know, dub two, 10 inches, 12inches, packed with cellulose.
Bill: 17:08 What you have is basically two parallel stud walls separated by a few inches. So the total dimension of the walls, as you said, is 10, 11, or 12 inches. Then the whole thing is, is packed with dry blown-in, dense pack cellulose. That’s a lot of insulation, you know, at at 3.2 inches, r-3.2 or 4 per inch or whatever, times 10 inches, you know, r-30 r-30+. So what you’ve got there is a whole lot of thermal insulation. So you’re stopping the energy heat flow, and you’re getting some air sealing, but it’s not a complete vapor permeable solution. So that if there are air leaks, which there will be a little bit, there always are. Interior, more moisture ladened air can get into that wall system, find its way to the exterior sheathing and potentially condense, because the exterior sheathing does not have the continuous insulation we talked about. So it’ll be colder. And if it’s colder and my interior air gets to it, the relative humidity goes from 50% to 100%, because the air goes from 70 degrees to 10 degrees. I can get some condensation issues, some dewpoint problems and end up with a big problem. So if you are going to do the double stud wall, there are some products in the market that are highly recommended to make it safe generically called Smart Vapor retarders that you would want to install on the inside of the wall behind your drywall to make that type of a wall system basically a safe wall system. Without that, it’s potentially problematic.
Robb: 19:05 And a rainscreen or vented siding is even more.
Bill: 19:08 In combination with the vented siding, the rain screen. Yeah. You know, we talked about the continuous insulation a little bit before. And what the continuous installation also does, just to sort of amplify that a little bit, is that it’ll take that same exterior sheathing and raise the temperature of it, because you’re putting your continuous thermal barrier between the outside weather and the exterior sheathing so that the temperature of that sheathing remains above the dewpoint under all design conditions. So, even if I have air leakage into the cavity with say, you know, two inches of xps or poly ISO insulation and say in r-19 or r-20, in a two by six cavity in most climate zones in the U.S., that’ll be a safe wall, because the sheathing stays above the dewpoint. So any air that leaks into that assembly still will not condense.
Robb: 20:05 It makes sense. I mean, it all makes sense. I see people wanting to stay away from petrochemicals and most of the rigid installation, continuous insulation is foam. But you said there are alternatives. There’s some mineral wool or there’s some cellulose based rigid insulation.
Bill: 20:22 There’s some European type products that are finding their way into the U.S. Market slowly that are essentially wood based, but they perform more or less like a mineral wool, rigid installation and they have the same drawbacks. You know, You don’t want to nail directly to them. You want to provide a capillary break, you want to give them a chance to drain behind your cladding and so on. But there’s plenty of options.
Robb: 20:45 And more and more. It’s hard to keep up with it all, but more options.
Bill: 20:54 More options is good, but includes more confusion on the part of the designers and builders that are using them. Because if there’s a lack of basic understanding of some of the building science involved they’re relying on your particular marketing information, which is always going to be a little bit incomplete because it’s designed to sell stuff. You know, let’s be honest.
Robb: 21:21 Yeah, definitely. All right. Switching gears a little bit, masonry, what’s different in masonry?
Bill: 21:30 Well, let’s start with what’s the same in Masonry? And in masonry, if we’re talking about new construction, you really want to employ the same strategies you’re doing in frame construction and that’s the continuous exterior insulation barrier outside the whole thing, jacket the whole darn thing. And some sort of an insulation jacket, be it xps, mineral wool or poly ISO, or something else, to the point where the inside surfaces of the masonry, if it’s a CMU block and plank building for instance, stays above that dewpoint. Same rules apply. If I don’t have enough continuous insulation on the outside, and my CMU backing gets below the dewpoint, my warm interior air at 50% relative humidity can get in there, contact that masonry, get down to 50 degrees, and 100% relative humidity, and I have a moisture problem. So there it’s the same. In terms of continuous air barriers and weather resistive barriers, the place to put that typically is on the direct face of the concrete masonry units. The CMU- I’m sorry for using the jargon, I get carried away sometimes. You know, that kind of a system you’ll see with either a brick veneer exterior, we’re seeing a lot more rainscreen cladding systems, metal panel systems, terracotta panel systems, we’ll employ that sort of a strategy. So we’re seeing a lot of different approaches that actually work now that the building science is a little bit better understood. And there’s more products to provide more variation that are, you know, desired by designers.
Robb: 23:21 So the weather resistant barrier, on the outside of the masonry, and most if not all the insulation outside of that.
Bill: 23:30 generally that’s going to be a higher performing approach. Yes. Yeah, absolutely. Yeah. I mean, you could, you could come in with some like age still framing inside your concrete block with some, you know, essentially auxiliary installation material, to get a boost in r-value or meet energy code or whatever you’re going to do with it. But it’s really that outside continuous insulation that’s performing most of the work.
Robb: 23:53 And this is typical in the northeast for multifamily, new construction, some commericial?
Bill: 23:59 In urban locations, yeah.
Robb: 24:00 And then in the rest of the country is, I mean, we’ve been talking about colder climates generally, does the same hold true? I mean, the r-values are lower required by code as you get further south, as you get warmer. I mean, are there different challenges, similar challenges?
Bill: 24:20 Well, the challenges are, are, are different all over. In general, the masonry buildings are going to be safer in terms of moisture performance cause they’re a little bit more idiot proof. You know, if we look at frame construction, when you get down into the south, the rules are a little different there. If I’ve got a cooling dominated climate, and the interior temperature is kept at say 70 degrees, and it’s 95 degrees and you know, 80% relative humidity outside, and I have any moisture drive at all, the thermal dynamics doesn’t change. It’s just that the flow of energy is now different, and it wants to flow from hot to cold. So in that case, it’s outside to inside. So now if I have air leaks, I’m drawing the exterior of the hot sticky air through that wall assembly to the inside. And if I have a vapor retarder on the inside, even if it’s by accident, like vinyl wallpaper, I have a big problem because that’s where the moisture stops. I’ll get condensation, I’ll get mold, I’ll have all sorts of problems. So it really is climate specific, how you deal with it. When you’re looking at climates in between, that can be both, say Washington DC, you know, it can get New York cold or it can get Florida hot. So, you know, you really have to understand the dynamics in designing high-performance wall systems, especially in those mixed climates.
Robb: 25:47 In Phoenix, the risks are lower, I imagine.
Bill: 25:51 In Phoenix or you know, most of California, the risks are lower because yeah, the enemy is water, and there’s just less of it in the air. So it tends to not be as much of a problem.
Robb: 26:04 So if your brother called you up, said he’s building a new house and asked you what kind of wall he should use in a single family home in Connecticut, in New York. What would you tell him?
Bill: 26:15 I just had this conversation with my brother yesterday. No, I didn’t. But I mean, you know, in a typical northeastern climate zone five scenario, likely be a two by six wall, it could be maybe 2×4 wall. Builders don’t want to do that anymore, but there’s nothing wrong with it because structurally it’s fine. And if I can get my continuous insulation on the outside, do my air barriers out there, do a continuous, r-15 on the outside, which is, you know, two and a half inches of XPM of poly ISO, three inches of xps and do an r-13 bat or an r-15 bat, that’s an r-30 wall and a two by four wall. Nothing wrong with that. So if you’re looking at low cost, high performance, that is probably a really good approach. A lot of builders will just say, I’m not going to do two by fours because blah, blah, blah. So they want to do two by six. And I’m like, okay, well then do you know r-10 continuous outside and then do r-19 batts. You want to make sure that the continuous insulation stays within a fairly high ratio of the total installation. You know, like 33% probably is a minimum for condensation control. So that’s, you know, basically a rule of thumb. You’d want to look at the actual attributes and parameters of your materials before making that decision. But that’s a pretty good rule of thumb to start with for basic planning purposes.
Robb: 27:48 Okay. And further south, is there a line where continuous is really kind of not really required? I mean, in Georgia, do you really need continuous insulation? The code doesnt require it?
Bill: 28:01 No, the code doesn’t require it. And you know, again, if you don’t do the continuous installation, you’re still gonna have the thermal breaks. The thermal bridges rather, but the thermal bridges will be less important because the temperature differential is less. If I’m 70 degrees on the inside, 90 degrees on the outside, that’s a Delta t of 20 degrees, not a big deal. In New York, if I’m 70 degrees on the inside and 10 degrees on the outside, that’s a 60 degree temperature differential three. So, you know, the, the thermal drive is three times greater, but you’re still not getting the complete performance package. If you do continuous insulation, you know, again, you gotta pay attention a little bit differently to the dynamics, but you can still do high performance wall. You could do a high performance wall without the continuous easier in the south than you can in the north.
Robb: 29:00 Yup. Okay. All right. So I mean, keep the weather out is number one. Bulk water management, siding, drainage, probably number one. Air tightness, probably number two.
Bill: 29:07 Agreed.
Robb: 29:12 And then from there it’s designing for forgiveability or if something does go wrong, you’d get a little bit of leakage or you get a little bit of water leak.
Bill: 29:32 Yeah. it’s understanding what the vapor control issues are. Dewpoint control, Condensation Control, vapor dynamics, and it all changes a little bit based on how much r-value you put in because of that very unforgiving second law of thermodynamics.
Robb: 29:52 Alright, so Okay. If we talk about this again in five years, what are we going to be talking about? Any new systems you think new systems, new products?
Bill: 30:01 Well, the physics will not change. I can Pretty much guarantee that. There’s always going to be new systems on the market introduced about half of which will make it, half of which will make it, half of which won’t, and 10 or 20%, that’ll be pretty good. The other 30% that make it, that aren’t pretty good, will make it on behalf of their marketing strength, but that’s okay too. I don’t see anything really new and upcoming that’s going to be earth shattering. There’s a lot of stuff that people always think is going to be, you know, we saw, you know, panelized construction, sip construction and all that kind of type of product. They’re fine. You know, you just have to understand how they work and where are they working, and what to avoid in terms of problems.
Robb: 30:54 But they haven’t taken over the world.
Bill: 30:55 Yeah, yeah. No, they’re, you know, the 2% market share or whatever it is and they’ve been around for a while so they haven’t, you know, there’s a reason they haven’t taken over the world. You know, but they’re perfectly fine product and perfectly acceptable for use in modern buildings. Whether something new comes along that, you know, usurps, all of that, you know, that’s crystal ball time. I don’t see it right now.
Robb: 31:17 All right. Awesome. Thanks very much.
Bill: 31:22 Hey, thanks for having me.
Speaker 2: 31:27 Thank you for listening to buildings and beyond. For more information about the topics discussed today, visit www.swinter.com/podcast and check out the episode show notes buildings and beyond is brought to you by Steven Winter Associates. We provide energy green building and accessibility consulting services to improve the built environment. Our professionals have led the way since 1972 in the development of best practices to achieve high performance buildings. I’ve production team for today’s episode includes Dylan Martello, Alex Mirabile , and myself. Heather Breslin, thank you for listening and we’ll see you next week.