How to Design a Wall

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How to Design a Wall

Common-sense advice for people who can’t decide how to build their walls

Posted on Feb 19 2016 by Martin Holladay
prime

Builders love to talk about walls. Almost all of us are willing to argue about the best way to build a high-R wall, and we love to debate whether certain wall details are environmentally friendly enough to be considered “green.”

Although these conversations can be fun, our obsession with wall details is often misplaced. Details that inflame our passions are often irrelevant. In most cases, we should just choose a relatively airtight easy-to-build wall with good flashing details — one with an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. in the range of R-20 to R-40 — and be done with it.

I probably shouldn't admit this fact, but it's true: when a GBAGreenBuildingAdvisor.com reader posts a question proposing a new type of wall assembly, I sometimes sigh. I wonder whether it's time to dial back our wall discussions and to spend more time talking about air barriers or windows.

Two popular approaches

Now that I’ve gotten my rant out of the way, I’ll provide some advice on walls. For readers who don’t have time to get bogged down in details, here’s the short version of my wall advice:

  • In most of the U.S. and Canada, there are two cost-effective ways to build a wall with an R-value that exceeds minimum code requirements. The first approach is to build a double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation.. The second approach is to build a 2x6 wall with a continuous layer of insulation (usually rigid foam or mineral wool) on the exterior side of the wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. .
  • Either of these approaches works very well, as long as the builder understands a few basic principles of moisture management.
  • Other types of wall assemblies, including walls using ICFs, SIPs, Larsen trusses, straw bales, or adobe, can be made to work, and in some cases have a few advantages for certain locations or applications. But these approaches tend to cost more than a double-stud wall or a 2x6 wall with insulation on the exterior side of the wall sheathing.

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Six Proven Ways to Build Energy-Smart Walls


Double-stud walls

Energy-conscious builders in North America have been building double-stud walls for at least forty years. (In 1978, for example, Gene Leger built an energy-efficient house with cellulose-insulated double-stud walls in East Pepperell, Massachusetts.)

For more information on double-stud walls, see the following GBA articles:

Double-stud walls have two parallel framed walls — either two 2x4 walls, or one 2x4 wall and one 2x6 wall. The total wall thickness is variable; while 12-inch-thick walls are common, it’s also possible to build a double-stud wall that is 9 inches thick or 14 inches thick.

If you plan to build a double-stud wall, you need to decide which of the two walls will be your bearing wall: the inner wall or the outer wall. Either approach can work, as long as you have a continuous load path from the bearing wall down to the foundation.

In recent years, some building scientists have suggested that the exterior wall sheathing of a double-stud wall is at risk for moisture accumulation in cold weather. For more information on this issue, see:

Experts advise that the most robust double-stud walls include the following features:

  • Any type of exterior sheathing material other than OSB — for example, plywood, diagonal boards, fiberboard, or fiberglass-faced gypsum panels.
  • A ventilated rainscreen gap between the siding and the water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material. (WRB).
  • A smart vapor retarder (for example, MemBrain) or vapor-retarder paint on the interior side of the wall — or (for builders who are following Joseph Lstiburek’s specifications) a layer of OSB or plywood sandwiched between the two 2x4 walls. (For more information on Lstiburek’s double-stud wall specifications, see Lstiburek’s Ideal Double-Stud Wall Design.)

Double-stud walls are considerably more expensive than walls with a single row of studs, so builders in warm climates (Climate Zones 4 and warmer) should probably stick with a conventional 2x4 or 2x6 wall, with or without exterior insulation.

2x6 walls with exterior rigid foam

Energy-conscious builders began switching from 2x4 wall framing to 2x6 framing in the 1970s. There are two problems with most 2x6 walls:

  • The nominal R-value of the insulation is limited by the depth of the stud space. In most cases, 5½ inches of fluffy insulation has an R-value of R-20 or less. In cold climates, that’s not much.
  • Because of thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. through the studs, the whole-wall R-value of a 2x6 wall insulated with R-20 insulation isn’t R-20 — it’s about R-12 to R-17, depending on the wall’s framing factor and how well the insulation is installed. (For more on this topic, see What is Thermal Bridging?)

The R-value of a 2x6 wall can be greatly enhanced by adding a continuous layer of rigid foam on the exterior side of the wall sheathing. For more information on this type of wall, see How to Install Rigid Foam Sheathing.

Builders who install rigid foam on the exterior of a wall need to make sure that the rigid foam is thick enough to keep the wall sheathing above the dew point during the winter. My article, “Calculating the Minimum Thickness of Rigid Foam Sheathing,” includes a table showing the minimum R-values for exterior foam in a variety of climate zones.

If a builder installs rigid foam meeting the minimum R-values shown that article, here’s what the wall would look like:

Builders in climate zones that are warmer than the zones shown in the table can install any thickness of exterior rigid foam they want, without worrying about moisture accumulation or condensation problems. In these warmer zones, many builders choose to install 1½-inch-thick Zip-R sheathing — a type of sheathing that combines OSB with R-6.6 of rigid foam.

While this table shows the minimum recommended thickness of exterior rigid foam in a variety of climate zones, nothing prevents a builder from installing rigid foam that is thicker than the examples shown in the table.

Walls with exterior rigid foam are designed to dry to the interior, so builders should never install an interior polyethylene vapor barrier or vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). wallpaper on this type of wall.

Of course, in warmer climate zones, where walls don't require the high R-values sought by cold-climate builders, 2x4 framing may be perfectly adequate. The thermal performance of a 2x4 wall (like the thermal performance of a 2x6 wall) is greatly enhanced by the addition of a layer of exterior rigid foam.

What if I want to install exterior mineral wool?

Mineral wool insulation can be substituted for rigid foam insulation on the exterior side of wall sheathing. One advantage of mineral wool over rigid foam: because mineral wool is vapor-permeable, it doesn’t inhibit wall sheathing from drying to the exterior. That means that builders can install mineral wool of any thickness on the exterior side of their walls. You don’t have to worry whether exterior mineral wool meets any minimum R-value requirement. (Of course, thicker insulation always does a better job of resisting heat flow than thinner insulation.)

For more information on the installation of mineral wool insulation on the exterior side of your wall sheathing, see these articles:

What type of insulation should I install between the studs?

Double-stud walls are always insulated with an air-permeable insulation — for example, fiberglass batts, blown-in fiberglass, cellulose, or mineral wool — rather than spray polyurethane foam. (Spray foam insulation is too expensive to install in such a thick wall.)

If you're building a 2x6 wall with exterior rigid foam, it's also best to avoid using spray polyurethane foam between the studs. When I hear that a builder wants to install spray foam between the studs of a wall with exterior rigid foam, here's what I advise: If you want to use foam insulation, you have to choose where you want the foam to be located. It can either be located between the studs, or it can be located on the exterior side of the sheathing — but it shouldn't be installed on both sides of the sheathing. Why? Because if you enclose the OSB or plywood sheathing with foam insulation on both sides, the foam can inhibit drying. If the sheathing ever gets damp, it won’t be able to dry out very quickly.

If a builder asks, "Where's the best place to put the foam?", I always answer, "On the exterior side of the sheathing." There are three reasons why this makes sense:

  • Spray foam insulation is expensive.
  • Most brands of closed-cell spray foam insulation are manufactured with a blowing agent that has a very high global warming potential.
  • Exterior rigid foam addresses thermal bridging through the studs, while spray foam insulation between the studs does nothing to reduce thermal bridging. The thermal bridging penalty is so great that it makes the high cost of spray foam insulation hard to justify compared to less expensive types of insulation. (For more information on this issue, see "Installing Closed-Cell Spray Foam Between Studs is a Waste.")

If a builder insists on using spray foam between the studs as well as rigid foam on the exterior side of the sheathing, I advise the builder to use open-cell spray foam (which is vapor-permeable) rather than closed-cell spray foam (which has a very low vapor permeance). That way, the sheathing will be able to dry inward if necessary.

No matter what type of air-permeable insulation (cellulose, fiberglass, or mineral wool) that you choose to install between the studs, it's important to make sure that the insulation fills all of the wall's nooks and crannies, without leaving any voids. This is easier to achieve with a blown-in insulation than a batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. .

If you decide to install cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. — a good choice, in my opinion — it needs to be installed using the dense-pack method. (For more information on dense-packed cellulose, see How to Install Cellulose Insulation.) Dense-packing a double-stud wall is trickier than dense-packing a 2x6 wall; special techniques are used to make sure that every part of the wall is insulated to the same density.

While experienced cellulose installers usually aim for a density of 3.5 pounds per cubic foot for 2x6 walls, a double-stud wall needs to be insulated to a density of 4 pounds per cubic foot to eliminate any possibility of settling.

Other options

What about other types of walls — for example, earthship walls, straw-bale walls, autoclaved aerated concrete (AAC) walls, adobe walls, Larsen truss walls, Klingenberg walls, PERSIST walls, insulated concrete form (ICF) walls, or structural insulated panel (SIP) walls?

All of these walls have their advocates. Some of these walls have definite advantages; for example:

  • Adobe walls use natural raw materials that can sometimes be gathered at the job site;
  • ICF walls are tornado-resistant; and
  • SIP walls can be erected very quickly.

If these characteristics are important to you, you can certainly choose a wall on this list. (In the “Related Stories” sidebar on this page, you will find links to many articles that discuss these options.)

That said, all of these walls either cost more, or perform worse, than a double-stud wall or a 2x6 wall with exterior rigid foam.

What do I need to know about vapor drive and vapor permeance?

Many green builders think that a wall has to “breathe.” While “breathing” is a poorly defined concept, it usually refers to a wall that is vapor-permeable.

The classic example of a vapor-permeable wall is a straw-bale wall. During the winter, water vapor moves from the warm, humid interior of the building towards the cool, dry exterior. In the case of a straw bale wall, the water vapor moves right through the straw bales. As long as the water vapor doesn’t encounter an exterior vapor barrier, it will eventually evaporate.

During the summer, the direction of the vapor flow reverses. Water vapor will move from the hot, humid exterior toward the cool, dry interior.

This type of vapor flow can sometimes cause problems. For example, the wall sheathing on a double-stud wall is cold during the winter. Plywood wall sheathing isn’t as vapor-permeable as straw, so moisture can accumulate at the cold wall sheathing. Whether or not this type of moisture accumulation is a problem depends on how quickly the wall sheathing can dry to the exterior in the spring.

Similarly, inward vapor flow during the summer can be a problem if an air-conditioned home has walls with an interior layer of polyethylene. In some cases, water vapor that is driven inward by the sun shining on damp siding can condense on the cool polyethylene, leading to puddles at the base of the wall.

Builders use a variety of techniques to limit problems arising from vapor drive. Here are a few important points:

  • There is no truth to the belief that vapor-permeable walls perform better than walls that include a vapor barrier. In fact, limiting the flow of vapor through floors, walls, and ceilings is often essential. (A wall with a vapor barrier can perform very well or very poorly. In general, vapor barriers that provide R-value — for example, foil-faced polyisocyanurate — usually cause fewer problems than vapor barriers that have no R-value — for example, polyethylene sheeting.)
  • Wood-framed walls with an adequate layer of exterior rigid foam do an excellent job of preventing problems associated with vapor drive. The exterior rigid foam keeps the wall sheathing warm and dry during the winter, so moisture can’t accumulate in the sheathing. Moreover, the rigid foam prevents inward solar vapor drive during the summer. Everything on the exterior side of the rigid foam dries to the exterior; everything on the interior side of the rigid foam dries to the interior.
  • As noted earlier, the sheathing in a double-stud wall is far more likely to get damp during the winter than the sheathing in a wall with exterior rigid foam. This wintertime moisture accumulation won’t necessarily lead to problems, however, as long as the builder remembers to include good details. Double-stud walls should include a ventilated rainscreen gap between the siding and the WRB, as well as a smart vapor retarder or vapor-retarder paint on the interior side of the wall. Builders should also consider installing a type of wall sheathing that is more vapor-permeable than OSB.

For more information on this topic, see All About Vapor Diffusion.

Every wall needs a water-resistive barrier

Building codes require, and building scientists recommend, that every wall include a water-resistive barrier (WRB). This WRB is usually located between the siding and the sheathing. Examples of WRBs include asphalt felt, Grade D building paperTypically referring to Grade D building paper, this product is an asphalt-impregnated kraft paper that looks a lot like a lightweight asphalt felt. The Grade D designation has come to mean that the building paper passes ASTM D779 (minimum 10-minute rating with the “boat test”) and different products are called out as “30-minute” or even “60-minute” based on D779 results. At times confused with roofing felt, roofing felts and building paper differ in two ways: felts are made of recycled-content paper, building papers of virgin paper; felts are made of a heavier stock paper; building papers a lighter stock. See also roofing felt., plastic housewrap, liquid-applied barriers, rigid foam, and Zip sheathing.

For more on this topic, see All About Water-Resistive Barriers.

Flashing details and rainscreen gaps

The most common way that walls get wet is when rain leaks past defective wall flashing. Typical problems occur at windows — especially at the two lower corners of windows that lack sill pan flashing — and at penetrations like cantilevered joists or porch railings secured to the wall. If you want your wall to stay dry, you need to have a thorough understanding of flashing techniques, and these techniques need to be carefully executed. (For more information on this topic, see All About Wall Rot.)

Wall flashing must always be integrated with the wall's WRB — so if you don't know where the WRB is, you can't flash the wall. If you're using asphalt felt, Grade D building paper, or plastic housewrap as your WRB, all horizontal WRB seams (and all seams between the WRB and flashing) must be lapped shingle-style to shed water away from the house.

In addition to paying close attention to flashing details, it's important to include a ventilated rainscreen gap between the back of your siding and your water-resistive barrier (WRB). This rainscreen gap adds a lot of forgiveness to your wall: even if your wall has a flashing problem, the rainscreen gap can often save the wall by allowing the water to drain and by accelerating evaporation. (For more information on this topic, see All About Rainscreens.)

Pay attention to airtightness

If you care about your home's energy performance, you need to focus on airtightness. Exterior walls are part of your home's thermal envelope, so it's important that walls be built to be as airtight as possible.

Most energy-conscious builders now realize that the easiest way to create an air barrier for your wall is to tape the seams of the OSB or plywood wall sheathing with a high-quality tape. For more information on this issue, see these two articles:

Of course, if you decide to create an air barrier at the wall sheathing, taping the sheathing seams is just one aspect of the job. All of the wall penetrations, including windows and doors, need to be sealed to limit air leakage. Pay attention to tricky areas that have traditionally been missed, including the gap between the bottom plate of your wall and the subfloor. (This crack can be sealed with an EPDM gasket installed before the wall is raised, or with caulk or tape after the wall has been raised.)

Conscientious builders often install two air barriers: one on the exterior side of the wall, and one on the interior side of the wall. For more information on this approach, see these two articles:

Which type of wall is best?

People who are planning to build a home sometimes ask me, "I can't decide between a double-stud wall and a 2x6 wall with exterior rigid foam. Which type of wall is best?"

I answer, "Either type of wall works fine. The best type of wall is the one your builder prefers."

Martin Holladay’s previous blog: “Insulating Walls in an Old House With No Sheathing.”

Click here to follow Martin Holladay on Twitter.


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Image Credits:

  1. Lucas Durand

1.
Feb 19, 2016 11:42 AM ET

Kudos
by C. B.

This is one of the best articles written on GBA in a while. Important. Detailed (without being overly so). Thorough.

In my humble opinion, this type of article needs to be seen more frequently on GBA and fewer articles on green energy philosophy.

Nice job, Martin!


2.
Feb 19, 2016 11:57 AM ET

Edited Feb 19, 2016 1:32 PM ET.

Response to C.B.
by Martin Holladay

C.B.,
Thanks for your feedback. A fairly complete list of all of my "how to" articles can be found on this page: How To Do Everything.


3.
Feb 19, 2016 3:25 PM ET

Edited Feb 19, 2016 3:27 PM ET.

I agree
by Malcolm Taylor

Great article. However if Martin thinks this will in anyway stem the flood of wall innovations from posters I think he is going to be disappointed.
The photo of Lucas Durand's build made me realize we never got to see the end result. I hope he drops in here sometime to update us on how it turned out.


4.
Feb 19, 2016 3:58 PM ET

Builder, architect, CPHC?
by Ethan T ; Climate Zone 5A ; ~6000HDD

Thank you for the information... Perhaps we could modify the assertion that "[t]he best type of wall is the one your builder prefers" to read "[t]he best type of wall is the one your builder, architect, or CPHC prefers. I've had too many builders value engineer all the extra insulation and fancy sheathing out of my walls to be comfortable with that assertion as currently written.


5.
Feb 19, 2016 4:24 PM ET

Edited Feb 20, 2016 5:02 AM ET.

Response to Ethan Timm
by Martin Holladay

Ethan,
If a residential construction project has a team that includes an architect, a builder, and a homeowner -- and in some cases a certified Passive House consultant -- then of course everyone has to get on the same page, ideally during pre-construction conferences. Everyone has to be on board: the builder, the designer, the homeowner, and any consultants who are part of the team.

If, in your experience, you have had jobs where the "builders value-engineer all the extra insulation and fancy sheathing out of my walls," you are talking about a project (and a team) with serious communication problems. On jobs like that, the team has to come together to resolve the communication problem, or you will end up with many more issues than a problem with missing "fancy sheathing."


6.
Feb 19, 2016 4:50 PM ET

NICE REVIEW
by KEVIN ZORSKI

Martin - Nice review article.


7.
Feb 19, 2016 5:33 PM ET

Edited Feb 19, 2016 5:33 PM ET.

A further simplification
by Eric Habegger

Martin, since we are on the subject of simplification perhaps a further simplification can be made. I'm sure some people will disagree with my assessment but here goes: If you live in zone 4 or warmer then ditch the double wall approach. It's a needless complication and expense. It's so obvious maybe you felt you didn't need to say it.

I would go one step further and use a sheathing in those zones that integrates the sheathing with the insulation, waterproofing, and an integrated flashing technique. Just choose the sheathing with the proper level of insulation for your zone and the level of insulation between the studs. One more thing, since it seems one can never depend on people using common sense, make sure that your team is up to date on the very slight changes in flashing required for a system like that, such as the zip system. Otherwise one negates all of the very real advantages of using that system. I can easily imagine that happening if one is too over confident.


8.
Feb 20, 2016 5:06 AM ET

Response to Eric Habegger
by Martin Holladay

Eric,
Excellent comments. I will edit my article to reflect some of your recommendations.

My only quibble: I'm reluctant to advise builders to use sheathing (Zip-R sheathing) from a single manufacturer. While all reports indicate that Zip sheathing and Zip-R sheathing are excellent products, it's hard to predict whether problems will arise with these products in the future, or whether Huber Engineered Woods will still be in business in 10 years.


9.
Feb 20, 2016 10:03 AM ET

Are they really so easy?
by Charlie Sullivan

I generally agree with the theme here, that we should stop re-inventing high-r-value wall assemblies for each job, and that people should pick one of these two, based on what the team (particularly the builder) is comfortable with and proceed. But the reality is that both are still tricky and can be unnecessarily expensive. I think there's a real need to keep refining the systems for each and sharing and discussing experiences.

For example, I used to be a true believer in dense packed cellulose, based on it's ability to "fill all of the wall's nooks and crannies, without leaving any voids," which I'd heard contrasted to the need for meticulous installers to install batts without major deficiencies, a very real problem. But cellulose doesn't magically install itself -- when I tried to get my deep double stud wall packed by what was supposedly the best crew in the area, it took more visits than I can count, plus the help of a nationally recognized expert (Bill Hulstrunk) before we got a a halfway decent result.

That experience increases my appreciation for the Lstiburek Ideal Double-Stud Wall Design, because the plywood in the middle divides the cavity into two smaller cavities. Each of those is easier to fill because it's more like what installers are used to and because you don't need as high density to prevent settling in a cavity that's not as deep. Furthermore, you can use an air-permeable netting (insulweb) to retain the cellulose on both sides, avoiding the challenge of blowing insulation into an air-tight cavity that has Intello membrane on the inside. And you have the additional real-world advantage that the mid-wall air barrier requires less coordination between trades in the process of getting to an air-tight wall.

And once you pick a wall construction method, you also need to figure out how to install the windows, which results in another round of re-inventing the wheel, and, once you decide what to do, a round of building complex stuff on site that I think would have been better to build in a window factory. Window manufacturers, particularly those that specialize in high-performance triple-pane windows, should realize that a lot of their customers are re-engineering the installation systems. They should offer the option of windows that come ready to install in deep walls, with a straightforward flashing system with pre-cut pieces that anyone can install right. I think that part of the reason they don't do that is that there are so many different approaches to high R-value walls that it doesn't make sense for them to engineer a different system for every one. Maybe the result of Martin's article will be more standardization of wall construction, enabling windows to be engineered to match.


10.
Feb 20, 2016 5:41 PM ET

Martin, what can we say? Many
by David Gadbois

Martin, what can we say? Many of the folks around here are often creative types thinking outside of the box. I am considering adapting a medium-gauge steel frame system with Joe L's perfect wall. No wood, no drywall. There is no blueprint or even precedent when you get creative. So we need to ask the smart folks here about it.


11.
Feb 20, 2016 6:08 PM ET

David,
by Malcolm Taylor

You wrote: "There is no blueprint or even precedent when you get creative."

I don't think that is true on any field, and certainly not in construction, which is an applied science. Creativity allows the advancement of any field of human culture by building on past achievements - and that can only be done if you have a firm knowledge of the foundations of the field you are trying to advance. It is a true for building science as it is in physics, software development, poetry or painting. The important innovators put in the hard work to understand what is at stake and what is at play. What you are calling creativity is simply idle speculation.


12.
Feb 21, 2016 4:34 AM ET

Edited Feb 21, 2016 4:41 AM ET.

Response to David Gadbois
by Martin Holladay

David,
If you plan to put all of the wall insulation on the exterior side of the sheathing -- following the PERSIST method -- then there won't be any thermal penalty from using steel studs. You can do that.

The main downside of the PERSIST approach is that some builders are tempted to skimp on R-value. The best PERSIST walls exceed code-minimum R-values. In cold climates, that requires a lot of rigid foam.

I'm not sure what you mean by "No drywall." I guess that means either (a) that you will finish the interior side of your steel studs with a material other than drywall -- perhaps plaster? -- or (b) that you want to look at the empty steel stud bays. If (b), make sure that it is legal to leave your wiring exposed -- and get ready for lots of regular dusting.


13.
Feb 21, 2016 4:37 AM ET

Response to Malcolm Taylor
by Martin Holladay

Malcolm,
In your comments, you're implying that David Gadbois may not have a firm understanding of building science principles. I'm inclined to give him the benefit of the doubt on that point, and to address his creativity without impugning his knowledge.


14.
Feb 21, 2016 5:30 AM ET

Response to Charlie Sullivan (Comment #9)
by Martin Holladay

Charlie,
You've raised some important issues. I'll address them one at a time.

1. The types of walls described in this article are "unnecessarily expensive." This is a real issue, especially for Passivhaus builders. These days, energy is cheap. Moreover, PV systems continue to drop in price. Old rules of thumb -- for example, "cold climate builders need to aim for R-40 walls" -- no longer apply. Trying to figure out whether to invest in an expensive wall when energy is cheap is quite tricky. The answer depends on many factors, including how long you expect to live in the house and your predictions for future energy costs. Suffice it to say that you're right -- we need to avoid unnecessary expense. In some cases, it may make sense to build a wall that barely meets code minimum R-value requirements -- especially if the builder is willing to pay close attention to airtightness.

2. In some areas of the country, it's hard to find contractors who are experienced at installing dense-packed cellulose. This is, indeed, a real problem. If you see signs that your insulation contractor is in over his (or her) head, it may be time to specify a different type of insulation.

3. Lstiburek's double-stud wall recommendations make sense. I agree with you, which is why I mentioned this approach and included a link to an article describing it.

4. Figuring out how to install windows is complex. I agree, and I said as much in my article on the topic (Installing Windows In a Foam-Sheathed Wall). I can't think of an obvious way to solve this problem, though, short of the manufactured housing solution. Lots of things in life are complex, including a few necessary skills (like, say, performing an appendectomy) that experts go to school to learn. Sometimes we just have to acknowledge that mastering a few complex skills is part of our job as builders.


15.
Feb 21, 2016 12:22 PM ET

Charlie
by Malcolm Taylor

Some of the window innovations you'd like to see are already occurring. Several of our local manufacturers offer "rain screen windows" where the flange is set further back so the frame covers the rain screen gap. I would imagine that once the innovation slows and several wall systems are widely adopted by large national builders, then manufacturers will begin to market products that will cover deeper layers of exterior insulation.


16.
Feb 21, 2016 12:43 PM ET

Martin
by Malcolm Taylor

My comments weren't meant as a criticism of David, but rather a common approach to building design I often see here on GBA, which I think works on flawed logic. The thinking seems to work something like this: Since existing building assemblies aren't energy efficient enough, that leaves me free to make up my own. This extends from wall assemblies to other elements of the house, and often to the architectural design of the house itself.
At the risk of hurting a few feelings, I wonder if it wouldn't be worth looking at all the important innovations that have occurred in building science over the last few years and seeing if any of them are the result of this "creativity"? Do the houses that come out of this end up better from a building science and design standpoint? Does each house need a different set of assemblies thought up by the owner, or would it be more fruitful to look to the people doing the hard slogging and see what they have come up with?


17.
Feb 21, 2016 1:25 PM ET

Response to Malcolm Taylor
by Martin Holladay

Malcolm,
You seem to distinguish between "people doing the hard slogging" and people who are interested in "creativity." I imagine, however, that a Venn diagram of these two groups would show a lot of overlap.

Here are GBA, we certainly do our best to explain the findings of building scientists. We're all learning: scientists as well as the rest of us (their disciples). If our logic here at GBA is occasionally flawed -- and I don't doubt it sometimes is -- someone almost always shows up to point out the flaws and urge that the flaws be corrected.


18.
Feb 21, 2016 2:07 PM ET

Martin
by Malcolm Taylor

Perhaps it's generational thing, but "creativity" and "talent" used to be attributes that were bestowed by others, not claimed by individuals or groups.


19.
Feb 22, 2016 2:16 AM ET

Edited Feb 22, 2016 2:27 AM ET.

To clarify
by David Gadbois

To clarify for Malcolm, my comment was responding to "when a GBA reader posts a question proposing a new type of wall assembly, I sometimes sigh."

I do believe that there are plenty of wonderful energy-efficient wall designs out there. What makes people want to go off the beaten path? Well, many things can. I'm in a mountainous area of California, where we have a wildfire hazard. So I favor non-combustible materials for most things. Second, the frame system I selected (Blue Sky steel frame, only in California so far) already puts me well off the beaten path. They have a baseline wall design they are used to...so I can adapt that and tweak it so that I have a Pretty Good House. Third, well, there are just certain materials I'd like to avoid, I don't need to rehearse the weaknesses of wood and gypsum board. To sum, there are many factors besides energy efficiency in play.

New products and new techniques will always drive creativity.

I am an aerospace engineer, BTW, who voraciously reads GBA so...I hope I am at least conversant in building science?

Martin, to your points I'm actually trying to find a fiber cement panel that is appropriate for interior walls. And, yes, the Roxul board will all be outside of the studs.

Cheers.


20.
Feb 22, 2016 8:54 AM ET

Maximizing useable under roof space
by Mitchell Costa

Martin, Excellent article and thanks for simplifying the choices. I'm building a home on a limited lot space and am looking for a good wall that uses minimum space on the main floor (walk out basement will be ICF walls because space isn't as critical down below). I'm in an official zone 5 area in CA, but really closer to zone 4 at the edge of the zone where we only very rarely dip below 25F, or go above 90F. Everything I've read indicates that 2x4s on 16" centers with OSB siding for shear strength provides a very strong wall, and I plan to top that with 2" EPS on the exterior. That gives R-8 from the EPS plus R-13 from fiberglass bats inside for an R-21 wall with great airtightness and little thermal bridging. Upgrading to polyiso takes it up to R-24 and adds better fire and water resistance, but also significant cost. The assembly comes out to mass production standard 2x6 wall thickness to simplify finishing somewhat. The extra useable space provided by 2x4s vs 2x6s in this assembly seems like it would be good for many situations. Is simplification the only reason you didn't include the 2x4 option, or is there another drawback to 2x4s that I'm missing?


21.
Feb 22, 2016 9:14 AM ET

Response to Mitchell Costa
by Martin Holladay

Mitchell,
Thanks for your valuable comments, and thanks for the reminder that 2x4 wall framing is often perfectly adequate in warmer climate zones. I have edited my article to reflect that fact.

One quibble: When R-13 batts are inserted between 2x4 studs, you don't end up with an R-13 wall. Depending on the framing factor and the quality of the installation work, the whole-wall R-value of a fiberglass-insulated 2x4 wall may be as low as R-9.7.

If you add R-8 of rigid foam to the exterior side of this type of wall, you end up with a wall rated at about R-18, not R-21.


22.
Feb 22, 2016 11:26 AM ET

David,
by Malcolm Taylor

You are right and I wasn't seeing it in that light. Of course we should modify our houses to fit the demands each region or climate makes on them. What I find unproductive is the starting from zero each time approach of so many posters. As an engineer you know your whole profession is built up on a carefully tested knowledge base. You don't just dream up components, you come from a place of experience and presidents. At least I hope you do, or I'm going to be a lot more nervous when I fly :)


23.
Feb 22, 2016 11:39 AM ET

Suggestion for David Gadbois
by Steve Knapp CZ 3A Georgia

Have you considered using MGO board (https://en.wikipedia.org/wiki/Magnesium_oxide_wallboard) as an alternative to drywall?


24.
Feb 22, 2016 1:26 PM ET

It's interesting when
by Eric Habegger

It's interesting when discussing building construction in fire country. I happen to live in Lake County Ca , where we had a conflagration this past summer which destroyed over a thousand homes and caused (I think) four fatalities. About 1/10 of the homes in our small county were destroyed. I don't speak of it much because though I wasn't affected directly if you live in this county then you are affected emotionally. You can't help but be.

On the subject of new construction in this county. Well, to me the best advice is not to live in the most rural parts of the county that is farthest from local fire fighting resources. The fire that engulfed the homes here was so hot that its not entirely clear that one could survive even in a home that was invulnerable to fire. That is simply because of the heat generated by the combustion of the forest, and other homes, that were next to you.

Even if your home survived that fire the very reason many people lived in these remote areas, the beauty of the scenery, no longer exists in many cases. To me it begs the question if one really wants to put all your resources into building a fire safe home. It seems to me that it might make more sense to mitigate the condition that cause these types of intense fires. And where that can't be done then stay out of those areas if you can.


25.
Feb 22, 2016 3:50 PM ET

Edited Feb 22, 2016 5:19 PM ET.

One more thing
by Eric Habegger

I don't think even concrete construction would save a home if the fire is intense enough. The homes that burned here burned all the way to the foundation with virtually nothing left. There weren't many partially burned structures. But the concrete foundations below those stick built homes were decimated. It's not obvious visually but on further inspection the foundations that are left mostly have no structural integrity. They now will just crumble from the damage done by the heat. A home does not have to combust to be damaged severely in a fire as long as the heat is intense enough.

The message: the common sense solutions to a fire safe structure do not work in all situations.


26.
Feb 23, 2016 9:55 AM ET

Non-integrated team approach
by Robert Swinburne

As an architect, I am often designing for owner-builders with not much experience. I also do stock plans for the national market and I do projects where I am not much part of the building process and where the builder may have little or no experience outside of 2x6 walls with fiberglass and plastic. (You do what you have to do to feed the family.) I find that sending links to informative and relevant GBA articles and super simple double stud or exterior foam detailing go a long way toward making these projects successful. I find myself preaching air sealing and ventilation much more than any particular wall system.


27.
Feb 23, 2016 10:07 AM ET

Response to Robert Swinburne
by Martin Holladay

"I find that sending links to informative and relevant GBA articles and super-simple double stud or exterior foam detailing go a long way toward making these projects successful."

Thumbs up! That's good to hear.


28.
Feb 24, 2016 3:04 AM ET

MAG board
by David Gadbois

Good suggestion, Steve. I'm open to mag board but I don't see it much around these parts...not that that is a deal-breaker. I admit I am not precisely familiar with the pros and cons of mag board vs. fiber cement.


29.
Feb 28, 2016 11:58 PM ET

Wall Data in Marine Zone 4C
by Burke Stoller

Many of the articles that are referenced to on GBA regarding building science studies on wall assemblies come out of the Northeastern US and Eastern Canada. I am a builder on Vancouver Island, and am therefore in a climate alike to Seattle. Can anyone offer any links to comprehensive building studies using test walls showing RH, temperature, moisture content, etc. of test wall assemblies built in the Marine 4C climate? I would love to see data about double-stud wall assemblies, since this is what we are most interested in compared to 2x6 walls with exterior mineral wool insulation (which we know are "safe" from a moisture standpoint, but which are more costly to detail and build than a simple double-stud wall). Thanks for any links to data/studies!


30.
Feb 29, 2016 6:02 AM ET

Response to Burke Stoller
by Martin Holladay

Burke,
I would start by reviewing the data gathered at the Coquitlam Test Hut. Here are some links:

The Vancouver Test Hut Facility

Mark Gauvin: Coquitlam Test Hut

Vancouver Test Hut


31.
Feb 29, 2016 11:24 PM ET

Vancouver Links
by Burke Stoller

Thanks Martin. Exactly what I was hoping for!


32.
Mar 1, 2016 3:26 PM ET

Pacific NW wall info
by Bill Dietze

Burke,
Please also see http://buildingscience.com/documents/reports/rr-1014-high-r-walls-pacifi...


33.
Apr 13, 2016 7:06 PM ET

Sheathing Options
by Douglas Epperly

Martin,

We really appreciate this informative article - thanks. So, for double stud wall sheathing with a rain screen is OSB a reasonable option, or should it still be avoided? Do Zip Panel's adhered WRB with a rain screen reduce/eliminate the potential for moisture to effect the OSB in Zip Panels?

Thanks in advance for your advice and thoughts.

Doug


34.
Apr 14, 2016 3:57 AM ET

Response to Douglas Epperly
by Martin Holladay

Douglas,
I stand by the advice I gave in this article. If I were building a double-stud wall, I wouldn't use OSB sheathing -- even Zip System sheathing.

The sheathing on a double-stud wall has to be robust enough to withstand regular moisture cycling. The sheathing is going to get damp every February, and then it will begin to dry out in April. You want a sheathing that can hold up to this type of moisture cycling for 50 or 100 years.

Of course, my advice is conservative. Plenty of people ignore my advice and install OSB on their double-stud walls. That way, they save some money. (If you decide to ignore my advice and install OSB, I agree that Zip System OSB is more robust than conventional OSB.)


35.
Apr 14, 2016 1:18 PM ET

Thanks for Sheathing Clarification
by Douglas Epperly

Martin,

Thank you for the clarification. I was just uncertain about whether the rain screen made enough difference. We'll start evaluating the other sheathing options mentioned in the article. Of those, do you have a recommendation?

Doug


36.
Apr 14, 2016 1:25 PM ET

Response to Douglas Epperly
by Martin Holladay

Douglas,
Plywood works fine and is easy to tape. If your sheathing doesn't have to be an air barrier, and if your local building code official has no objection, you can use diagonal board sheathing from a local sawmill.

For more information, see Wall Sheathing Options.


37.
Apr 14, 2016 1:34 PM ET

Thanks Martin!
by Douglas Epperly

Appreciate your time and for letting me know about the Wall Sheathing article. Best wishes.


38.
Jun 13, 2016 1:12 PM ET

Edited Jun 13, 2016 1:14 PM ET.

Mineral wool
by Tony Tibbar

I like mineral wool. It's quality material and bugs hate it or at least don't like it as much as foam insulation.

I can't find any (nearby) store that sells Rockwool.
Home Depot sells Roxul Comfort batts. When bought at 15 pack quantities good stuff for a fair price. (meaning I haven't found a cheaper place)
So I'll use Roxul Comfort batts as cavity insulation.
I would also like to use Roxul (or Rockwool) on the exterior side of my wall.
Unfortunately I can't find a place where they sell the boards. Roxul support doesn't even bother to answer emails. And if they do, they want to send a sales droid; not their exact wording :-)

Are Roxul Comfort batts also suited for the exterior side of the wall? Under siding ofcourse.

Mineral wool insulation can be substituted for rigid foam insulation on the exterior side of wall sheathing. One advantage of mineral wool over rigid foam: because mineral wool is vapor-permeable, it doesn’t inhibit wall sheathing from drying to the exterior. That means that builders can install mineral wool of any thickness on the exterior side of their walls. You don’t have to worry whether exterior mineral wool meets any minimum R-value requirement. (Of course, thicker insulation always does a better job of resisting heat flow than thinner insulation.)

Does the above mean no vapor retarder/barrier is needed?


39.
Jun 13, 2016 1:25 PM ET

Response to Tony Tibbar
by Martin Holladay

Tony,
The trickiest part of installing mineral wool on the exterior side of wall sheathing is the squishiness of the mineral wool, which makes installing furring strips a little tricky. The denser the mineral wool, the easier it is to install. There are lots of articles on the topic on GBA -- start with this one, and then read all the articles that show up in the "Related Articles" sidebar on the same page: Installing Mineral Wool Insulation Over Exterior Wall Sheathing.

As you know from reading my article on vapor retarders (where you posted another question), interior vapor retarders are required by code in colder regions of the country. In general, a vapor retarder like vapor-retarder paint or MemBrain causes fewer moisture problems than polyethylene.


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