What Is the Best Way to Build a Wall? Not a Simple Answer

Voltaire wrote Le mieux est l'ennemi du bien, often translated as "The perfect is the enemy of the good;" he might well have been talking about residential construction. You run the gamut from the typical American 2x4 frame wall all the way to Passivhaus construction with 12" of insulation and incredible care in detailing and construction. Proponents keep saying that Passivhaus only costs 10% more than conventional construction, but they are not talking about Pulte and KB Homes, which is what I consider conventional. How do we upgrade the standard builder spec to build a high performance wall that doesn't cost the earth or reinvent the wheel?

Architect Greg Lavardera has been thinking about this, and has done some interesting work. But first, let's look at what's out there.

hand drawings by Lloyd Alter; excuse the quality, it has been a few years

The Standard American Wall

The wall everyone knows how to build is the standard 2x4 stud wall with fiberglass insulation, sheathing on the outside and a poly vapour barrier under the drywall on the inside. It has a nominal R value of 12; when you build the same thing with 2x6 studs, it has a nominal R value of 20.

But it never really does; the studs have a lower resistance to heat transmission than the insulation and act as thermal bridges. The insulation is never absolutely perfect because there are wires in the cavity and you need to be really careful about installing around them and the electrical boxes.

But more importantly, research has shown that air infiltration and leakage is more important than the insulation, and that vapour barrier is shot full of holes for wires, boxes, misplaced nails and just general sloppy workmanship that comes from having people in a hurry working with an unforgiving system.

There is also a real problem where it meets the floor, and how the floor sits on the foundation; it is tough to really seal these well.

The "Canadian" Wall

An improvement is what I will call the "Canadian Wall", developed in the seventies by Canada Mortgage and Housing Corporation. Instead of plywood or OSB sheathing on the exterior, it uses up to 2.5 inches of extruded polystyrene. This effectively eliminates the thermal bridging through the studs, significantly increases the insulation value of the wall and solves the foundation connection problem by running right past the floor. You can keep going right to the footings if you plan it properly.

Although this wall has been used for decades in Canada, there are concerns. There are essentially vapour barriers on the inside and the outside; moisture that gets into the wall has no place to go. It could condense inside the wall and cause mould and rot. It sure could use a really good vapour barrier, but has the same one as the standard wall, a sheet of poly full of holes.
Why no spray foam options?

Spray foams, either polyurethane or evenicynene that I used to love, are conspicuously absent here. Lately I have been reading a lot of anecdotal stories of people having to move out because of fumes; they are difficult to deconstruct; many are extremely flammable; they are not in common use among conventional builders.

Insulated Concrete Forms

Others have suggested that we should abandon the wood frame wall altogether, and go with solutions like the insulated concrete form. The ICF manufacturers (like this one) claim that their product is green, providing 70% energy savings "compared to traditional building methods using wood." Of course, they say that it contributes to LEED points. I have been seriously criticizedfor suggesting that they are not green.

But I continue to take the position that a polystyrene and concrete sandwich cannot be green; they are both fossil fuel hogs. The polystyrene is treated with flame retardants that should not be in a house, or even in the country. The manufacturers claim that a life-cycle analysis shows that the carbon footprint of their manufacture is paid off in a few years by the energy savings; that is only true if you compare it to a 2x4 wall. If one compares it to a frame wall with the same R value, there is in fact no comparison in the footprints.

Then there is the fact that by today's standards, they are not even that good of an R value. On their own, they vary between R 16 and R 20, and one needs to add more insulation to get any higher. At BuildingScience, they write:

ICF construction is more expensive that standard construction and is usually prohibitively expensive in residential housing....Generally, ICF construction alone cannot achieve a high R-value and will require other insulation strategies in combination for cold climates, which is commonly done in practice. ICF is generally only used in multifamily and mid rise buildings, and not in residential housing.

It will find customers in high-end custom work and in tornado alley, but it is not a mainstream wall.

Structural Insulated Panels

image credit: Postgreen

Structural Insulated Panels, or SIPs, are another sandwich, made of polystyrene or polyurethane on the inside and OSB (oriented strand board) on the outside. They can be made almost any thickness, and are great insulators. They work best with very simple geometries; complex shapes like the popular pseudo-tuscan gablegablegable suburban designs would be tough. But on simple boxes like those that PostGreen builds, they are an interesting solution. BuildingScience wrote:

The cost and simple geometries of SIPs houses are two of the main reasons why this technology is not used more often.

I will also confess to some conservatism here; I am just not convinced that gluing two pieces of board to a slab of styrofoam makes a wall. Does the glue never dry out and give? How do you fix it? I confess that it makes me a bit nervous using them as structural elements. Some, like Tedd Benson, have used them as cladding on top of timberframe; I can understand that.

The USA New Wall

Finally, let's look at Greg Lavardera's USA New Wall. It does a couple of things really well; it uses conventional materials that are familiar to anyone, but adds a horizontal furring strip to separate the drywall from the vapour barrier, and to provide a chase for electrical wiring that is not in the main insulated wall, the major cause of insulation discontinuities. After the wiring is done, more insulation is added in the furred out space, increasing the R Value of the wall. It isn't fancy and doesn't use a lot of high tech materials, but it makes sense. Greg writes:

Why not use new materials and techniques? How can you make a New Wall that everybody is going to know how to build? We want to create a wall that can be widely adopted, something that any builder can start building tomorrow without any new training, without finding any new suppliers, with out changing the way they run their business. If we want the greatest number of builders to build more efficient houses we need a wall they understand immediately, we need a wall that they can purchase materials for from their existing suppliers, use their existing sub-contractors, and a wall that is familiar enough for them to reliably price and schedule. New materials and new techniques throw off all of this and become barriers to adoption. We don't want barriers. We want everybody to start building more efficient houses.

There are other methods that one can use; Chad of Postgreen writes "I'm still not quite convinced a double 2x4 is not less expensive and easier for the trades", but I have built those and found them to be a pain to frame at windows, and that vapour barrier still is within reach of errant screws and nails during drywalling.

I think Greg is onto something here. I asked him a few questions about it:

This is not the "greenest" or the "best" wall. Why did you develop it?

Thats right. It not the best performing wall you could build, but its not about that. Its about creating the best wall system for widespread adoption. That means something any builder can build with the skill set they have now. It means they get to buy materials from the same vendors, hire the sub-contractors they already know and trust, it means they are already competent at it, they can estimate it and price it reliably, and they know how long it will take them to build.

Why develop it? I believe we need efficient wall designs that the industry will embrace. In the end we will benefit more from a wall that performs at around 75% of the best that we can do, but can be implemented 90% of the time, than we will from a wall that performs 95% of the best we can do but would only be adopted for 2-3% of the houses.

Have you actually built one yet?

No I have not, but very similar wall systems are the norm in Sweden. They took this approach 40 years ago and now every house in Sweden is built this way. So I would say that this has already been spectacularly vetted for build-ability and common sense.

What do you think are the biggest problems in getting widespread implementation?

The biggest issue is communication - letting builders know about this. It takes very little explanation to understand. Once they see it they will know what to do. Reaching them is the challenge. The second biggest problem is the resolve to improve our building without being forced to by codes and taxes. This is something we can do today. Its a more complex wall, and there is no free lunch - it delivers more value, and it costs more to build. But we can pay for it today with a simple trade off with our expectations. We trade some size for better performance and reduced ongoing energy cost. The way we appraise houses has to start recognizing the performance side of home values.

Where do you stand on cellulose/glass/rock wool vs foams, polyurethanes etc?

Without writing a book? Blown-in dense cellulose has gained much more popularity with green builders than I ever expected. But the wider housing industry has not embraced the blown-in installation. I think that remains a barrier to wider adoption. I think the best place for blown-in to gain ground in the market is deep attic insulation - say up to 24". Its fast and easy for this and could become the de-facto way to insulate above our heads.

Fiberglass I have some issues with. First, its given batts a bad name among green builders. Poor installations are the culprit, and frankly getting batts into a wall crisscrossed with wires is too much of a challenge. Compounding this is relying on integral vapor retarders - these will never make a tight wall. Integral batt vapor retarders are good for one thing - the manufacturers who sell it. If you want a tight house, you need a separate sheet. My last gripe against fiberglass is the big manufacturers already make better performing, higher R-value batts. You can get them in Canada. They won't sell them here. Shame on them.

Mineral Wool is my new favorite. Right now its the highest R-value batts you can get - R23 for 2x6 walls, and R28 for 2x8 walls. Its now widely available in the US under the Roxul brand, however you may need to order it. Big box retailers like Loews and Home Depot offer it as well. I think builders will find its actually much easier to work with than fiberglass. It cuts easily, and it has a firm composition that does not sag and makes it easier to fill every void.

Foams have their place in construction. I just don't believe that its on the cold side of the wall. Any foam insulation will form a vapor barrier. If you put it on the outside of the wall in a cold climate, well then you've potentially trapped moisture within your wall. Not that this can not be done successfully. You can leave your wall cavity to dry to the inside, but you have to design it carefully for your climate to ensure your dew point is not in the cavity. Rather it should be in the foam layer. But beware of out of range temperatures that can cause condensation in your wall space. I have a conservative view on this, I realize. There is good reason to insulate on the face of the wall. It breaks the thermal bridge of the studs. But there are good alternatives to foam for this location. Mineral wool has been used for cavity insulation in commercial construction for many years. It sheds water and passes vapor. In time we'll learn that foam's best use is with insulating edge forms for slab on grade construction, and monolithic below slab insulation for the same.