July 4, 2013

Exterior insulation has been adopted more widely in some situations than in others: in commercial construction vs. residential construction, for example, or in Europe vs. North America. Should all insulation be on the outside? Why isn't it? Joe Lstiburek leads a discussion of these issues with John Straube and Industry Experts Achilles Karagiozis, Mac Sheldon and Gary Parsons.

Are we a nation of sweater wearers or sweater eaters? The pushback we get when we are asking for exterior insulation is:

  • It's harder than interstitial insulation.
  • It costs more money. Actually, it doesn't if we're talking about steel frame but it is true if we're talking about wood-frame buildings.
  • The third reason that comes up occasionally is fire codes: I have to use foam plastic and so therefore I have a problem.

Those are the three categories of issues I get.

We also have zoning issues such as city mandated setbacks that are often to the face of the cladding not to the face of the structure. People don't want to add to the thickness of their walls because of real estate value. Another issue that is sometimes raised is water being trapped in the wall. There's a lot of misinformation about that one still. What's interesting to me is that we have vapor open insulation, we have vapor closed insulation, we have vapor in-between insulation, and all of them can work. It's a design process, not a materials process. If you choose certain materials, you have to do certain designs. Literally that's all it is, and I think that can work in any climate. You just design for the materials you want to use. Use only materials that work with a chosen design.

It reminds me of the discussion of vented or unvented attics. They are not a performance or code requirement, they are a design choice, and the choice of that particular design approach leads you to the specific methodologies in order to meet that choice. There are no bad materials, only bad applications. So the question of insulation location comes to the functionality versus the cost. People choose to sacrifice functionality for cost. We have to deal with that issue, and it's really case by case. There's no universal solution for that.

I wanted to add that codes are another factor here. Right now, we're in a transition: codes are not requiring thermal resistance analysis for the whole enclosure at a level that would push us to exterior insulation and to have to focus on thermal bridging in most places in the country. But they're changing. So, as long as there's an option not to, we'll stick with current low-cost solutions, and as soon as codes move further in that direction, that will have to change.

No one disagrees that thermal bridging is avoidable but as long as it's allowed in the code the problem will likely be ignored. Perhaps the Thermal Metric Project will so conclusively prove the benefit of at least some of the insulation being applied to the outside of the framing members the code will finally change.

The production builders are in a difficult position. They operate on such thin margins that it is difficult for them to add the few hundred dollars to the job to really do the insulation package well. The irony is that insulation in a typical house represents about 2 – 3% of the house value. It costs less than carpet, countertops, cabinets and plumbing fixtures; all materials that can be and often are changed every 10 or 20 years in the home's hundred-year life. The insulation and air-sealing in the walls we're usually stuck with for all time. Attic insulation can be changed or enhanced but it rarely ever happens. Mostly old houses that suck more than their share of energy will continue to suck for all time.

The physics and the politics of code changes are – how shall I put this – quite interesting. I was recently in California, and in their California Energy Code, they actually have an incredibly detailed list – an appendix – of many enclosure assembly variations and their impact on typical thermal bridges. I thought it was completely comprehensive. It's listed in tables and says "look, with these 2-inch Z-girts you're going to lose more than 50% of your R-value" and yet many buildings are still built assuming steel Z-girts are not the massive thermal bridges they are. It's all there in the code tables, and of course the science is relatively easy for us to support, but it does seem to me that a massive education program is required before it will be actually enforced, even in a state that has a code. It's in the code and people don't know it. So I think there's massive outreach needed to get people to be aware of thermal bridging, and then it will be more deployed.

One thing we hear on the street from builders is the structural performance, going from an OSB or some type product to a rigid foam insulation, they question the structural integrity of it, versus a nice piece of wood that they can you know put in their hands. It is a matter of providing background information to help them understand the structural performance to ease their mind.

I think that's a valid point, but it doesn't have to be connected. The exterior insulation can go over the OSB if you want OSB. We remove the OSB in some designs as a value engineering exercise. However, something similar that we hear often is integration issues of exterior insulation with the windows and cladding. "I understand why I need to put the insulation out there, I'll do it, I just can't find a cladding manufacturer who will allow me to do it over 2 inches, or more, or with furring strips, and then how do I do the windows, and there's no support from the manufacturer there either".

I'd also like to mention durability, exterior insulation improves the overall temperature and moisture conditions within the cavity. That is, wood components within the protection of the exterior insulation layer will be warmer and drier in general compared to thicker walls where the outermost wood components are essentially at the exterior conditions. BSC has shown this very well through various projects, most recently the Vancouver Test Hut Project. The OSB in the wall samples that were protected by exterior foam insulation looked pristine during the assembly deconstruction analysis – much better than the walls that did not include insulation. The durability of walls is difficult to characterize experimentally and we will have to learn these lessons over time by looking at failures.