Wednesday, November 24, 2010

Thermal Bridging

I've done some research on limiting thermal bridging. 25-27% of the interior surface in a typical stick-built building in the US consists of the face of the studs, headers, and other structural elements that extend through the walls from the inside to the outside. Our exterior sheathing is really simple due to the fact that the house was build in the 1970's before people understood about these things: one layer of plywood on the outside and tarpaper on the inside of the plywood to reduce moisture contact from rain and condensation. So the studs extend between the exterior sheathing and the interior drywall, acting as a major thermal bridge. Thermal bridging reduces the R-value of the walls by a considerable amount. A 2x4 stud is around R-4 while a 2x6 is around R-6. Even if the cavities between the studs are insulated with R-6/inch closed cell foam (for a cavity R-value of R-21 for a 2x4 wall or R-33 for a 2x6 wall), the impact on the wall assembly is a reduction in R-value of around 33%. For example, an R-19 insulated wall is reduced to R-12.8 by thermal bridging. Condensation on the interior surface of the studs and mold growth is a possibility. In fact, we found some minor mold growth on the interior studs of the back hallway wall. This wall gets no sun in winter at all, so it is possible that some condensation occurred during an especially cold winter night and never dried out the next day.

Thermal bridging is most evident in thermal imaging pictures. Consider this image taken of the master bedroom wall (unfortunately not being re-insulated at this time since we are living in it):


If you are interested in major problems, the bright blue along the corners where the walls and ceiling meet obviously draws the eye. These are places where the fiberglass batt insulation was improperly installed or has sagged with age (it is the former in this case, we had this room remodeled only 4 years ago). Major thermal leaks are occurring. But the thermal bridging effects of the studs can be seen in the dirty blue lines extending downward from the ceiling. These are the thermal traces of the studs. Reinsulating the wall with closed cell foam will get rid of the bright blue along the corners, but not the dirty blue below.

One very nice solution to thermal bridging is a product called Thermablok. The product is 4'x1.5"x0.4" strips of aerogel. The strips have a sticky side like tape, and are designed so that the backing can be removed and the strips simply stuck to the face of the studs. Aerogel is the best insulating material known, R-10 per inch, short of full vacuum. The R-value for a Thermablok strip is around R-4, effectively doubling the insulating power of a 2x4 stud. The cost is $1/linear foot or $4/strip, on the expensive side compared to materials like fiberglass batt or even spray foam.

I looked into Thermablok as a possible solution to thermal bridging in our house. A couple years ago, I made a thermal model of our house which required measuring all the walls. It is not perfectly accurate, but I think it gives a good estimate. The estimate of the area for the currently removed thermal envelope is 2520 sq. ft. At 25% stud area, the amount of stud area is 630 sq. ft. This would require 1261 4'x1.5" Thermablok strips, which, at $1/linear foot would be $5041, in addition to the labor to install it. And this price does not include the studs under the floor. This is pretty expensive. Besides that, as Paul mentioned when I brought up the topic in one of our meetings, rigid spray foam is rarely flush with the face of the studs, so there would be additional bridging  through the rims of the studs that are exposed above the foam.

In an effort to find a cheaper solution, I googled around a bit for some more information. I found this reference  which describes how to handle thermal bridging with normal insulating materials. Ideally, the external walls should be sheathed with extruded polystyrene board on the outside, but of course we can't do that. The recommendations that we could follow are:
  • On the ceilings, 2" foil-faced rigid polyisocyanate board as sheathing on the inside held in place with 1x4 furring strips, 16" on centers
  • On walls if you can't use  exterior sheathing , 1" polystyrene board (XPS) held in place with 1x4 furring strips, but I think maybe polyisocyanate could be used there too since I think it is a greener material than polystyrene and has a slightly higher R-value.
In addition, the gaps between the rigid boards must be sealed, and special attention  must be paid to the headers and footers. The link above has a couple of points about thermal bridging around headers and footers, and I've seen some about window sills, but mostly these seem to be for new construction, since they involve installing insulation between the structural members before they are fastened together. Buildingsciences.com also has some papers on thermal bridging too, but no dedicated reference and mostly it is about new construction.

The reference above doesn't mention under the floor, but I think we get thermal  bridging through the floor too. So the same treatment as for the  ceilings would probably be appropriate though it might be possible to use thinner strips to hold the rigid board in place and a 1"polyiso would be sufficient, since the crawlspace is already pretty tight and heat loss downwards isn't as serious as heat loss upwards.

I think the HRV chase upstairs would need a different  treatment, because there simply isn't any space to get polyiso board in, besides which, it might crack and come loose when trying to crawl back  into the space, for example, to service the HRV unit. I think that area  could be done with Thermablok since it is relatively small. The area is  around 38 sq. ft. and it would be about $76 for Thermablok. So I think  it should be possible to install the Thermablok by hand and use some  touch-up foam along the studs where the spray foam didn't cover to the  top of the stud.

We also opened up an  area from the outside under the living room window where we could do exterior sheathing. There is a large, exposed footer that will act as a major thermal bridge. I don't know if  enough clearance exists for 1" rigid board, though, since the window assembly is already in place. Thermablok could probably be used in this area, since the amount would be relatively small. Since Thermablok is only 0.4" think, there should be less problem with clearance.

Unfortunately, we can't do anything about the footers and headers from the outside in the rest of the house because the siding is in place. We can mitigate bridging  through the headers from the inside somewhat by ensuring the rigid board is sealed along the headers, and that gaps are filled with touchup foam, but we can't do anything about the footers because the floor is in place. We also can't do the wall between the garage/attic and hallway/kitchen because we had that redone several years ago and I was primarily interested at that point in simply reducing air infiltration to improve air quality, and not in reducing heat loss to the maximum extent possible.

Eliminating thermal bridging is one of the treatments necessary to build according to the German Passivhaus standard. The Passivhaus standard aims to get away with really minimal additional active heating and cooling, and uses HRV quite extensively. We are never going to turn our house into a Passivhaus, but I thought we could learn from what they do and try to get our carbon footprint down even further. So it was with visions of reducing our carbon footprint from heating by 50% instead of 30%  dancing in my head that I emailed Paul with the above suggestions for eliminating thermal bridging in our job.


Unfortunately, The Lovely Wife brought those visions crashing down. Already annoyed by the length of time the job is taking and my tendency to want to make things right, she pointed out that the insulation contractors who Paul was soliciting for bids had probably never done thermal bridging elimination before. And the drywall contractors probably hadn't dealt with trying to fasten drywall when the walls were sheathed in polystyrene board. Considering the fact that our HRV contractor screwed up some fairly simple aspects of the installation which he could have had right by calling the manufacturer's technical support (see forthcoming post), the probability that the insulation contractor and drywall contractor would completely screw up any thermal bridging treatment was very high. In addition, all the electrical outlets are already in place, so if we put in 1" polystyrene sheathing, they would need to either be moved outward or extended somehow, to say nothing of the places were the drywall was not removed.


Sadly, I sent another email to Paul and told him to forget about the thermal bridging treatment, except on the back wall of  the  hallway. The mold indicates a major problem, so we'll confine the treatment to just that wall. Because it is such a limited space,  Thermablok to cover it should not be so expensive, and any areas where the spray in foam failed to cover the stud can be touched up by hand with can foam.

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