Insulating old house from interior zone 4A
Hello,
My project is located in D.C., climate zone 4A, mixed-humid.
We are renovating our old home for different reasons, including energy performance. My question is specific to the building envelop and improving the thermal performance as well as making sure our ‘improvements’ don’t create a moisture problem inside the wall. Removing the siding seems cost prohibitive, plus the cedar shakes are still viable, so I’m looking at reducing thermal bridging as much as possible from the interior.
After doing a lot of reading, here and elsewhere I am considering the following assembly:
Existing: plaster, 2×4 studs @ 16″ o.c., diagonal sheathing, 15#felt (assumed), painted cedar shakes.
Retrofit: some kind of cavity insulation (see question below), 1-1/2″ continuous foil faced polyiso rigid board on interior side with taped seams, 1/2″ drywall, 1×3 horizontal stripping.
questions:
1) what kind of cavity insulation would you use to reach R-20? thoughts on open cell/closed cell spray foam in this application?
2) would taping the ISO provide sufficient air barrier? would spray-in foam provide the air barrier?
3) how do you recommend insulating the rim joist and electrical boxes?
4) how will the new insulation (rigid board and cavity) effect the ability of the wall to dry to the exterior if the sheathing will presumably be colder than before the added insulation?
5) would having rigid on the interior prohibit future installation of exterior rigid should we one day need to replace the siding and have the opportunity to add more insulation?
I appreciate your answers to my specific questions, as well as additional advice and ideas that I may not have considered.
Adrienne
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Replies
Adrienne,
You've asked a lot of questions, and I probably won't be able to answer them all at once.
The best insulation to use to fill the empty stud bays of an older house is dense-packed cellulose, not spray foam. Spray foam costs a lot, requires full access to the stud bays, and doesn't prevent thermal bridging through the studs (making it hard to justify the higher cost compared to cellulose). For more information, see How to Install Cellulose Insulation.
For more information on your idea of installing rigid foam on the interior side of your walls, see Walls With Interior Rigid Foam.
Q. "Would having rigid on the interior prohibit future installation of exterior rigid should we one day need to replace the siding and have the opportunity to add more insulation?"
A. Possibly. The best rigid foam to install on the interior of your walls would probably be unfaced EPS, since unfaced EPS is more vapor-permeable than other types of rigid foam. For more information on this topic, see Choosing Rigid Foam.
Interior vapor barriers (like the foil-faced polyiso you plan to use) are less problematic in very cold climates than in hot climates. Since you are in Washington, DC, a more vapor-permeable rigid foam will probably give you more options in the future in case you want to install exterior rigid foam some day.
many thanks for your quick response, Martin! I had already read your article Walls with Interior Rigid Foam and I was looking for even more information...seems like an uncommon detail and I have been hesitant to commit to this assembly. The main reason I am considering interior rigid foam is that it would address the issue of thermal bridging without removing the siding. Is there another way?
we will have full access to all the bays...would that change your opinion about dense packed cellulose vs. spray foam? My understanding is that spray foam acts also as an air barrier...
further to my last comment, the other reasons I am partial to closed cell spray foam for the wall cavity is that the existing wall is 2x4 and I can achieve R-20 without losing floor area by building a double wall. Additionally, we are doing an unvented cathedral ceiling (2x6 rafters) and my understanding is closed cell spray foam is best in rafter cavity due to problems with wet roof sheathing. We plan to install rigid on top of the roof sheathing. The ratio between interior cavity insulation and continuous exterior rigid is important to consider, correct?
again, my appreciation in advance for those of you who weigh in on this discussion with your expertise!
whoops, I see my error above in suggesting sandwiching roof sheathing between rigid and closed cell foam.
...and closed cell foam in wall cavity would not allow future exterior rigid board if and when we replace the siding.
hmm, so if code requires R-20 then my choices are:
1) closed cell foam in 2x4, no future exterior rigid, deal with thermal bridging on the interior (or not at all)
2) build double wall inside for enough thickness of dense packed cellulose
3) replace siding now and install dense packed cellulose in existing 2x4 wall and rigid on exterior...
thanks, if you have taken the time to read my thought process here. hope to have your thoughts and comments.
Reply to Adrienne:
I think you may have been misinformed about closed cell spray foam being a panacea. My understanding is that any advantage in closed cell insulation value per inch over others is quickly negated by other factors. The main one is my understanding is that you cannot shave closed cell foam to make it come out evenly at 3.5 inch thickness (for 2x4 walls). That's because a lot of the extra insulation value of closed cell over open cell is the fact that it "skins over". If you destroy that by shaving it to thickness then you lose that extra insulation value. What this means is that the foam sprayers have to be very conservative so that the foam never expands thicker than 3.5 inches thick. Usually it will only be 2/3 of that value, usually a lot less.
Dense packing a 2 x 4 wall with cellulose is your best bet and can be done by any competent contractor. I even did it myself just using a rental blower. It turned out well, though I'm not advising other homeowners to go that route. It has quite good air sealing properties if done right. It also has the lowest embodied energy of any insulation, bar none. Take Martin's advice on this.
If you want a little more insulation than you'd get from just the wall thickness you have, but don't want the limitations that come with interior foam, and don't want to lose a lot of space by doing a double wall, a possible solution is a Mooney wall. On another recent thread, Malcolm wrote:
That's from this thread: https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/59366/rigid-insulation-interior-face-wall
"am partial to closed cell spray foam for the wall cavity is that the existing wall is 2x4 and I can achieve R-20 without losing floor area by building a double wall. "
R20 center cavity for a 2x4 wall DRAMATICALLY underperforms a 2x6 R20 wall to the much much lower R value of the ~25% framing fraction. In an R20 2x4 wall the 25% framing fraction is only R4.2-ish, which is U0.24 while the 75% fraction R20 or U0.05, or only about 1/5th the heat loss per square foot. 25% x U0.24= 6 and 75% x U0.05= 3.75 for a total of 9.75. That means the heat flowing through the framing fraction is 6/9.75= 62% of the total, well over half.
With 2x6 R20 wall the framing fraction is about R6.6 or U0.15. 25% x 0.15= 3.9, the 75% framing fraction is 75% x U0.05 = 3.75, a total of 7.65. The framing fraction is still over half, but barely: 3.9/7.65= 51%.
The heat loss through the 2x4 wall is 9.75/7.65= 1.27x that of the 2x6 R20 wall. That's 27% more heat transfer, at the same center-cavity R value.
This is all just long-winded napkin way of pointing out the obvious: Save the expensive high R/inch for the continuous layers. The framing fraction robs SO much of the potential performance of the closed cell foam that in a 2x4 wall R20 closed cell fill is the thermal equivalent of adding continuous 3/8" XPS siding underlayment under the siding (or under the wallboard) with only R13 cavity fill. Given the cost of each (including the heavy environmental cost), which is really the better value?
Many thanks...I'm much clearer now about the choices and issues at hand because of each of your input. I'm convinced...we're going with dense packed cellulose for the wall cavity. we will consider exterior continuous and the Mooney wall as well.
Best,
Adrienne