More Best Practices
When I purchased my small 1950s vintage Cape Cod home in 2018, I knew there was a lot of work to do to improve its performance and interior comfort. I also knew I would be doing the work alone, and I was planning for a deep energy retrofit—as opposed to a shallow energy retrofit—that would take several years to complete. Given the scope, I decided to approach my experience as a case study and to share lessons from my first-time endeavor, which is, in part, what I’ll do here. I will also talk more generally about installing exterior insulation on existing houses.
One of my goals was to improve the insulation levels of the walls and roof. I went back and forth trying to decide if I should add a layer of continuous insulation to the exterior. After all, I was going to remove the siding and replace all the windows—typically the ideal time to add what some builders call “outsulation.” In the end, I decided against it. The gable ends of my roof have no eave, so adding exterior insulation would have required rebuilding a portion of the recently re-shingled 12/12 pitched roof. I now regret that decision, but more on that later.
The reasons for including continuous insulation have been more fully discussed here on Green Building Advisor and other forums, but the biggest benefits are comfort, durability, and energy/cost savings. Recently I wrote a two-part article on continuous insulation, where I concentrated on new construction, though much of the information also applies to existing homes, the subject of this post.
Where to start
With this type of project, I start by figuring out how the existing wall is assembled. I identify the products and techniques used at the time it was built. I also…
Weekly Newsletter
Get building science and energy efficiency advice, plus special offers, in your inbox.
This article is only available to GBA Prime Members
Sign up for a free trial and get instant access to this article as well as GBA’s complete library of premium articles and construction details.
Start Free TrialAlready a member? Log in
13 Comments
Excellent breakdown. I'm slowly embarking on an energy retrofit of my own home built in the 40's. I think I'll strip the cedar claps as carefully as possible and reuse them as best as possible. My current assembly is drywall or paneling (Depending on the room), questionable density cellulose, board sheathing, tar paper, cedar claps, tar paper, asbestos shingles, typar, and vinyl siding. I have my work cut out for me... My biggest concern is the windows, but I've studied enough details to have a plan.
I've been considering residing and adding CI to my 1970's house in zone 4c but since I would be doing the work myself don't see being able to get it done during the dry season here in willamette valley. Curious if anyone has tried doing a project like this in stages? Doesn't seem like the kind of thing that could be done one wall at a time, but happy to hear other thoughts on the matter.
I am struggling with figuring out how to retrofit continuous exterior insulation to an existing building without creating condensation problems inside the wall. I don't know if the existing walls have an interior vapor barrier in none, some, most, or all of the walls. The mock-up at the top of your article shows what appears to be low-perm exterior insulation. How is the wall going to dry out if it gets wet due to condensation somewhere in the middle?
jimg13,
Moist interior air moves outward through the walls until it hits a surface below the dew point and condenses. That surface is usually the sheathing. If the exterior foam is thick enough it keeps the sheathing above the dew point and the moist air does not condense - much as it doesn't on any other surface in your house.
The IRC gives the levels of foam necessary for the wall to stay safe for each climate zone, whether there is or isn't a vapour-barrier, and how much permeable insulation is in the stud bays. Martin explains how this all works much better than I have in this blog: https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
Thank you, Malcolm for the extremely quick reply. The article by Martin that you referred me to was one of the many that I had read previously. It left me discouraged. Here is a key quote from the article:
"To sum up, there are two important points to remember about foam-sheathed walls:
The rigid foam must be thick enough to prevent moisture accumulation (“condensation”) in your sheathing or framing; and
This type of wall must be able to dry inward, so it’s important to avoid low-permeance layers like polyethylene, vinyl wallpaper, or closed-cell spray foam on the interior."
I cannot guarantee that the interior does not have a vapor barrier. Therefor, per Martin's comment it would be unwise for me to use rigid foam insulation on the exterior. But I have read elsewhere (I think) that this may not always be so. Thus, my quandary.
jimg13,
Yes, as a general rule every wall should have a good path to drying, but the experience of walls with exterior foam and interior poly seems to show it isn't as important in that situation as it might be in others. Martin and others rowed back a bit in this follow up article: https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness
With adequate exterior foam the wall cavities should maintain a similar RH level to the rest of the conditioned space. Moisture accumulation would only be a risk from leaks through the cladding, rain-screen gap, foam, and WRB. Build those layers diligently and I don't see a problem.
Another option which removes any risk is to use mineral wool board. Being permeable it allows drying to the outside. It is a lot more expensive though.
Thank you again for your quick reply. Yes, this article was one of those that I referred to above, that said interior poly may not always be a no-no when using exterior foam. BUT I decided to look more closely at mineral wool. I am still doing that, but am leaning a little toward 2" EPS, because it gets me to R-10, and I think I can install vinyl siding right on top of it (without furring strips). I'm unsure about that, and unsure if I should install Tyvek or similar over the EPS before the vinyl siding. My original intention was to do so. Do you have any thoughts on any of these ideas?
jimg13,
I'm afraid that I don't know what the practical limit is for direct fastening through foam is. I suspect it's less than 2". The manufacture should have some guidance in their installation guide. Here is what was said then this came up a few years ago: https://www.greenbuildingadvisor.com/question/vinyl-siding-over-2-foam-no-strapping
To me a layer of house-wrap over the foam offers a few benefits. It provides extra protection against water intrusion, allows you to run any flashing on the surface of the foam rather than having to go back to the sheathing layer, and means you aren't only relying on taped joints for the longevity of your WRB
Thank you for your generous help. I agree with your reasoning regarding the house wrap. I'll keep digging regarding the vinyl siding.
All the best!
jimg13,
Good luck with your project!
Malcolm,
What is the recommended method for attaching house wrap to two inch foam board? Tyvek puts out a silent animation that shows it being stapled over thinner foam board. It specifies that the staples must penetrate into the wood sheathing underneath. Two and one half inch staples are beasts. They don't seem appropriate for this task. What are your thoughts?
Cap nails. You only need enough to hold it until the strapping or siding goes on.
Thank you AGAIN.
Log in or become a member to post a comment.
Sign up Log in