Exterior insulation
Hi,
I have a 1930s home in New York with traditional 2×4 framing and am pretty sure the insulation isn’t adequate enough to make it a comfortable home in the cold months. I do not want to rip out drywall because it is in good condition but I do want to replace windows, siding, and roof. I want to rip out the siding and apply a fluid applied product called Prosoco and fill all joints in the sheathing. I want to install foam board on the exterior to provide more r value but am reading mixed comments about it. I was planning on filling all the joints on the foam board and installing furring strips on top the provide a rain screen. Would this setup work or will this setup block airflow from the interior of the home and cause moisture to form in the wall cavity. Which foam board should I use since I am in a cold climate but does get hot and humid in the summers and what thickness of board do I need?
Thank you
Russ
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Replies
Raworks,
These articles should answer most of your questions. Of particular importance is to add enough exterior R-value (thick enough foam) so that condensation doesn't form inside the framed wall.
Also noteworthy is that XPS (one of the most common board foams) carries with it a much higher 'global warming potential' impact due to the blowing agents used in it.
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-sheathing
Also probably worth mentioning is that blocking airflow is actually your friend to a resilient and efficient design. Air carries with it moisture and letting that air traverse through the thermal boundaries of a wall makes it prone to condensation/accumulation risks. This type of wall assembly is not designed to dry to the exterior, which means diffusion of vapor to the exterior isn't required and movement of air isn't desired.
Air movement behind the siding IS good, however. So the rainscreen set-up is a good idea.
Hi Tyler,
I am not 100% clear on what you meant can you give me a bit of a better explanation I am not too familiar with all of this. I did check out the link you sent me and there seems to be no information on the thickness of the insulation. I am in climate 5A New York Putnam County. Thanks for your help and patience.
Yes, Is there a specific part you are looking for clarity on?
If its in regards to calculating the required exterior foam, using your climate zone (5) and referencing the chart found in the IRC, with a 2x4 wall, you need a minimum of R-5 exterior to your framed wall.
The link Jon provided likely will allow some variables to be adjusted and get a more accurate number.
That said, since you are retrofitting an older structure, there are a lot of variables to consider in your detailing. You may want to try searching 'deep energy retrofits' here on GBA since what you are proposing falls into that category. There are a lot of articles to browse through.
The topic has breadth and depth, and honestly it is a bit beyond my expertise to advise on all the potential pitfalls of such a project.
Some people concerned with drying issues around older structures resort to a vapor permeable exterior continuous insulation such as Rockwool. Its a bit pricier and not as rigid, but will allow exterior drying if there are particulars that require that.
I'll also just copy paste two important points Martin makes in the article I linked above:
"—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."
Here is a calculator you can use:
https://www.appliedbuildingtech.com/fsc/calculator