Which wall cavity insulation should I retrofit into my walls?
I’m interested in hearing what everyone thinks the best type of retrofit insulation is for existing wall cavities. I own a great 1930’s brick home in Oklahoma City. The wall section is pretty typical of the era: 2-3/8″ brick exterior, approx. 1″ air space (I consider it to be unventilated, but there are places in the attic the open top of the wall is exposed), tar paper, 3/4″ solid wood plank sheathing, empty 2×4 cavity, and lath and plaster interior finish. While I love my house, a deep retrofit is completely out of the question for both cost and personal sanity reasons. However, I am excited to fill those empty stud cavities with insulation.
I have done some research of my own. Since blowing or spraying in insulation is mostly a one shot opportunity, I’m leaning toward the foam kits that actually pour into the wall cavity and expand to fill it up. These seem to only come in a closed cell foam and would add approx. R-21. The foam is expensive stuff, but the combined air tightness and high R-value seem like an ideal fit. I know I’ll get comments about blowing in cellulose, so I’ll openly admit I could use it. However, for my situation the cellulose has a big drawback, R-value per inch. Cellulose would only get me R-12. The wall cavity can only be filled once and a brick homes doesn’t leave many other easy options for additional insulation.
Additionally, I’m struggling to understand the if I’ll be causing unwanted side effects by installing insulation in my wall cavities. I’ve tried my best to learn from this website and BuildingScience.com, but don’t feel confident enough to come to a sure conclusion. Oklahoma City is classified as a mixed climate which makes vapor migration an interesting subject. Installing either cavity insulation will alter the temperature of the exterior plank sheathing possibly making it more susceptible to condensation. The foam insulation would create an air barrier and an additional vapor retarder while the cellulose will do neither. Furthermore, the brick cladding is a reservoir material that will push vapor into the building when heated by the sun. As you can see it’s quite challenge to understand what will happen! Any help here would be greatly appreciated.
Brad
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Replies
Brad,
Your analysis is good. You seem to have a good understanding of the advantages and disadvantages of your two insulation choices.
To sum up: compared to cellulose, foam:
Is more expensive
Is less environmentally friendly
Is trickier to install
On the other hand, compared to cellulose, foam:
Has a higher R-value per inch
Provides better vapor control
Pluses and minuses. You get to make the call.
Thanks for the response Martin. So do think I shouldn't be concerned about any "side effects" from the new insulation? Last night after posting, I read a buildingscience.com article about reservoir cladding that specifically called out not installing a interior vapor barrier and to allow inward drying. I'm afraid closed cell foam insulation with it's semi-impermeable nature will trap all the moisture in the cavity and mold and rot my sheathing. That is unless I ventilate my brick, which I'm just now realizing wouldn't be terribly hard to accomplish by cutting out mortar head joints at the top and bottom every 24" or so. Any thoughts here?
You should look into Homesulate foam, which is a non-expanding foam specifically for retrofit projects. Go to http://www.Homesulate.com for details. Watch the video to get an idea on the product application. I am an energy consultant in Dallas, so I have no financial interest, but I have worked with this product & it is quite impressive.