Foam board on rafter extensions?
I recently bought a 100-year-old house and had huge icicles over the winter (Michigan’s Upper Peninsula, zone 7). It is a 2-1/2 floor, brick construction. The attic has soffit vents but no ridge vents. The gable vents were covered with siding at some point by previous owners. The roof is made by wooden planks on top of 2×4 rafters with added 2×4 on OSB gussets. I don’t think there is an air-barrier membrane below the shingles. The rafter cavities are 12″ deep, in total, and is stuffed with R-45 fiberglass batts stapled to rafters.
The attic feels drafty and the fiberglass batts are filled with dust. The ceiling is filled with loose vermiculite and covered with wooden planks. Previous owners have piled several layers of old vynil-like flooring and old carpets.
I want to prevent icicles and ice dams, reduce my heating bill and improve the aesthetics and thermal comfort of the attic. After reading several articles here, I think my best option is to reopen the gable vents, install gable louvers, create ventilation baffles with 1″ EPs sealed with foam and tape, replace the fiberglass batts, and add drywall (with 3′ kneewalls and 3′ flat ceiling). Regarding the floor, I am planning on leaving the vermiculite untouched and just add 1/2″ OSB and carpet. Will it work? Am I overlooking something?
My question right now is: shall I stuff 1-1/2″x4″ strips of EPS between the original rafters and the extension? The roof assembly in those areas is 4″ wood+4″ air+4″ wood, which is way lower than the 2″ air+1″ EPS+9″ fiberglass, so it makes sense to me to add EPS to reduce thermal bridging.
A second question is, can I reutilize the old fiberglass batts? Is it a problem if I compress 12″ batts into 9″ cavities?
I would appreciate any comments. I learned as much as I could before posting my questions but I could find an answer to these. Thanks.
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Replies
Hi Josue.
Your plan will work. I think it is a good idea to insulate the air space between the rafters if that is possible. Reusing and Compressing Fiberglass Batts is Okay (the R-value per inch goes up, though the total R value goes down because they are now compressed into fewer inches). You want fiberglass batts to completely fill the cavity they are used in and they should have an air barrier on all six sides. Have you read this: How to Build an Insulated Cathedral Ceiling?
Hi Brian,
Thanks for the article about compressed batts. I had read elsewhere they shouldn't be compressed, so it is reassuring to learn that's a myth. The article on cathedral ceilings is a good one, I got the idea of using EPS for the ventilation cavity from there.
You don’t want to compress the batts as you install them if that means empty spots in the cavities. Think about this as a “poorly fluffed” insulation. It is ok to compress the ENTIRE batt EVENLY to make it fit in a thinner cavity.
Basically it’s more important than the batt completely fill the cavity without any voids than it is that the batt be compressed to any specific dimension or not.
Bill