Faced Fiberglass Batt in a knee wall with foam fastened over studs BEHIND it
MichaelAndrew
| Posted in Energy Efficiency and Durability on
I have a gambrel roof where the exterior wall of the conditioned space has no sheathing behind it. how should I insulate that wall? and what should I secure to the studs behind it to add R value/protect the insulation between the studs from wind washing?
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Michael,
The advice usually given is it's better to have the insulation follow the roof line, rather than the knee walls - but I'm assuming that is difficult in your situation, so I'll just stick to answering the question you asked.
You can add any type of rigid insulation to the exterior side of the wall as long as it is sufficiently thick that it says warm enough so that condensation does not form on the interior face. The safest way to do that is to follow the recommendations in this blog:
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
the apartment is above a garage and when i purchased it it was just a square over a gambrel rounded over roof.. so just the conditioned space was insulated. I am remodeling and in a few areas I am actually cutting into the vertical wall and putting in a header to allow more floor space which will make my wall closer to the sloped side roof area. doing this where the bed headboard will be, toilet, tv in living room and one other storage area. near the floor space it provides like 2
so in those areas I am considering fastening foam insulation board directly to the inside side roof rafters leaving a 3.5" vent channel from soffit in most areas and 1.5 " in one area where there is blocking between rafters. with reflective side facing the roof sheathing to get some radiant barrier as well as R value in the foam board (if good do I run this all the way up to edge of top plate/header and seal it all or leave room at top for next wall insulation to breath vent up there. then I would build another wall right in front of it with the foam board acting as the "backing" to enclose the back side of the insulation. I could do this all over but I am hesitant to cover it all up and not be able to see potential leaks. Had the roof done 2 years ago and while doing this work actually found a leak at the transition from lower roof to upper roof and have to have my roofer come back.
due to the roof leak, cost, & just all the intricacies of the manufactured trusses spray foaming the side attic areas seems like a bad idea... that is why I am leaning towards keeping it vented there is a 1.5" to 2" gap at the top plate of the walls to allow air to upper roof which has gable vent on each end and ridge vent..
in the vertical wall areas putting foam board behind the wall to avoid wind washing of insulation in the wall will help with thermal bridging correct? also I can't read that article completely maybe I'll have to join the membership thing i guess
on this topic.. could you put foam board on the sloped roof area and also inside the vertical wall/knee wall.. is it ok to have two insulated walls like that close to each other but not directly having the insulation in contact? would the knee wall attic area be halfway conditioned due to this? or is it a bad idea? will both their R values still help?
Michael,
Splitting the insulation is a problem for a couple of reasons.
- It's ineffective if the space in between is vented.
- Semi-conditioned spaces are unpredictable in terms of where the dew p0int ends up and which surfaces may attract condensation.
ok what is your belief on radiant barriers? could I put radiant barrier on the sloped roof section and then insulation inside the vertical wall away from it?
what is the best product to put behind a vertical wall that is a interior wall but the end of the conditioned space so it does not have sheathing and siding on the back side of it? I was thinking foam board to add R value but I've also seen house wrap etc..