Insulating finished attic kneewall space
I have snooped around these forums for a bit now and have gathered enough knowledge that I have started tackling insulating my kneewall space. My house is 1907 colonial revival (I think) and it has several hips and dormers making the job difficult. I am located in St. Louis, MO.
I have opted to insulate the knee wall, slope, and top attic, instead of the roof as it seems a bit more feasible.
Currently, I am air sealing the floor joists with rigid foam and spray foam. I am also putting in batts in the kneewall along with foil faced rigid foam. For the top attic, I will blow in insulation and also down into the slope. I plan to put baffles in the slopes and then seal them to keep airflow but also get some R-value. Last I’ll blow in insulation to the attic portion behind the knee wall the best I can. Here are some questions I have and would truly appreciate any advice. Thanks!
1. If I use Rockwool in the wall will the foil-faced foam provide the vapor barrier or do I need a separate one?
2. Will putting rigid plastic baffles up the slope be enough airflow to the top attic?
3. For dormers or hips where I have next to no access, should I just blow in insulation and hope for the best?
4. Should I add some vents down in the lower portion of the attic?
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Replies
1> In US climate zone 4A ( St. Louis) a vented attic kneewall you won't need a vapor barrier, but it doesn't hurt anything as long as there is just one.
2> Code requires 1" minimum, which most pre-made baffles will provide.
3>Stuffing large spaces full of insulation comes with some risks, but if the insulation is cellulose the moisture burden would be shared between the structural wood & insulation. Venting hipped roofs doesn't really work- insulating from the exterior is the safest, followed by closed cell foam on the underside of the roof deck (which you can't really do if you don't have sufficient access.)
4>If there is a ridge vent or gable vents on the micro-attic above the sloped ceiling, yes soffit vents into the kneewall attics can help.
Kneewalls, dormers, and hips are all tricky. These articles may be helpful:
https://www.greenbuildingadvisor.com/article/what-architects-need-to-know-about-attic-kneewalls
https://www.finehomebuilding.com/2011/07/14/a-crash-course-in-roof-venting
https://www.finehomebuilding.com/membership/pdf/4618/021205059.pdf
https://www.finehomebuilding.com/2012/09/06/two-ways-to-insulate-attic-kneewalls
Thanks guys, I really appreciate the responses.
Despite the fact that I have insulated about 25% of the knee walls I am not thinking that insulating the roof with rigid foam is the best route. I need new shingles anyway and I have ductwork in the micro attic.
Does that seem to be the best route since I can't get at these slopes very well?
Moving the insulation to the roof is always a good option in trick situations like yours. And, if you are re-roofing, you could consider adding some rigid foam above the roof deck. Here's some info on doing that right:
https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing