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Community and Q&A

Insulating finished attic kneewall space

HistoricSTL | Posted in General Questions on

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. Expert Member
    Dana Dorsett | | #1

    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.

  2. HistoricSTL | | #3

    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?

  3. Brian Pontolilo | | #4

    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

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