adding insulation to knee walls

Old 10-27-17, 12:41 PM
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adding insulation to knee walls

Hi Folks,

We are looking to add insulation to our 1925 small home in Portland, OR. The 2nd floor is finished in the middle with two attics on either side--knee walls. I have access to both attics. There is paper-backed mineral wool batts in the bays of the knee walls. A couple people have said that the mineral wool insulation was put on backwards with the paper facing out into the attic rather than facing toward the living space.

I'm going to do air sealing around the electrical outlets that go through the knee walls (all new romex wiring in plastic boxes). I'm thinking that with that and the multiple coats of paint on the walls on the finished side I don't have to worry about the vapor barrier and can replace the mineral wool where it got ruffled by the electrical work then put fiberglass batts (maybe rotated 90 degrees) over top the existing mineral wool batts (which I heard is a real pain to remove).

The roof and gable walls are uninsulated inside the attic (apart from some styrofoam insulation under the vinyl siding on the gable walls). I'm going to also seal the openings in the floor joists that run under the finished space with rigid foam board sealed with the expandable stuff. Then more blown in and call it good (the knob and tube wiring is dead, tra la!).

There are no soffit vents on the outside and a gable vent on each end in the very peak of the roof (not accessible).

Thoughts about the vapor barrier question?


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Old 10-27-17, 02:14 PM
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If it ain't broke, don't break it. Sure, by today's standards the vapor barrier is backwards so they could have added it after the fact but if there aren't moisture issues by now I wouldn't worry about it. I was in a [very] similar situation with a Cape Code house although mine was fiberglass with the vapor barrier facing inward (irrelevant though to the process).

Your thoughts on right on track with what I did:

1. Cut foam (Dow blue board) to fit in each joist cavity under the wall. Moved/cut the insulation so I could put it in there, foamed around it, replaced insulation
2. Get some R19 backed roll insulation
3. Cut it to length for the height of the knee wall stud bays
4. Took a knife and cut several large diamonds in the vapor barrier of the new insulation. Basically negating the vapor barrier but keeping the staple tab on each side present and solid
5. Install the insulation with the (now semi) vapor barrier facing inward and stapling the vapor barrier tab to the end of the exposed wall stud
6. Installed R30 perpendicular to the floor/ceiling joists and backed myself out

You could install the R19 horizontally on the wall like you mentioned. It would prevent the gaps like I had for the studs so it's a good idea (wish I'd thought of it at the time).

Make sure to get a good mask as you'll be sucking in a lot of newly disturbed junk and new insulation fibers.
Old 10-27-17, 05:09 PM
Join Date: Oct 2008
Location: New England
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You've probably seen this but I'll add it anyway.
Two Ways to Insulate Attic Kneewalls - Fine Homebuilding

If you decide to add any low ventilation they make under-shingle vents or you can add low wall vents on the gables. You will also want an air path from the side attics up above the insulation in each rafter cavity.


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