Our 1926 house has a finished room in part of the attic that is completely uninsulated. It is miserable up there. We are starting the process of insulating it. We've just about finished sealing up the spaces between all the floor joists. There are a few things i'm not sure about, though. How can I get the fiberglass batts to stay in place on the knee walls? Should I tack at the top with a few staples, then cover it up with some rigid foam?

I have decent access to the flat part of the room's ceiling, so I'll either do batts or blown in up there. I'm not sure what to do about the sloped part, though. The gap is only the thickness of the roof rafters and there are occasional nails sticking through from the roof. Should I block off the bottom of the sloped section with some rigid foam and try to fill the cavity with loose insulation? Or should I try to stuff some batts in there? Either way I'll be using some rafter vents - there is a dormer vent in the "upper" area above the attic room.

Hope this all makes sense. Thanks for any help/suggestions!

 

In the sloping roof area you might do better to install a foil faced foam board between the rafters and leave a gap of 1"-1 1/2" for venting purposes. The foil will diminish the heat build-up caused by solar gain. You could then add fibrous insulation below the foam to increase the R-value .

If the working space provides sufficient room, you could also run the foam board right up the rafters to the ridge line above the flat ceiling area and then add the blown-in material to the flat ceiling. Again, this will diminish the effects of solar gain which is keeping it uncomfortable in the area of the proposed room.

 

So I would face the foil outwards towards the roof to reflect the radiative heat away from the room? Any suggestions on how I would seal the edges of the board to the rafters, considering the sloped portions are 3'+? I've just been using the retail cans of expanding foam. Maybe I should get a cheaper "pro" expanding foam gun and shoot it in that space to seal the board to the rafters?

 

The pro gun would be the better application method.

 

Any tips on getting the insulation to stay in the stud bays? On another forum someone suggested using spray adhesive. I'd probably cover that with some solid foam (maybe foil faced?) for good measure and to act as the air barrier.

 

You could install foam on the back of wall, again with foil facing to outside as radiant barrier. Staple batts into place between studs and cover with drywall for air barrier on interior.

I don't know exactly how cold your temperatures get to but probably 1/2" foam on the walls would be fine with a 3 1/2" R-13 or R-15 batt.