Love this website . I am learning from everyone.
We have all come across poorly insulated knee walls with poorly fit batts falling from the joists.
I was wondering how you like to recommend improving existing knee wall insulation. Do you like to encase them? With what? etc. I like the idea of encasing in foam board but that will not fit through many access points.
A NEW question about knee wall insulation.
This is in regard to a 1907 house with no insulation in the knee wall areas. The floors of the knee wall space are finished. The triangle spaces on top of the knee wall are stuffed with rock wool. There is minimal rock wool in the attic with minimal attic ventilation.
There are 5 knee wall areas. Two will be used for storage.
1. For those knee wall areas where want to save the floor space for storage the plan for spray foam insulation under the rafters is too expensive. Are baffles + faced fiberglass under the rafters reasonable? There will be no ventilation.
2. For the other knee wall spaces the plan is for fiberglass on the walls and the floor with a vapor barrier on the warm side.
Would appreciate advice. Thanks.
The rafters in a house build in 1907 are unlikely to be on 16" centers, so there may be a problem with getting the batts installed tightly. If you use Kraft-faced material there will be possible issues with getting the batts fully enclosed on all six sides to optimize their performance and minimize the potential for condensation on the underside of the roof. If you use unfaced batts and cover them with Tyvek it's hard to get a good seal at the perimeter. If you don't air seal the space between the bottom of the knee wall and the ceiling below, warm air will move to the outside through what is probably balloon framing unless you seal that floor cavity at the intersection with the exterior wall. There is no reason I can think of to install baffles unless there's an attempt at ventilation (never mind the fact that it's not at all clear to me why ventilation in such a situation is ever added).
In the areas that will not be used for storage, the fiberglass on the floor will only work if it's in contact with the ceiling below and if the space under the wall is air sealed.
Unfortunately the methods you propose do not have a very good track record of success. How do I know this? Because I tried them over and over again and they worked somewhere between poorly and marginally over and over again.
The sad truth is that knee wall attics are stinkers to air seal/insulate, and knee wall attics in pretty old houses are really stinkers. It's a sad truth, but it's a truth none the less.
In knee wall areas with floor boards, if you are going to keep the area unconditioned and ventilate the area, you need to rip one up the floor boards next to the knee wall to gain access to air seal under the knee wall.
If you are going to seal the area off and bring it inside, making it conditioned space, then you need to rip up the floor board towards the eave to air seal it out on that end.
Probably one of the better options for insulating side attic rafters is polyiso foam board air sealed in place and then covered with a fire resistant rated vapor barrier like FSK. But depending on the jurisdiction, you might have to paint intumescent paint over it or install some kind of ignition barrier.
A cheaper option might be to install tyvek first up on the underside of the rafters, then install fiberglass batts, and then install a fire resistant vapor barrier material like FSK over that. Tape up all the seams with Tyvek tape or a compatible tape. You will get a decent air seal out of that although it won't be as good as regular drywall. But if you are in a really cold climate, 4" of fiberglass might not be enough R value to prevent ice daming.
Ed is right about the spacing between the rafters. They vary from but most are 18+ inches.
Someone offered to blow cellulose into the interstitial space between the knee wall floors and the ceiling below but I may have live knob and tub wiring there so that was probably ruled out (also gave a very high quote).
Thanks for the advice guys.