Insulating Second Story of Cape House
Hello all
I am in the process of getting quotes to insulate the 2nd story of a 1950’s era cape cod style house in NY (zone 5a). I have been trying to get smart on how to go about this so I can select a reasonable contractor for the job. Here were some of the articles I found helpful:
One of my goals with the house was to use as little spray foam as possible for environmental reasons and to make future renovations easier. I was originally trying to insulate the full slope of the roof as recommended in the above links, but none of the contractors I spoke with would do that with anything other than spray foam. I have a contractor I “think” will do a good job, but I wanted to check with the brain trust hear to get a second opinion on what they are recommending. I have summarized their work below:
Knee Wall – Rock Wool
Floor behind Knee Wall – loose fill cellulose
Main attic (ie above 2nd floor) – loose fill cellulose with baffles between rafters
Cathedral slopes – dense pack cellulose
Air seal – plumbing, wire chases, and wall-framing gaps in attic
Basement rim joist – closed-cell spray foam (obviously not the attic, be added for completeness)
Here are my questions
1. The floor behind the knee walls has sub-floor in place so you cannot see the floor joists. When I asked about insulating under the knee wall to keep the cool air from getting under the floor, this was the contractor’s response. “Would need to see floor joists to know if we are supposed to insulate only the floor or the entire space to R48. If we need to block the floor cavity we will, if it is a floored attic space the dense packed cellulose will act as the barrier“. So are they saying they are going to drill holes in the sub-floor and dense pack under the knee wall? Is that typical, or do most contractors pull up the sub-floor, add blocking under the knee wall, and loose fill cellulose everything.
2. There is currently no blocking at the bottom of the cathedral slopes. The contractor said they were going to install “Fiberglass as the blocker” Is that a common material to use in this application? In my readings, I have not heard of that before
3. For the knee walls, does anything need to be installed on the opposite side of the rock wool to keep it in place? I know mineral wool is stiffer than fiberglass batt, but wanted to make sure it was going to stay where it gets put.
Anything from what I summarized above seem odd or out-of-place? Any other questions I should be asking?
I appreciate the feedback!!
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Replies
I'm in a similar situation to you and not an all an expert. Tried to post a question about it, but it's sitting waiting for approval for some reason. It doesn't sound like the plan you've described includes an air barrier behind the rockwool, which would be a problem from my understanding. I don't think you should have an issue with the batts falling out, but without an air barrier, convection will move heat right through your insulation. There's another thread on here about air permeability of dense pack cellulose (https://www.greenbuildingadvisor.com/question/is-dense-packed-cellulose-an-air-barrier). So I think it's *not* technically an air barrier, but will resist a fair amount of air movement (unlike rockwool) and is therefore probably fine without an air barrier on top.
If you include links in your posting it will get held for moderation. Sometimes forever.
Ah indeed I did. Thanks for the tip. I'll try again without the links
There are no pending comments on the backend (I check daily). Unless they are winding up somewhere only the web development team has access to (I doubt that), so I'm not sure what is happening with your post. You are welcome to email the question and I will see if I can either find it or I will publish it for you--and then investigate further. My email is [email protected].
Huh, odd. I reposted without the links as https://www.greenbuildingadvisor.com/question/compromise-solution-for-insulation-of-attic-with-kneewall and deleted the old post. The original link was https://www.greenbuildingadvisor.com/approve/question/218628 if that helps you in debugging efforts, but it 404s now that I deleted the question.
Why are you chousing to reject the solution in this photo that is at the front of the article you linked to?
The problem with your plan is it is about insulation and insulation is useless until you stop the air from blowing thru and around it.
I will start by saying I despise half story houses and spray foam. If you have made the choice to buy an old drafty half story and they are all very drafty you have little choice but to spray foam it if you have any hope of stopping the drafts.
The problem is without spray foam it is all but impossible to keep the warm air under the floor from finding a way to escape into the unconditioned triangle attic you have.
The second problem is the limited rafter depth means that without the foam you have no hope getting close to the R38 you need.
Fiberglass as the blocker sounds like magic beans to me. Unless the fiberglass has been saturated in resin and is as stiff as a board it is not stopping air flow just filtering it.
Walta