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Community and Q&A

Improving attic insulation in 1.5 story Cape

jxwZPBYPxB | Posted in Energy Efficiency and Durability on


I just bought a Cape in zone 5A in NH which has two rooms upstairs, splitting the attic into three zones. The side attics, and the space above the rooms. The space above the rooms is vented with gable vents on the ends and a continuous ridge vent. The side attics are my concern. One side appears to be completely sealed, and has ‘balsam wool’ insulation in the rafters, completely filling the rafter depth. It seems to be in decent shape but there are some holes and insulation falling out here and there. Under the floor boards there are some batts but the coverage isn’t perfect.

My initial thought was that I would just blow in AttiCat on the floor, but now after reading tons of material on the internet and know this is an unvented attic space I see that this isn’t going to do much. I have seen tons of contradicting information about vented vs unvented. Some people claiming unvented is bad, others say it’s great, etc etc. I had considered converting to vented, but in the one side attics the roof joins with the roof of a 3 season porch, so that complicates adding soffit venting I would imagine.

So, my research leads me to believe that removing the balsam wool and spraying closed cell foam would be an improvement. I have seen people talk about using baffles or 1x1s and thin plywood to create air channels under the roof deck also. I’m not sure if this is worth the effort if I have no way to get soffit vents connected to the channels? My other concern is the spray foam would be much more expensive than blow-in on the floor.

My question is, would it be a bad idea to to a 1-2 inch layer of closed cell foam, use some furring to increase rafter depth, then some sort of fabric or netting, or perhaps even rigid foam and blowing in AttiCat or cellulose in that space? In my mind you’d have the vapor and air barrier of the foam, and increase your R value with the blow in… all at a much cheaper cost…. but is there an issue with moisture/condensation or some other issue with this layering? I would then use more blow-in in the vented attic space above the rooms (on the floor of course).

Secondly would I want to put any kind of insulation behind the kneewalls?

Perhaps I would be better served to just do the blow-in above the rooms, and the other side attic (which I believe is vented, has no balsam wool, but I need to check it out further), and spray foam the rim joint of my basement for now?

Thanks in advance for opinions and suggestions!

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  1. GBA Editor
    Martin Holladay | | #1

    The entire exterior of your house needs an air barrier as well as a layer of insulation. The triangular kneewall attics can be detailed as outside of your home's thermal envelope or inside of it. I strongly urge you to make this area inside your home's thermal envelope -- the details are easier.

    In your case, it makes sense to insulate the sloped roof, and to make this an unvented roof assembly. To learn how to insulate an unvented sloped roof, see this article: How to Build an Insulated Cathedral Ceiling.

    The short version: by far the easiest method of insulating this type of roof is with spray polyurethane foam.

  2. jxwZPBYPxB | | #2

    Ok, so my my method of closed cell foam and then AttiCat blow in fiber would work as long as the spray foam was R20 for zone 5? And I shouldn't put air channels in below the sheathing? R20 would take 3 inches of spray foam.. I know this is what code specifies but is it truly necessary? It would be much cheaper to do 1 inch of foam and several inches of blow in...

    Also, are the DIY foam kits like the 'foam it green' any good/cost effective for this type of application?


    "If you want to install a combination of closed-cell spray-foam on the underside of the roof sheathing and air-permeable insulation between your rafters — an approach sometimes called “flash and batt” — the building code requires that the spray foam be “applied in direct contact with the underside of the structural roof sheathing” and that it meets the requirements “specified in Table R806.4 for condensation control.”"

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Ok, so my method of closed cell foam and then AttiCat blow in fiber would work as long as the spray foam was R20 for zone 5?"

    A. Yes.

    Q. "And I shouldn't put air channels in below the sheathing?"

    A. I don't recommend it.

    Q. "R20 would take 3 inches of spray foam.. I know this is what code specifies but is it truly necessary?"

    A. Yes, if you want to avoid problems with condensation and moisture accumulation.

    Q. "It would be much cheaper to do 1 inch of foam and several inches of blow in."

    A. Cheap but very risky.

    Q. "Are the DIY foam kits like the 'foam it green' any good/cost effective for this type of application?"

    A. It depends on the volume of foam you need. Do the math: calculate how many spray foam kits you need, and compare that cost (and the hassle) to the bids you receive from local spray foam contractors.

  4. jxwZPBYPxB | | #4

    Great, appreciate those answers. One last thing. Do you see an issue with having the side atiic sealed and insulated like this, and the 'above room' attic vented as is, with additional AttiCat blown in on the floor to ~R50? I know this will probably create a temperature variation across the roof, but if the top attic is sealed from the conditioned space properly I would think its portion of the roof would be colder, which should avoid ice damming?

  5. GBA Editor
    Martin Holladay | | #5

    Q. "Do you see an issue with having the side attic sealed and insulated like this, and the 'above room' attic vented as is, with additional AttiCat blown in on the floor to ~R50?"

    A. No.

  6. Foamer | | #6


    If you bring the side attics inside the conditioned envelope as Martin suggests, all your insulation must be in the roof. AttiCat will not work for this. It is loose blow material for attic floors.

    Your roof more likely than not has 2x6 rafters, which leaves you with a dilemma. To vent the roof properly, you need to run continuous baffles from your eave vents all the way to the upper attic space, which will eat up about 1.5 inches of the depth you have available for insulation. I am with Martin on this one - don't vent it and get as much r-value as you can using spray foam.

    If your home is like most we work on here in the Midwest, the sloped part of the finished ceiling is somewhere between 3 and 5 feet from side attic to flat ceiling. A foam contractor can fill these cavities with material and give you a system that really works. DIY with kits will never accomplish that. For your contractor, a very significant part of the project cost will be the labor involved in cleaning out the old insulation and getting ready for spraying. If you want to save money, this would be the place to do it. When all is said and done, I think that you will find that the costs are a lot closer than you think.

    Good luck

  7. jxwZPBYPxB | | #7

    Hi Torsten,

    The atticat wouldn't go on the floor, it would be filled in cavities below the foam. I'd spray (or hire someone to spray) the foam ~3", then furr out to make enough rafter space, cover them with fabric, then blow the atticat into the void... basically flash and fill. The sloped finished parts of the ceiling are about 3 feet in length with the closets.. I admit I had overlooked that detail. I'm guessing the blow in would just go down into those cavities, but maybe I should do a low expansion closed cell foam in there since the venting is via gable/ridge? I'm guessing there is either nothing, or some batts jammed in there currently. I haven't gone all the way up there to inspect things yet.

  8. Foamer | | #8

    Hi Dustin,

    I see but it still sounds like an awful lot of work for no functional benefit and little savings compared to an all foam approach. We do a lot of roofs like yours (Northern Ohio, Zone 5) with a medium density foam, which allows us to achieve R-27 in the enclosed parts and obviously as much as you want elsewhere. Cost runs about 10 cents per "R" per ft2 - more if access is abnormally difficult.

    Filling unvented rafter cavities with air permeable insulation such as Atticat is not recommended because of moisture control issues. Foam works but fiberglass doesn't.

    As far as the upper attic goes, you can insulate the ceiling and leave that vented through the gables as it is now or you can make it conditioned by running the foam all the way to the ridge. Obviously the end walls will then be foamed as well.

  9. jxwZPBYPxB | | #9

    I see what you're saying as well Torsten, but my thinking is I may have more atticat than I need for the area above the rooms when I buy the minimum amount I need to get the machine for free. Plus I could DIY everything but the foam, maybe even the foam depending on quotes. I guess it will depend on the side attic space on the other side, which I think is vented but I need to check it out more.. Unfortunately I'm on the road working right now and can't go check.

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