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Attic insulation advice needed

bjh85 | Posted in Energy Efficiency and Durability on
​I’d like to vastly improve the attic insulation in the old home (~110 years) that I purchased a few months ago in Minneapolis (zone 6 for insulation). Currently there’s only about 2 inches of very old insulation in the attic! I’m hoping to get some advice on the best strategy.
Note: I am planning to have a steel roof product installed this summer. I mention this because that might affect the best insulating strategy.
We’re considered 3 main insulation options: cellulose insulation blown onto the floor of the walk-up attic, closed-cell spray foam insulation on the bottom side of the roof boards, and insulating the top side of the roof boards (like Hunter’s “cool vent” product: https://www.hunterpanels.com/product-over/product-listing/cool-vent; I’ll call these “hunter panels” for simplicity ) when the roofing work is done. If there’s another option that I’m overlooking, let me know!
Cellulose is the cheapest, but it makes using the attic space more difficult. If we went this route, then I’d probably create a raised platform in the middle to give myself a little bit of cold storage.
The other two options seem somewhat similar to me in that they’re both more expensive, and I was imaging both as “hot roof” (unvented) options. It seems like they’re roughly the same cost. The hunter panels are probably better for the environment, they don’t have the thermal bridging through the 2×4 rafters, and they have ventilation built-in. However if I used the hunter panels then I’d still need to insulate the 2×4 walls of the attic (perhaps I’d use spray foam).
One thing I worry about with the hunter panels is that will be some gap between some of them (eg sloppy roofer work), and then I have a very hot spot in my roof. On the other hand, if I use spray foam on the bottom side of the roof boards then I worry about any roof leak going undetected. I like the idea of being able to visually inspect my roof boards.
In terms of the amount of insulation, here’s approximately what I was thinking:
cellulose: roughly 17″ or whatever it takes to hit about R50
hunter panel: 3.5″
closed cell foam: either 3″ so I still have access to the 2x4s or 4.5″ if I fir the rafters out so that I can get that thermal break
The ceiling isn’t very high in the attic so I’m hesitant to lower it in any way. I’m 6′, and I’m able stand straight basically only at the highest point. But if I were willing, another option seems like it would be to put polyiso foam insulation on the bottom side of the 2×4 rafters. This would provide a thermal break. Doing this might make more sense that firring out the rafters so that I could add more spray foam (since spray foam is more expensive).
In terms of roofing, I’m leaning towards a steel shingle over standing seam for 3 reasons: 1) price, 2) air flow built into their design, and 3) we have some convexity in our roof. Boral has a nice produce that I’m considering: https://www.boralroof.com/wp-content/uploads/product-selector/4DCP93185HD-Cottage-Barclay.pdf
lots of questions!
1) What am I overlooking?
2) What would you do? Convince me! 😉
3) Now if I only have R20 of spray foam between my rafters, do you think i’d be in danger of ice damns?
4) Is there any combination of insulating both the top (via hunter panel) and bottom (via foam or other) sides of the roof boards that makes sense? I worry about trapping moisture there.
thanks!

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Replies

  1. Andrew Bennett | | #1

    You've got a beast of a project there. If you were going to blow in insulation it would make sense maybe to pull the attic floor up. If you pull the attic floor up you might as well do some air sealing. If you are going to do some air sealing you need a big shop vac to suck up those 2 inches of old insulation (assuming it is blow in in as well). Now...if you did all that then you could also raise the height of the attic floor by however much you wanted by running 2x of desired thickness perpendicular to the current joists and put the floor boards back. Now you've air sealed and have appropriate insulation. You lose a little head room, but you maintain storage space. You can visually examine the underside of your roof. The exterior roofing details become easier (not that they were overly complex) and pretty close to typical for most roofers.

    I'm not big on spray foam myself...or foam boards....just not a foam fan. I'd use blown in rockwool instead of cellulose.

    1. bjh85 | | #3

      Thanks Andrew. I know that insulation is pretty light, but do I need to have any concerns about the pressure that this could put on my very old plaster ceiling if I took the floor boards out? What do you like about rockwool over cellulose?

      1. Andrew Bennett | | #6

        Lack of flameability and water resistance. Those really come into factor more with batts than blown in. For blown in I don't think it matters much. There's no HVAC stuff up there to catch on fire and if there's a water leak it's gonna be a big rip out anyway. So, in your situation, I just don't know that it matters.

        I don't think there would be any concerns about any type of blown in being too heavy. If there is any degradation of the plaster all the moving around and work in the attic isn't going to help, but if you have chunks fall it was only a matter of time anyway.

  2. GBA Editor
    Brian Pontolilo | | #2

    HI bjh85.

    I'm typically in favor of air sealing and insulating attic floors with blown-in cellulose as the most cost effective approach in many situations like yours. But as Andrew pointed out, you'd have some work to do to prepare for that (Is that a stairwell in the photo? If so, you'd need to insulate and air seal that area too).

    There are lots of options for insulating your roof and adding continuous exterior insulation in combination with rafter cavity insulation is a good option (without trapping moisture). Check out this article for all of the possibilities for insulating the roofline: How to Build an Insulated Cathedral Ceiling.

    1. bjh85 | | #4

      Yes, that's a stairway in the picture.

      I've read that in climate zone 6 one should have a vapor barrier below the insulation. Would this just get laid over the 2x4 joists in an uneven way (most of the time being directly on the top of the plaster ceiling, and then jumping over each joist)?

      I assume that I could substitute polyiso boards for any blown in insulation in any portion of the attic where I want to conserve vertical space. In particular I wouldn't want to raise the storage platform by a significant amount to conserve head space.

  3. Expert Member
    Dana Dorsett | | #5

    >"I've read that in climate zone 6 one should have a vapor barrier below the insulation."

    A vapor barrier is not really, if the attic ventilation is up to snuff, and a sheet of polyethylene could create condensation issue during the cooling season if the house is air conditioned. With a vented attic that isn't usually necessary south of the middle range of climate zone 7.

    If you're uncertain, re-paint the ceilings with vapor barrier latex primer. But really air-sealing is the first, biggest, and most critical aspect of keeping wintertime indoor humidity from ending up in the attic's structural wood.

    1. bjh85 | | #7

      Thanks Dana! I'm tempted to put a minimal amount of spray foam (eg 1") between the 2x4 joists to reduce the number of air leaks I might miss while air sealing. Do you have any thoughts on that? Or would you dislike that for the same reason you don't favor a vapor barrier?

  4. Walter Ahlgrim | | #8

    First let me say I think sealing the air is at least as important as insulating.

    You do understand that roof is likely to have twice the surface area of the ceiling and heat loss is function of square feet times R value. If you double the top area that will make the house have about 20% more heat loss. If the attic was to be finished space it could be worth the 20% higher fuel bills and the thousands more you will spend insulating the roof but for a nicer storage space I do not see the value.

    If you read the questions and answers on the site you will see a lot of questions about cathedral ceilings/ insulated roofs, because they are difficult to get right and easy to do wrong and end up with a rotting moldy mess.

    Walta

  5. bjh85 | | #9

    I wanted to follow up and thank everyone for their perspectives. In the end I pulled up the flooring, thoroughly air sealed, and built a platform in the middle for storage. There's only about 12" of cellulose below the platform and 16" elsewhere. Just in time for MN winter.

    1. Andrew C | | #10

      Tidy looking job. That should make an enormous difference. In the houses that I've lived in that got additional blown cellulose on top of existing insulation (after air-sealing), the reduction in outside noise in the house was a big bonus on top of improved efficiency.

      Make sure you seal up the attic access door/stairway, but I'm sure you've done that already.

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