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Efficiently and cost-effectively insulating an old attic with 2×6 rafters

nonstoptr | Posted in Energy Efficiency and Durability on

Hi, I recently became the owner of a 104 year old home in zone 5 (Western New York). As part of our commitment to lessening our environmental impact we are planning on insulating the home to the best of our abilities. When we moved in, there was blown in rockwool in the attic joist bays covered with r13 faced fiberglass. As this was installed over knob and tube, and many mice had made a home in it over the years, we made the decision to remove the insulation and start from scratch. The attic has an air handler for air conditioning installed and we’d like to keep that. In order to maximize the efficiency of our attic ductwork we plan on insulating the attic ceiling to bring it into the building envelope. I have read several of the articles and forum posts on here and Fine Homebuilding but wanted to see if there were suggestions for my particular situation.

Our home has 2×6 rafters which make it difficult to reach r49 in the attic. We also plan to keep the roof deck vented with 1 inch of airspace regardless of insulation method chosen. We have been quoted for both closed cell foam and open cell options that would bring us to r30-34 in the rafter bay. The closed cell foam would not cover the rafters to address thermal bridging, but the open cell option would go about 2 inches over the rafters. Both quotes also include spraying the gable ends and rim joist to r16. Closed cell and open cell are $8600 and $5900 respectively.

The other options I am considering based on my research here are rigid foam over the roof deck at roof replacement time coupled with rockwool batts in between the rafters, but I feel this will be cost prohibitive based on the modifications that would need to be done to the roof framing and eaves to make it look right (inside the attic is a standard gable roof but the outside was done as though it was a gambrel style if that makes sense). The final option was venting (homemade baffles with 1/2 inch reclaimed polyiso r3), with r16 rockwool between the rafters and two layers of r13 polyiso foam board fastened underneath for a total value of r45.

Any suggestions on how to proceed? Trying to be cost effective as well as environmentally conscious.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    nonstoptr,

    Another option which may be simpler and cost effective would be to fur down the rafters by adding 2"x4"s to the underside attached by plywood gussets. You can increase the depth of the rafter cavities to include any size of ventilation channel and amount of batt insulation you choose by using longer gussets. This a pretty DIY friendly method that leaves you with a well insulated, ventilated, foam-free, conditioned attic.

    1. nonstoptr | | #2

      Thank you! I have looked at that as an option, unfortunately when I start getting towards the top sill of the walls space becomes a lot less available due to the pitch of the roof, so I'm not sure I can fully extend those areas. How would you suggest air sealing that type of assembly?

      1. Expert Member
        MALCOLM TAYLOR | | #4

        nonstoptr,

        Depending on whether you will be doing much cooling, I would air-seal with poly or a variable-perm membrane.

  2. Expert Member
    BILL WICHERS | | #3

    If you run that furring perpindicular to the rafters, you can put your second layer of batts up across the rafters to greatly reduce thermal bridging. If you do this, I would put in the vent baffles, then the first layer of batts, then install the horizontal strapping with a screw at each rafter (screws are better for pullout strength for something like this), then the next layer of batts, then your interior finished surface material. This isn't much more work than just furring down the rafters, and gains you reduced thermal bridging. It should be cheaper than rigid foam, but maybe not by much, so I'd price doing the project both ways.

    I would put a small flat ceiling near the peak, and leave that open to the ridge vent. Insulate that as a sort of miniature vented attic. This should solve the issue with the tight clearance near the peak of the roof, at the price of slightly reduced headroom in the center of the attic space.

    Bill

    1. nonstoptr | | #5

      Thank you for your reply Bill,

      Sorry if I confused things, the confined space is more at the base of the roof where it meets the top sill of the wall of the floor below. I have plenty of space towards the peak. I'll have to take some measurements of how much space there is but it is pretty tight.

    2. nonstoptr | | #10

      Bill, your suggestion had me think of one more idea, which might be crazy and more work than it's worth, but figured I would put it out there anyways. Would creating knee walls for the areas that aren't near the air handler and blowing loose fill insulation in those, then creating a smaller central room that just covers the air handler and working space make sense, or is that just crazy?

  3. woobagoobaa | | #6

    If you are leaving 1" for air space under the roof deck and have true 2x6 rafters, 6" of CCSP will provide your air seal and r4o-ish plus an inch coverage to mitigate the rafter thermal bridging.

    1. Expert Member
      BILL WICHERS | | #7

      Using near R7 per inch for spray foam is a little optimistic. R6 per inch is a better estimate for longer term performance. Assuming a typical 2x6 of 5.5 inches deep (a “true” 2x6 only gains a 1/2” here, so not much), and a 1” air channel, you have about R27 worth of spray foam space if you don’t overfill and cover the rafters. It would be a good idea to overfill here though, and get up to code levels of insulation. Overfilling in this type of assembly would only be an issue if you wanted to put some type of finished ceiling up over the insulation.

      Bill

      1. nynick | | #8

        This is what my group of consultants (energy consultant, GC and architect) came up with. A full 6" and then a 1" over fill to cover the rafter edges. We haven't discussed the first 2" being CC and the last 5" being OC yet.

        1. Expert Member
          BILL WICHERS | | #9

          Sometimes there isn't much difference in price to do a closed cell / open cell mix like that, be sure to price the mix AND just using all closed cell. Closed cell is better R value for the same thickness, and a little safer for moisture too. I'd probably try for a minimum 3" layer of closed cell too, since I find that the installers have a difficult time getting an even layer in thinner layers, and you want AT LEAST 2" thick. 3" gives some extra margin for thin spots to not be "too" thin.

          Bill

          1. nynick | | #11

            THX Bil.

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