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Roof insulation on new build — newbie question

Sharon & Wayne Collum | Posted in Energy Efficiency and Durability on

We are building a 1200 square foot house, cape cod style, 1.5 story (so upstairs has one bedroom, storage, and some general living space). The house is located in climate zone 5 and close to climate zone 6 in the Pocono mountains area of PA. We are building the home ourselves – not as green as we would ideally like because of our limited budget and desire to have as little debt as possible, trying to stay on the green side where we can manage it.
The question – we used 2 x 10 rafters when building the roof and later realized that we cannot quite achieve the code-required R-38 with batt insulation (oops). We were wondering whether it would work to put R-30 batts in and then add rigid foam insulation sufficient to bring our roof R value up to 38 or slightly better. We would then attach the drywall to the rafters through the rigid foam.
If this is a good idea what type of rigid foam is our best option (considering that it is an interior-ish application) and do we then use faced insulation batts or unfaced?
Or should we just suck it up and rip down 2x4s to the right dimension and attach them to each rafter so we have space for the R-38 batts?
Any other suggestions would be appreciated – these are just the ideas we came up with.
Thank you!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Sharon and Wayne,
    The first piece of advice comes too late to help you, but it may help other GBA readers. Here's the advice: Plan ahead! For more information, read this article: "Plan Ahead for Insulation."

    The short answer to your question is that you should read the following article, which lists all of the different ways to address your problem: "How to Build an Insulated Cathedral Ceiling."

    Your biggest decision concerns whether to create a vented roof assembly or an unvented roof assembly. If your roof has any hips, valleys, or dormers, you can't create a vented assembly -- an unvented approach would be necessary.

    Assuming that a vented assembly is possible, you can either deepen your rafters with 2x4s or Bonfiglioli strips, or you can install a continuous layer of rigid foam on the interior side of your attic. If you install rigid foam, it's usually best to install 1x4 strapping, 16 inches on center, on the interior side of your rigid foam, to make things easier for the drywall crew.

    For more information on Bonfiglioli strips, see "Breaking the Thermal Bridge."

  2. Sharon & Wayne Collum | | #2

    Thank you Martin. Yes, our roof is just a basic gable roof so venting assembly is the way we are going. We certainly should have planned better. We are really doing almost everything ourselves and 2x10s were all we could manage (weight) for the rafters - building it ourselves has it's own challenge (only hires so far have been septic inspector and an excavator to dig the trench for the electric line). After reading your response and a few articles we are going to go with deepening the rafters. The Bonfiglioli strips are a great idea, I think that will help. Thank you again!

  3. Expert Member
    Dana Dorsett | | #3

    Even with deeper rafters you can't hit an IRC 2018 code-min R49 with dimensioned lumber.

    With 2x8 rafters you can hit R30 with rock wool, and with 3.5-4" of roofing polyiso up top it would be sufficient for dew point control at the roof deck. If using relaimed roofing foam that can be pretty cheap. Both Repurposed Materials Inc and the Insulation Depot have facilities within reasonable trucking distance and often advertise here, and there are usually others:

    https://philadelphia.craigslist.org/mad/d/used-poly-iso-insulation-sold/6729648291.html

    https://newjersey.craigslist.org/search/sss?query=rigid+insulation

    https://philadelphia.craigslist.org/mad/d/iso-rigid-foam-insulation/6733229119.html

    https://allentown.craigslist.org/search/sss?query=rigid+insulation

    If you have a place to store the materials, keep searching and find the deal. Depending on condition you may need 15-25% of excess, but it's usually less than 1/3 the cost of virgin stock foam. Two layers of 2" is fine, so is a single layer of 4" or any other combination that is at least 3.5" (4" is better.)

    1. Sharon & Wayne Collum | | #4

      Thanks Dana :)
      I had heard of Insulation Depot but haven't contacted them yet - I wasn't sure if our relatively small home would warrant enough rigid foam to be worth it to them. Doing a search on craigslist is a great idea too. I will check into Repurposed Materials Inc also - we are all about repurposing building materials and fixtures when we are able to

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