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Double check my insulation plan!

Tyler Tervooren | Posted in General Questions on

I’ve just finished air sealing the attic of my home and am now creating a plan to install more insulation. Would like to get some feedback before moving forward.

The house is in zone 4c (Portland) and the roof is fully hipped, 4/12 with the “not enough space at the eaves” dilemma common in older homes. It’s vented (12 vents spread around perimeter) and some existing older fiberglass loose fill (the semi-chunky white stuff).

Here’s the plan so far:

1. Test air sealing w/ blower door test. Fix problem areas before proceeding.

2. Replace vent baffles and block around baffles w/ rigid foam.

Also block around rafter bays adjacent to vents to prevent wind washing or accidentally filling soffit vents (1-2 bays to each side of vent).

3. Re-spread existing fiberglass. It was universally a few inches above the joists before air sealing, but all my digging and rolling around to air seal has created lots of mounds and compressed areas.

4. Wrap chimney in rockwool to maintain separation from new blown-in.

5. Blow new cellulose to (at least) R-49.  Attempt to fill all the way to the outside of the exterior top plates without filling the soffit.

Where I could use clarification:

1. Do I need to baffle all the way around the entire perimeter to keep the new blown in out of the soffit? Or just around the vents? The old blown fiberglass extended all the way out without any baffling. Not sure how they achieved that. Wind-washing issues seem irrelevant except near the vents.

2. Any harm in insulation contacting roof sheathing? Thinking I need to pack the eave areas as much as possible to best counteract the lack of space.

3. Should I spread the existing fiberglass evenly before adding new cellulose? Right now, I have it raked back from the eaves so that I could air seal the exterior top plates. Thinking I should leave it that way so there’s more space for cellulose. And, throughout the attic, my digging and rolling around has created both areas of compressed fiberglass and giant, fluffy mounds of it.

4. Any use in insulating over the (unconditioned) attached garage?

5. Planning to insulate to r-49 but maybe r-60 is worth the additional time and materials? Not sure how to find the sweet spot.

Thanks so much.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    >"1. Do I need to baffle all the way around the entire perimeter to keep the new blown in out of the soffit? Or just around the vents?"

    The purpose of the baffle is to keep the insulation from contacting the roof deck, limiting it's ability to dry. Baffle everywhere.

    >"2. Any harm in insulation contacting roof sheathing?"

    It's a code violation, and a potential wintertime wet-spot. You'd probably get away with if it were only a few limited spots and the attic was amply ventilated. (Any ridge ventilation, or is it soffits-only?)

    >"3. Should I spread the existing fiberglass evenly before adding new cellulose? "

    It's not critical, as long as the total depth adds up to your target R value, and there is at least 3" of cellulose on top of any fiberglass (for to provide a modicum of air retardency, limiting convection currents through the insulation.)

    >"4. Any use in insulating over the (unconditioned) attached garage?"

    Not necessarily, but check that the garage ceiling is a sufficient fire barrier.

    >"5. Planning to insulate to r-49 but maybe r-60 is worth the additional time and materials?

    If you can't hit at least R49 all the way out to over the top plates of the exterior walls heaping the middle to R60 isn't going to provide much additional performance. If you have 18" between the roof deck and top plates of the exterior walls (sufficient for R60 cellulose with the requisite 1" air gap between insulation and roof deck), by all means, do it all at R60 (~17" initial blown depth).

    1. Tyler Tervooren | | #3

      >"Any ridge ventilation, or is it soffits-only?"

      There are (11) 50 sq. in. box vents spread across the ridge.

      >"check that the garage ceiling is a sufficient fire barrier."

      1/2" taped/mudded drywall

      >"If you can't hit at least R49 all the way out to over the top plates of the exterior walls heaping the middle to R60 isn't going to provide much additional performance."

      Good to know.

      >"If you have 18" between the roof deck and top plates of the exterior walls"

      More like 4-5"...

      1. Expert Member
        Dana Dorsett | | #5

        If there is only 4" of space for insulation when maintaining the 1" space there may not even be a rationale for R49 in the middle. At 4" of cellulose/loose fiberglass looking at about R14 at 8" it's barey more than R25, so the perimeter wedge where it's 8" or less is losing more than twice as much heat per square foot as the ~R50 where the insulation depth can be 14" or more.

        At 4" of HFO blown closed cell foam is about R27, and at 8" it's over R49, so it may be "worth it" to spray foam the perimeter wedge with closed cell foam up to where the depth hits 8" and do the rest with cellulose. Simply making the site-built insulation chutes out of 4" polyiso instead of thin stock would work pretty well too for a lot less money if you discount your time.

        1. Tyler Tervooren | | #7

          I crawled up there again to get the measurement. It's too tight to reach with a tape, but the real clearance from top plate to sheathing can't be more than 4." Maybe closer even 3.5". So that's only 2.5" when maintaining a 1" air channel.

          And then it's 18" inward from outside of top plate to get to 8+1" where I could switch to cellulose.

          If I went that route, how do I figure out what the right amount of cellulose is for the middle area?

  2. GBA Editor
    Martin Holladay | | #2

    Tyler,
    You might want to read this article: "Insulating Tight Spaces at the Eaves."

    1. Tyler Tervooren | | #4

      Thanks. I had read it previously, but the "good solutions" (insulate @ roof deck or lower ceiling) are not practical for me.

      I briefly considered the closed cell foam approach, but have assumed it would be prohibitively expensive. I guess it doesn't hurt to get bids, though.

      1. Joshua Van Tol | | #6

        I did something very similar with a hip roofed house. Footprint is 24' x 28'. We sprayed the "wedges" at the edge of the roof with closed cell foam. The job cost about $1500, including spraying a 1" pass over the rest of the attic floor to seal numerous cracks and whatnot. Yours could be more or less depending on lots of factors, but there's a data point for you.

        Our roof is steeper, so the depth of the wedge didn't need to be all that deep, perhaps 12" from edge of top plate to the point where it reached an 8" thick wedge.

        1. Tyler Tervooren | | #8

          Cool, thanks. So you had about 108 linear ft of eave space filled to about 12" out. I have about 210' of eave that would need to go to 18" out.

  3. Expert Member
    Peter Yost | | #9

    If the eave space is tight enough, you may ask your spray foam installer to consider injection foam formulation rather than spray (injection still expands to 30:1 but has different catalysts that slow the rise of the foam, making it easier to get full fill and formulation out to the very edge of the eave. See this article: https://www.buildinggreen.com/blog/foam-place-insulation-7-tips-getting-injection-and-spray-foam-right.

    Yes, it is a pain in the butt and an extra step for the installation, but getting full height and cure at the eave is really important.

    Peter

    1. Tyler Tervooren | | #10

      Thanks, Peter. I'll add this to the list of questions I ask the contractors I call.

      Do you guys have any articles that would help me figure out what is actually attainable as a system r-value on a house like this? With only 3" or so of space at the outside of the top plates, I think that's about R-19.5 at best for CC foam at the edge, and then increasing until I can switch to cellulose.

      Assuming this method is going to be significantly more expensive than just blasting cellulose all over, I'm trying to figure out what increase in benefit I'm actually going to achieve for the cost.

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