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Community and Q&A

Economical way to insulate small attic

bac478 | Posted in Green Building Techniques on

I have a small 8×12 entryway attached to the front of my house. The attic space above it is accessible from the second floor. I want to use it as conditioned storage space.
When I removed the covering and insulation from the rafters during a gut rehab I noticed that the top half of the sheathing was wet on the north side of the attic. I believe this was caused by moisture from the interior air condensing on the cool sheathing surface. I think solar heating on the south side was enough to keep the sheathing above the dew point during the day and to get rid of moisture that condensed at night. Since the insulation was removed, the sheathing is no longer wet, even during and after rain, precluding the possibility of a roof leak.
The rafters are 2×4. I was planning on strapping it with 2x3s for added insulation depth. The total roof deck area with the single gable end is about 150 sqft. I got a quote at about $5/sqft for 5.5″ deep closed cell foam. I cannot add foam panels above the roof deck because the ridge for this small entryway is just barely below the eave of the rest of the house (ridges perpendicular). Adding to the roof deck would cause the two roofs to intersect only near the ridge and I think it would look really odd.
Is there a more cost effective way to insulate this small area? air permeable insulation means I have to vent the assembly which chews up insulation depth. I don’t want to add more than 1.5″ to the existing 2x4s because it decreases attic space. This is not enough area to rent the cellulose machine again, at least not in my opinion (I probably should have done this when I blew-in the rest of the second story, but I hadn’t discovered the problem yet)

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  1. GBA Editor
    Martin Holladay | | #1

    Considering the limitations that you list, I think that spray polyurethane foam is your only option.

    (By the way, your explanation of the damp sheathing on the north side of the roof was clear and accurate. It's fairly common for gable roofs to have damp sheathing on the north and dry sheathing on the south, as you observed, due to the beneficial effects of the sun.)

  2. Expert Member
    Dana Dorsett | | #2

    With 2x4s + 2x2s you have 5" total. The roof deck is completely protected with as little as 1" of closed cell foam, but pretty much everywhere with 2". The maximum depth that can be safely applied in one pass is 2", with a cooling off period between passes, so you can stop there.

    If you give it a 2" shot of foam and fill the remaining 3" with a compressed R13 batt it will have nearly the same performance as a full fill of closed cell polyurethane.

    If instead of 2x2s you gained the extra depth with an rafter-edge strip of 1.5" polyiso and filled in with a compressed batt it would exceed the performance of 5.5" of closed cell foam.

    You don't state your location, but with 2" of foam and a compressed (to ~3") R13 you would have sufficient dew point margin at the foam/fiber boundary even at the warm edge of US climate zone 7. In climate zone 4 or lower you'd be fine with 1" closed cell foam and a compressed R20 (or R19, if you're REALLY cheaping out.)

  3. bac478 | | #3

    Old house, so true 2x4s. 4+1.5=5.5in. Closed cell is rated at about R7/in. fiberglass is about R3.6/in. I'm going to ask the obvious question: 7*5.5>7*2+3.5*3.6 so how will the foam+fiberglass have equal performance to full thickness closed cell? Unless we are talking moisture (or lack thereof) performance?

    Climate zone 5, zip 01301.

    Unfortunately froth pak and equivalent only come in 200 bf and 600 bf sizes. I need about 300 for 2". A 600 bf kit is just north of $600. I prefer rock wool over fiberglass and that is going to add $120. That puts me back at $720 vs. $750 to have someone spray it. Fiberglass is 50% cost of rock wool so that would put me at $660 which is still relatively close to $750.

    If I strap the wall horizontally with 2x3s I will get very small areas of thermal bridging through whatever insulation I decide to use and won't have to worry about attaching drywall through 1.5" of rigid over the studs.

  4. GBA Editor
    Martin Holladay | | #4

    I think that Dana's point is that thermal bridging through the rafters tends to have a strong influence on the thermal performance of this type of roof assembly, so that the thermal advantage of spray foam is much less than most homeowners and builders assume.

    This is a small roof over the entryway of your house; don't overthink it. I think that convenience and cost are important considerations, so you can choose your approach based on those criteria (as long as you obey the basic building science rules outlined here: How to Build an Insulated Cathedral Ceiling).

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