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Struggling to find the right insulation contractor and approach

Matthew Kelch | Posted in Energy Efficiency and Durability on

Last year I purchased late 1970s house in south central PA with a large unfinished walk in attic space which I am planning to insulate and begin finishing in the coming year. This space has since also had a Mitsubishi air handler and ducting installed behind the kneewalls, making it all that much more critical to get insulation in.

The current assembly is as follows:
– Open fastener steel roof
– 1×4 Purlins
– 30# roofing felt
– 3/4″ plywood
– 2×6 rafters

The roof pitch for the ESE roof is ~6/12 while the WNW pitch is ~11/12.

Due to the lack of rafter depth and limited ceiling height at the low end of the ceiling, creating an unvented assembly is the only practical approach to achieve a reasonable R value (other than re-roofing and adding exterior foam).

Based on the research I’ve done it seems the best balanced approach between safety and cost would be to use a ‘flash and fill’ or ‘flash and batt’ approach with ~2″ of closed cell spray foam directly against the sheathing. Unfortunately even just 2″ of spray foam over approximately 1700sqft of roof deck ends up being quite expensive.

So far I’ve not had great luck with finding the combination of insulation contractor and approach that I trust completely while also remaining  financially viable. The most recent contractor I spoke to proposed simply using fiberglass bats in the joist bays even after I voiced concerns (ugh!).

Another contractor who performed an energy audit on the home last year has strongly advocated for simply using 3lb/ft^3 dense-pack cellulose on the basis of it’s cost effectiveness. I know this approach has been used successfully in some areas, but there is no doubt based on the research that it is inherently more risky.

My questions at this point:
1. Does the existing assembly have any outward drying potential, or is it effectively nil due to the metal roofing?

2. Assuming there are no penetrations, why would adding a continuous vapor barrier (or even carefully sealed rigid foam sheets) under the rafters not be sufficient to prevent issues resulting from interior humidity?

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Replies

  1. Expert Member
    Akos | | #1

    Lot of metal roofing with ribs is self venting. The ribs are pretty decent channels for air flow, provided you have some intake and exhaust area, it will dry much better than a shingled roof.

    I had a similar dilemma for a roof. 2.5 story with the usual uglies: knee walls, dormers and flat roof. Pretty much SPF should have been the only way to go.

    Since it is my place and to save on cost I ended up going with batts instead. The way I figure, if I end up with moisture problems, I can just fix those areas, it will still be cheaper than SPF over the whole roof.

    What I did is try to get as much air flow between rafter section that couldn't be vented. Lot of drilling holes into rafters. Than installed 1x1 strips into the corners of the rafter bays. With real 2x6 and the 1x1, an R24 mineral wool batt fits in with a decent gap to the deck. Installed a well detailed vapor barrier and air tight drywall over that. NO pot lights.

    Not very high performance but much better than the original empty rafter bays. The place is comfortable in summer and winter and so far no moisture issues.

    It is still a risky assembly, in hindsight, going with a smart VB probably would have been a good idea.

  2. Matthew Kelch | | #2

    I'm not sure how self-ventilating my roof is despite the air gap. I know the bottom edge of the panels sit on a gasket which is shaped to the profile of the panel, but I'm not sure about ridge.

    My most immediate goal is really to bring the air handler and ducting into semi-conditioned space.

    At this point I'm basically considering three options:

    1. Unvented Dense Pack Cellulose (Maybe with Intello Plus?)

    Pros:
    - Relatively affordable

    Cons:
    - Poor R-value of about R-20 without accounting for thermal bridging
    - Somewhat risky in terms of durability

    2. Unvented flash & batt or flash and fill (~ 2" of closed cell foam)

    Pros:
    - This is a proven assembly in terms of durability
    - Up to about R-28 without accounting for thermal bridging'

    Cons:
    - Even just 2" of closed cell foam is pretty expensive

    3. Unvented Cut and Cobble Rigid Foam
    Pros:
    - Cheap (DIY w/ reclaimed polyiso)
    - About R-27 without accounting for thermal bridging

    Cons:
    - Getting the air sealing details right is critical
    - Questionable durability due to movement of framing creating leaks

  3. Expert Member
    Akos | | #3

    Unvented cut and cobble is a bad idea, no matter how careful you are it will never be sealed well enough, roof failure waiting to happen.

    You can do a vented cut and cobble, but usually the whole assembly R value bump is just not worth the cost/labour.

    Unvented dense pack is a questionable roof see here for testing:
    http://nesea.org/file/11407/download?token=4smhIvIF

    The roof profile is simple enough for standard venting in the unfinished attic area. You would have to fur out the ridge support wall to have enough room for a 1" went channel and some insulation. Do a saw cut on the cladding near the ridge and cap with some rolled vent+flashing, this way you don't have to touch the metal roof.

    If you want higher R value, go with batts and layer of rigid foam on the inside or fur out the rafters and go with thicker batts.

    Looking at the options, I think it is just simpler to bite the bullet and go with the flash and batt. Cost is more but you can get it done in a couple of days instead of turning into a couple week project.

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