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Is my re-roofing strategy adequate?

Dielectricunion | Posted in General Questions on

I just visited the supplier that I’m planning to get my foam board and metal roofing from. I was struggling to decide my strategy before, but now I’m really confused.

I an reroofing a 118 year old bungalow in central Indiana, zone 5. My plan is to tear off shingles to the wood board sheathing, repair, wrap in air barrier/membrane, 2 layers of 2″ foam board (probably foil faced polyiso) seams taped and staggered, then horizontal purlins attached with 7″(+) screws. I’m installing exposed fastener metal roofing on top.

After talking to the supplier, he’s trying to lead me in a totally different direction. I had a hard time getting him to understand the whole unvented, “hot roof” assembly. He said I should- “Leave the single layer of existing shingles, lay horizontal purlins over them, then cover everything in that foil bubble wrap stuff. Then metal roof on top. In the rafter bays, cut 2.5” polyiso to fit. He says “I’ve been working in this industry for 30 years, you cant believe everything you read on the internet. Ha.

Anyway… I’ll probably stick with something similar to my original idea. The awesome thing is, this supplier has salvaged foam board that’s basically in near mint condition for 4x8x2.5″ @ $10 ea and other similar prices.

He said I should definitely use a vented closure, letting air between the metal and the purlins. Is vented closure a good or bad idea?

Thanks if you actually read all of that rambling… I have to do the work 100% my self, I’m on a tight budget, and I’m kind of freaking out

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  1. GBA Editor
    Martin Holladay | | #1

    Anytime I hear someone tell me to install foil-faced bubble wrap, I tend to cross that person off my list of experts. (You can read more on the topic here: Stay Away from Foil-Faced Bubble Wrap .)

    Your two layers of polyiso (totaling 4 inches) will give you an R-value of about R-20 to R-24, depending on how much you want to de-rate the R-value to account for its poor performance in cold weather. That's obviously less than minimum code requirements (R-49 in your climate zone), so I assume that you plan to supplement the rigid foam with some other type of additional insulation.

    In your climate zone, R-20 above the roof sheathing is enough to keep your roof sheathing above the dew point during the winter, assuming that you plan to install about R-29 of fluffy insulation under (and in direct contact with) the roof sheathing. You probably don't want to install more than R-29 of fluffy insulation, however, or you will throw the ratio of foam-to-fluffy out of whack.

    Here is a link to an article with more information: Combining Exterior Rigid Foam With Fluffy Insulation.

  2. Expert Member
    Dana Dorsett | | #2

    A continuous ~R36 above the would meet code minimum performance on a U-factor basis with NOTHING between the rafters, which would be 6" of virgin stock foil faced polyiso, or 6.5" of reclaimed roofing polyiso. With insulation in the rafter bays you can get by with less.

    Cut'n'cobbled foam in the rafter bays is a waste of foam & labor, since the thermal bridging of the rafters severely undercuts the performance of the foam. How deep are the rafters? If the rafters are full-dimension 2x6s 24" on center you can trim some crummy R25s (8" manufactured loft) and compress them into the 6" space where they will perform at about R22-R23.

    With R22 in the rafter bays and a continuous ~R24 above the roof deck (4" of virgin stock foil faced, or 4.5" of reclaimed fiber faced polyiso) it should still meet code on a U-factor basis, despite being less than R49 at center-cavity.

    Reclaimed foam is out there at salvage yards etc at a fraction of the price of virgin stock goods. A quick craigslist search comes up with both national and more local options:

    I don't know how much material the outfit in Spencer has, but the price is about right.

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