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How should I insulate my roof?

steven_gubkin | Posted in General Questions on

I bought a house in Northeast Ohio which I am hoping to retrofit. Here is a link to some pictures of the house:

I am using

as the general contractor. They do have experience with DER and building new homes which have met the passive house standard.

I am going to be doing my retrofit in stages as I save money. Currently, I have enough money saved to redo the roof.

I want to get the roof to R-60, and replace asphalt shingles with standing seam metal roof.

Most of the roof is a cathedral ceiling. The plan the GC was comfortable with was to use fiberglass batt insulation in the rafters, and use XPS foam on the exterior. I would like to avoid XPS because of the high GWP of the blowing agents. However, the GC is worried about the large quantity of EPS which would be needed.

Can anyone give recommendations? I have read the article on insulating cathedral ceilings, but I am still somewhat confused.

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  1. Expert Member
    Dana Dorsett | | #1

    The contractor needs to be reminded that most the XPS manufacturers will warranty only 90% of labeled R, and it will hit that mark in 20 years or less. Before the standing seam roof needs replacing the performance of the XPS would be about the same as EPS of similar density, and from a responsible design point of view you have to factor that in.

    The roof is good for 50 years, at which point you could probably take the warranty claim on the XPS (as if anybody is going to sample and test it forcing them to make good on that claim), but they won't replace your possibly punky roof deck if you designed too close to the margin by assuming R5/inch or any of the labor, just the foam.

    So that's an optimistic R4.5 per inch for XPS compared to a real lifetime performance of R4.2 per inch for EPS. That only adds 7% to the thickness. In reality it's imprudent to assume more than R4.2/inch @ 50 years.

    Think polyisocyanurate, if thickness is the issue.

    I suspect the real problem is fixating on R60 as a number. R49 is code min, but U0.026 (R38.5 "whole assembly", which calculates the thermal bridging and the R value of all layers, not just the insulation, including indoor and outdoor air films, etc.) With the exterior foam thermally breaking the rafters you can have significant margin on code max U-factors even at less than R49.

    The more important number is the R-ratio of the foam /total, which needs to be at least 40% foam for dew point control in your climate zone 5 location. Less than 40% the north side pitch is at some risk of mold or dry-rot setting in. Let's assume you have either 2x8 or 2x10 rafters.

    A 2x8 rafter bay accommodates an R30 rock wool batt, a 2x10 rafter bay accomodates a mid-density R30 fiberglass batt.

    You need at least R20 above the roof deck for dew point control on R30 in your climate zone.

    At a derated R4.5/inch that takes 4.5" of XPS, R4.2/inch that takes 4.75" of EPS (only 1/4" more than XPS) R5.7/inch it takes 3.5" of 2lb fiber faced roofing polyiso R6.0/inch it still takes 3.5" of foil faced sheathing polyiso

    Any R50 center-cavity solution with R20 of thermal break over the rafters will perform about the same or even better than R60 blown fiberglass on an attic floor.

    If you have 2x12 rafters, a low density fiberglass R38 performs at R37 when compressed to the 11.25" cavity depth, it would need R25 above the roof deck for dew point control- add about an inch to the above numbers, and you'll be pretty close to your R60 number, not that it matters. The additional energy savings going from R50 to R60 at center cavity are tiny.

    Since saving money is one of your goals, take advantage of the multiple vendors selling RECLAIMED and factory seconds polyiso in your area at less than half the cost of virgin stock goods:

  2. steven_gubkin | | #2

    Thanks Dana! The contractor was concerned about using polyiso because it performs more poorly in cold weather. Do you know what r-value we should use in worst case scenario?

  3. GBA Editor
    Martin Holladay | | #3

    Here are links to some relevant articles that you should read:

    "Choosing Rigid Foam"

    "Three Code-Approved Tricks for Reducing Insulation Thickness"

    "Combining Exterior Rigid Foam With Fluffy Insulation"

    "Cold-Weather Performance of Polyisocyanurate"

    In the fourth of the articles listed above, you can find the answer to your question.

    Q. "The contractor was concerned about using polyiso because it performs more poorly in cold weather. Do you know what R-value we should use in worst-case scenario?"

    A. In my article, I quote building scientist John Straube, who says, "One option [to the question of the cold-weather performance of polyiso] is to stick with polyiso and just make it thicker. If we do that, let’s call polyiso R-5 per inch."

  4. Expert Member
    Dana Dorsett | | #4

    At 40% of the total-R on the exterior side of the assembly in a zone 5 climate R5 /inch would be the worst-case winter time average for 2lb roofing iso (typically labeled R5.5-R5.7/inch.) Foil faced 1lb goods will do slightly better. Dow claims to have beaten that issue with Thermax.

    See the derating curve for roofing polyiso in Figure 2 of this document, which is only labeled R5.2/inch @ 75F mean temp through the foam (which is lower then almost any product out there):

    The mean outdoor temperature over the winter in your area is about +30F:

    So at a mean indoor temp of 70F, mean outdoor temp of 30F you have a 40F delta, so the average temp of the warm side of the foam is about 0.4 x 40 = 16F warmer than the mean temp at the cold side, or 46F, which makes the mean temp through the foam about 8F warmer than the cold side, or 38F.

    A the 38F mark in the derating curve of the world's worst polyiso in Figure 2 it's performing at about R4.8/inch. So for the R20 needed for dew point control on R30 batts it would take 4.25".

    So if you're looking to ABSOLUTELY worst-case it, for unlabeled unknow reclaimed roofing iso call it R4.5/inch, and install at least 4.5", the same thickness that it would take for XPS at the "90% of original performance" warranty level.

    You'll note that in the curve it beats fresher-than-fully-depleted XPS at any mean temp through the foam warmer than +47F, so in fact the drying season for the roof deck starts sooner and ends later than with even fresh XPS. Fully depleted XPS or EPS performs at about the same as the mineral wool curve, which crosses the world's worst polyiso curve at a mean temp of 37F.

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