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Community and Q&A

Best roof assembly on a budget?

Chris Klingenberg | Posted in Energy Efficiency and Durability on

Thank you for your website and for the exceptional information available here. I am an active student of GBA and a big fan.

I live in Central East Texas, Hot Humid, and am in process of remodeling a mid-century modern home with low sloping roof lines: 1.5/12 to 3/12. Essentially the entire home and roof assembly will be a vaulted ceiling setup since the interior ceiling is the actual roof deck. Roof is about 50 square.

We have budget constraints given the local economy, real estate market and neighboring home values but Im willing to spend what it takes to get everything right. Local builders and contractors are also not aware of many of the techniques discussed here which adds a few kinks to the initial plan.

The roof I envisioned starting from the interior is 5-7 inches of open cell spray foam on the current 1×6 tongue in groove roof deck (circa 1956) that is still in good shape. On top of that would be full Ice and water shield coverage. Next would be 1.5-3 inches of poly-iso insulation in two layers with staggered and taped seams. Then 1×4 or 1×6 purlins strapped on the diagonal to hold the poly-iso down and create 3/4 vent space. This would be topped off with Peterson 24 gauge standing seam 16 inch panels.

Fascia would have Coravent intakes at the eaves built into the trim, but no open upper ridge vent given the lower pitch sections (1.5/12)

My questions are:

Given the low pitch and inability to safely vent the ridge, should i abandon purlin/vented approach altogether and just deck over the poly-iso with 3/4 CDX plywood and call it good? That would make the details much easier for our roofer who has never tried this before.

If I applied the upper decking Plywood and properly taped the seams with something like Siga tape could I also avoid the more expensive step of ice and water shield on the initial original roof deck? As I understand it, air migration here is key and skipping this step may cause significant problems if it is not controlled properly elsewhere.

If the plywood decking approach is used, do you need a framing structure below the upper panels that is blended with the poly-iso sheets to keep the assembly straight and even. One concern from my contractor was that over time the upper plywood panels may become uneven given the more pliable nature of the foam.

My general thought now is that keeping the assembly simple may minimize potential errors and ultimately be the most cost effective.

Any advice you may offer would be greatly appreciated.

Thank you,

Chris K

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  1. GBA Editor
    Martin Holladay | | #1

    You didn't tell us whether you plan to retain the current 1x6 tongue-and-groove ceiling or whether you intend to install a new ceiling (perhaps also tongue-and-groove boards? or drywall?).

    When your wrote that you plan to install "open-cell spray foam on the current 1x6 tongue-and-groove roof deck," I suspect that you meant to write that you plan to install "open-cell spray foam under the current 1x6 tongue-and-groove roof deck." Is my guess correct?

  2. Chris Klingenberg | | #2

    Sorry, will clarify the details. Yes, the open cell will be applied directly to the underside of the current (original) 1x6 tongue and groove, filling each interior rafter bay. Below that would be standard sheetrock and breathable interior finish applied to the room edge of each rafter.

    So the full sequence is: Sheetrock, open-cell to fill rafter bay that is applied to the underside of original roof deck, ice and water shield applied to exterior surface of original roof deck, two layers of poly iso insulation with staggered taped seams.

    Above that are the two options.

    1. Build a lathe system on the diagonal to create a 3/4 vent space with Coravent intakes at the fascia like Matt Risinger demonstrates in his blog. No ridge vent would be possible given the low pitch. If this approach was used the foil face of the upper most poly-iso layer would be diligently sealed with foil tape to act as a secondary water control layer. This allows the foil face of the insulation to be exposed to an air gap and would act as an additional radiant barrier. Above that would be the standing seam metal panels with correct trim and flashing.

    2. The other option is to just build essentially a poly-iso sandwich. So assembly would be as described for everything below the original roof deck. Above the original deck would be ice and water shield, two layers of staggered and taped poly-iso insulation, new plywood roof deck, then finished off with either standard 30 lb felt or synthetic roof underlayment as preferred by the roofer. The roofing panels would then be screwed directly into the upper plywood and be laying directly against either the 30 lb felt or synthetic membrane.

  3. GBA Editor
    Martin Holladay | | #3

    You didn't tell us your climate zone, but I'm guessing that you are either in Zone 2 or Zone 3. So your plan includes an adequate thickness of rigid foam above the roof sheathing.

    None of us here can determine whether your idea to install furring strips on the diagonal is a good idea. The only person whose opinion matters (on this point) is your roofing contractor. Some contractors will accept the challenge; others will refuse the job, because roofers like to know where the fasteners belong, and diagonal furring strips complicate that issue.

    In your climate zone, you are aiming for a total R-value for your roof of R-38. Your proposed layers don't reach that goal.
    5 inches of open-cell foam = R-18.5
    1.5 inch of polyiso = R-8.7
    Add these up, and you get R-27.2

    5 inches of open-cell foam = R-18.5
    3 inches of polyiso = R-17.4
    Add these up, and you get R-35.9

    If you can make sure that your spray foam contractor gives you 7 inches of spray foam, not 5 inches, and if you can install 3 inches of polyiso rather than 1.5 inch, you can be the proud owner of a code-minimum roof.

    It's perfectly OK to make this roof unvented -- I don't think that you have to worry about ice dams in your climate -- but try for a higher R-value.

  4. Nate G | | #4

    Note that roof venting in Texas primarily serves a different purpose: keeping out the heat. Especially with a metal roof, if you have ventilation channels that allow the heat that builds up underneath the metal to escape somehow, the cooling performance will be increased beyond what the R-value alone would suggest because the temperature difference across the insulation will be lower. Needless to say, in order for this to work, you'd need both inlets and outlets so that the hot air can actually get out. It's also not clear that this kind of ventilation will work as well as usual with such a low-pitch roof. You'd likely need to make those channels a few inches tall to get the benefit. If the roofing material itself will not be visible from the street, it might be cheaper and simpler all around to go with an adhered membrane rather than standing seam metal.

  5. Chris Klingenberg | | #5

    Yes, we are climate zone 2. Thank you very much for the advice.

    We will maximize the interior open cell to 7 inches and the above deck poly-iso to 3.5 to 4 inches. That should get us over R-50.

    With that much above deck poly-iso we will just go with the plywood overlying deck instead of purlins/lath/vent channel concept. Our roofer will be much more comfortable with that approach.

    Would it be possible to skip the ice and water shield step with that much interior open cell and several layers of taped poly-iso above the deck? Thats a significant savings given roof size of 50 square.

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Would it be possible to skip the Ice and Water Shield step with that much interior open cell and several layers of taped poly-iso above the deck?"

    A. Yes.

  7. Expert Member
    Dana Dorsett | | #7

    Nate- the amount of roof deck cooling you get from venting even on roofs with substantial pitch is a second order factor, and on low-pitch roofs like this it's downright miniscule, literally "in the noise"- the local average wind speed on hot days is a bigger factor.

    For low roof pitches the solar reflective index (SRI) of the finish on the netal roof will be a much bigger factor in the roofing temperature than any venting scheme. An SRI greater than 50 would be great, but even something in the high 30s or 40s would be a good start. It doesn't have to be white or off-white to have an SRI north of 35, within Peterson's standard palette:

    In zone 2 R38 fully between rafters/joists is code min, or a stackup of U-0.30 code-max. With continuous insulation above the deck and half-inch gypsum ceiling, with empty rafter bays (buying some air-film-R), adding in the 1x6 roof deck you can meet code on a max U-factor basis with only about R30 above the roof deck, which would be 5" of polyiso. With 3.5" of polyiso above the roof deck you could get there with 3.5" of open cell foam in the rafter bays after factoring in the trapped air films and the other layers.

    The deepest you can safely spray o.c. foam in one pass is about 5"-5.5". If you do 7" in one pass you have both foam quality issues and a potential fire hazard during curing. It's cheaper / safer / better to spec 3.5" of open cell foam and fill the rest with R13s (kraft or unfaced, but not foil faced) between the open cell foam and the ceiling gypsum, or limit the open cell foam to the 5.5" safe-installation limit, either of which is well ahead of code.

  8. Chris Klingenberg | | #8

    Thanks everyone. Your insight is greatly appreciated.

    Nate, we looked into the adhered membrane but the roof is a prominent feature when viewed from the street. There were good color options with the TPO and some cost savings, but it came down to a design decision. The standing seam really works well with the modern lines of the home.

    Dana, thanks for putting that into perspective. We will probably go with the Cityscape color that has a SRI of 49.

    Thanks again,

    Chris K

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