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Insulating unvented flat roof with multiple materials

Jimmy Black | Posted in General Questions on

I have 6″ joists under the flat roof to work with, I’m in Climate Zone 2A – Tampa, FL. I recently installed a TPO roof with tapered foam, the taper goes from 0.5″ to 4″, so there is some insulation on top. My deck is 1×12″ shiplap solid wood.

After reading the articles it seems like I have a few choices (1000sqft):
a) 5.5″ of closed cell foam, $5500, R36
b) 1.5″ closed cell + 4″ of open cell, $3100, R24
c) 2″ closed cell + 3.5″ fiberglass, $2300, R28

1) Can I use a wood ceiling which is non sealed under all of these options and that will be ok? My understanding is since I’m using closed cell foam that is my vapor and air barrier and I do not need a sealed ceiling.

2) I’m also insulating a cathedral roof, it is 1×12″ shiplap deck with a cement tile roof above it, from my understanding the same applies to it as the flat roof correct?

3) To save money and foam, should I use less open cell or fiberglass underneath where the taper is thicker, to achieve a more even insulation over the whole area? The tapered foam goes from a R2 to R11.

4) When calculating the R value of my entire flat roof, do I use the average of the tapered R value?

5) Should I consider any other insulation options?

I’d like to use option C as it’s cheapest. The house had R19 throughout before the remodel and it was more than sufficient. Thank you for everyone’s time and expertise, I’ve found this website to be very informative, well organized, and an excellent resource.

I read the following GBA articles and found them pertinent.

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  1. GBA Editor
    Martin Holladay | | #1

    According to the 2012 IRC, in Climate Zone 2 you need a minimum of R-38 insulation for your roof assembly. Even if your local code authorities don't enforce the code, it's a good target to aim for.

    That's a minimum. In my estimation, you need this minimum R-value at the side of your roof that has 1/2 of rigid foam on top of the sheathing. (That's where the minimum R-value occurs). You don't tell us what type of foam you have up there, but let's call the 1/2 inch of foam R-2. That means that you need a minimum of R-36 of insulation on the underside of your roof sheathing.

    Option (a) is the only option that comes close to meeting your minimum R-value goal -- and even that option estimates an optimistic R-value of R-6.54 per inch for the closed-cell spray foam.

    If you beef up the depth of your framing cavity by adding strips of wood on the underside of your existing rafters -- either perpendicular to the 2x6s, or scabbed on to the 2x6s -- you can improve the R-value of the assembly.

    If you installed 1.5 inch of closed-cell spray foam on the underside of the sheathing, you'd get about R-9. With the R-2 above the sheathing, you've got R-11. What you need is room for R-27 of fibrous insulation (fiberglass, mineral wool, or cellulose) under the rigid foam. That's about 7.5 inches of fibrous insulation -- so you need to make your rafter bays about 3.5 inches deeper than they are now to go this route.

  2. GBA Editor
    Martin Holladay | | #2

    You guessed correctly: moisture problems related to cold roof sheathing are far more common in cold climates than hot climates. In Florida, it's rare for your roof sheathing to get very cold. (That said, thicker insulation will lower your energy bills.)

    If you don't want to install R-38 insulation, you don't have to (as long as your local building code inspector approves your plan). It's your house.

    In theory, if you want to combine exterior rigid foam and interior fluffy insulation, the rigid foam layer needs to have a minimum R-value of R-5 in Florida. For more information on this topic, see Combining Exterior Rigid Foam With Fluffy Insulation. With tapered insulation, you are probably safe if you cheat a little -- especially if your fluffy insulation is thin. (Thin fluffy insulation moves the assembly in the direction of safety, not in the direction of risk.)

  3. Jimmy Black | | #3

    Well seeing as adding more fluffy insulation without using closed cell foam will potentially cause more moisture problems, and going from R19 to R28 by using $2000 of closed cell foam will save me $16/year, it would seem stupid to used closed cell foam. Also given that the old system of R19 fiberglass lasted for 40 years with no moisture issues, I am going to simply replace the insulation as it was.

    The reason for the minimal savings with added insulation is I'm using an extremely efficient mini split system, and then add to that my house is fully shaded 50% of the day, and I have a cement tile roof, actually I would guess the savings would be much less than $16/year!

    Thank you again for your advice, it just saved me $2000, of course your articles were the main reason I was considering the $2000 closed cell foam option haha.

  4. Jimmy Black | | #4

    Thanks Martin, yes I could add a few inches of ceiling to get close to R38, I'm not concerned though because the ceiling previously had only R19 with a vapor barrier and it was comfortable and I had no cooling or heating issues, in fact no moisture or roof deck problems either. This confuses me because the GBA articles say I should have all kinds of moisture problems with my decking. My first thought this was because of my tile roof was providing insulation and protection, but the same insulation and decking is were my flat modified bitumen roof was (now it is a TPO). Then my thought is maybe it is because of the temperament climate?

    Regardless, because the house is 40 years old, and hasn't had any moisture or decking issues caused by the current ceiling insulation, don't you think I'd be fine simply replacing the fiberglass like it was and saving $2000 not having to used closed cell spray foam? (I would drop the ceiling a bit to get close to R38).

  5. Expert Member
    Dana Dorsett | | #5

    With R36 in the cavities you don't need R2 above the roof deck to meet IRC chapter 11 thermal requirements. The foam-R above is not thermally bridged by rafters, and is "worth" more per R than the stuff between the rafters. It only needs a U-factor of 0.030 or less, which is ~R33 "whole-assembly", after accounting for all thermal bridging, and the thermal performance adders of the roofing, the sheathing, the ceiling gypsum, the indoor & outdoor air films, etc.

    You can get there with as little as R28-R30 of insulating materials under the roof deck, but maybe even less if done on a whole-assembly U-factor basis.

    An alternative solution would be:

    d: 1" of closed cell foam + compressed R23 rock wool batts (alternative: R21 fiberglass, if you can't get the rock wool), which should come in under $2000, and is quite a bit greener.

    You only need 1" of closed cell foam under the roof deck to moisture-protect the roof deck, and then only in places where the foam above is less than R5, unless you don't have a true air barrier above the ship-lap roof deck (but I would think the TPO roof qualifies.) Installing rock wool R23s under the 1" foam and compressing them to 4.5" would in combination with the R6 foam brings it up to about R25-R26, which may in fact be good enough. Even though the U-factor at the thin edge of the tapered foam would be slightly higher than code-max, where it's 4" thick it would be well-under code-max, and would likely be sufficient thermal performance when averaged over the entire roof area.

    When utility regulations get updated to reflect the 21st century grid in FL (or maybe even before then) you can apply the savings on insulation toward a rooftop PV system and save a heluva lot more than the marginal savings difference than a slightly below-code roof and one that costs more than $2000 more. FL has some of the lowest cost solar (but fewest subsidy incentives) of any state in the US. A couple grand should be able to buy you 600 watts of PV panel + inverter.

    If you shade that roof with the solar array it marginally lowers the cooling load directly, not just at the electric meter.

    Is the tapered foam EPS, or is it polyiso?

  6. Jimmy Black | | #6

    The tapered foam is GAF EnergyGuard PolyIso. The code requirements for R value in FL make no sense when you have a high efficiency HVAC. There is little value in going over R20 when you have a mini split system. Going from R20 to R40 on 1000sqft for example will save me $24/year, which would NEVER be paid back due to the added expense of building out a ceiling, and especially if you use any kind of spray foam (way too expensive).

  7. Expert Member
    Dana Dorsett | | #7

    Higher R values in codes isn't always based on net-present-value of lifecycle energy savings. Some of it is about resilience too, but there are also comfort aspects to consider independent of code.

    A really efficient air conditioner might be able to keep the average room temperature in the right range without spending a lot on power, but a hot ceiling radiating at you isn't exactly very comfortable. It's like having your head in the fire while you're butt's on a block of ice- the AVERAGE temperature may be fine, but that's not exactly the definition of "comfort" you're looking for out of a house & air conditioning system.

    GAF EnergyGuard roofing foam is rated R5.7/inch, so at the 1/2" end of the taper you're looking at R2.85, not R2. Anywhere the foam is 0.9" or thicker it meets the Chapter 8 prescriptive for minimum R5 above-deck for dew-point control.

    Clearly more than 80% of the roof area is R5 or higher, and with ~R2.85 at the very thinnest edge of the tapered foam you won't really have to be concerned about the R5 minimum prescriptive, especially if you used cellulose for the cavity fill, which will wick & safely redistribute any moisture that accumulates in a cooler-than average winter. If the inspectors balk (or if you're nervous), using 2 mil nylon (Certainteed MemBrain) under the ceiling gypsum would be just fine, even with rock wool or fiberglass in the cavites.

    With only R3 above the thin edeg and R20-23 cavity you will have a measurably warmer ceiling temp at that edge on hot sunny days, but if the TPO is nice light color with a high solar reflective index it won't be very bad at all compared to an old-school black torch-down.

  8. Jimmy Black | | #8

    Thanks Dana, yes the TPO is very reflective and I noticed a difference immediately. With the old bitumen roof on a hot day the underside of the decking would be very hot to the touch, I would guess around 120F, you could feel heat radiating down. After the TPO went on the underside it was room temperature, not noticeably warmer at all. Amazing at the 4" side the foam is R23, after thinking about it more, the edge of the interior wall is actually 2' from where the foam starts (because of the soffit and overhang), so the foam over the living space is actually 1" thick, giving an R5.7.

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