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How to vent a PVC low-slope roof and prevent condensation

SJDG | Posted in General Questions on

I’ve owned for 3 years a 1952 house in No. Calif. near SF Bay.  It has a low-sloped flat-top roof (almost flat/slight slope).  It has fascia-venting in every other bay, fiberglass insulation in the bays, and the slightly sloped flattop ties into a pitched composition shingle on the other end.  When I go into the attic of the pitched composition I can see inside the flattop slightly so I believe the roof vents from the fascia vents into the pitched composition and out the o’hagan’s of the pitched roof.  

The flattop is NOT leaking nor does it have any alarming issues as I have used a moisture meter on the interior gypsum ceiling and it’s all dry, except, in 2 small areas the gypsum registers moist because the bathroom fans don’t exhaust out the roof (they currently exhaust into the flattop, however the moisture from the bathroom fans is only affecting about a 1 foot round area of the gypsum near the fan locations).  I know this is not good to vent bath fans into a flattop which is one of the reasons I’m going to get the entire roof replaced so I can add roof exhausts to the bathrooms, plus the roof is old and I want to be proactive.  

I’m going to replace it with a IB PVC 60mil roof, I’m also going to replace roof deck with all new 5/8″ T/G CDX plywood, and roofer says 1/4″ non-prime dens deck over the plywood.   My concern is I read that some PVC flat and low-sloped roofs may condensate. That is the LAST thing I want is to put on a brand new roof and create a different problem.  I want to be pro-active and educate myself to figure out what is best to prevent eliminate that from happening.  

My certified IB roofer says he can add some wood to create slightly more space between the fiberglass insulation and the roof deck and add couple of roof vents (in addition to what I already have).  IB says to choose a darker color (not white).  IB says I could add 1″ polyisocyanurate on TOP of the plywood roof deck to keep the plywood warmer.  I’m worried that the polyisocyanurate may not allow my house underneath it to breath.  Isn’t a breathing house important?  Also if I do use polyisocyanurate, what kind? what thickness? I read low-perm and  I read if it’s faced that that can cause problems, so I assume it must come un-faced? Also I suppose it needs to be sealed with tape.  

While my roof is open, should my roofer seal around canned lights? fan openings? etc? Is dens deck mold resistant or is there something better to use? Please advise on my many questions, I don’t want to spend $14k on a roof that will cause a condensation issue, I know PVC won’t leak for 30+ years but last thing I want is a new roof with a new issue.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    It becomes a lot more readable with paragraph breaks (please edit!)

    >" My concern is I read that some PVC flat and low-sloped roofs may condensate. "

    The verb you're looking for is "condense". "Condensate" is a noun that refers to the condensed liquids. Technically the real problem is adsorb, not condensate- the water affecting the roof deck or gypsum is never liquid until the material is saturated.

    >"My certified IB roofer says he can add some wood to create slightly more space between the fiberglass insulation and the roof deck and add couple of roof vents (in addition to what I already have)."

    Venting a flat/very low slope roof doesn't really help purge moisture, and if anything the space between the fiberglass and roof deck should be reduced to ZERO, with sufficient insulation above the roof deck for dew point control. After sealing the gypsum board penetrations and sealing the vents, blow it full of fiberglass or cellulose right over the existing fiberglass. (If cellulose, dense packing to 3lbs per cubic foot recommended.)

    How thick is the existing fiberglass, and how deep is the vent gap?

    >"IB says I could add 1″ polyisocyanurate on TOP of the plywood roof deck to keep the plywood warmer. I’m worried that the polyisocyanurate may not allow my house underneath it to breath."

    The roof assembly is NEVER going to dry toward the exterior through a PVC roof- the polyiso doesn't affect that in the slightest. In a zone 3 climate 1" of polyiso is sufficient dew point control for up to R39-R40 of vapor permeable insulation under the roof deck (and probably even more) using the most commonly used roofing materials. With a white PVC roof you may want to bump that to 1.5" or even 2", depending on how much other insulation is in the stackup. For a "walkable" roof (not "STOMPABLE") 2lb density fiber faced roofing polyiso is the standard. (The lower density foil faced wall sheathing type polyiso is easily damaged by walking on it.)

    See "TABLE R806.5 INSULATION FOR CONDENSATION CONTROL in the IRC" :

    https://up.codes/viewer/utah/irc-2015/chapter/8/roof-ceiling-construction#R806.5

    >"While my roof is open, should my roofer seal around canned lights? fan openings? etc?"

    The ROOFER is normally only responsible for sealing around the ROOF penetrations, not ceiling penetrations. Insulation contractors would normally be responsible for the latter. That said, it IS a good idea to make the ceiling plane as air-tight as possible, for both thermal and moisture transport reasons.

  2. Jon R | | #2

    Also note below, which apparently allows you to use no foam. In such a case, limited venting will purge some moisture and, from the data I've seen, will result in drier sheathing than no vent. Not clear what you currently have, but the same design should continue to work well.

    =======

    b. Alternatively, sufficient continuous insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45°F (7°C). For calculation purposes, an interior air temperature of 68°F (20°C) is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months.

  3. GBA Editor
    Peter Yost | | #3

    I am curious how you are measuring moisture content in your ceiling gypsum wallboard. Sounds as though you are probably using a surface reading moisture meter?

    For more information on moisture meters, and particularly surface readers, see this GBA blog: https://www.greenbuildingadvisor.com/article/pinless-moisture-meters.

    Peter

  4. SJDG | | #4

    Peter, I understand how to use my surface moisture meter, but those are good tips for beginners because people can unnecessarily panic when metal flashing and even nail heads sets it off. My bathroom fan vents into my ceiling, I can see the where the end of the coil pipe sits when peeking my head in the fan's hole when we replaced it not too long ago and that's exactly where the underside of the gypsum registers 75% to 100% wet depending on how many people just took showers accounts for the varying degree in wetness.

    Jon, I agree that venting helps result in dried sheathing that's probably why the rest of my ceiling registers completely dry. I think if I'm going to make certain the sheathing doesn't form condensation then I'll need to spend the extra money and add polyiso ontop of it.

    Dana, thanks for clarifying a pvc roof won't breath outward, of course that makes sense so adding polyiso won't make it any worse. So to confirm you feel 1-inch polyiso in my northern california location would be enough for dew point control if I use a color other than white? Won't the 1/4" Fire Densdeck also help to reduce dew point? I would prefer not to use a super dark color, if I'm using let's say gray, would 1" polyiso keeping into consideration there will also be 1/4" dens deck be enough to control dew point? I really don't want to add additional insulation, the house seems fine as it is, never to hot or too cold. I believe I have 6 inch fiberglass insulation up there no however, I won't really know until the roof is torn off (I can see it through the fascia/soffit vent).

    You say "2lb density fiber faced", I read roofing polyiso is available with a variety of facings and PVC sites recommend non-faced, what is your recommendation?

    1. Jon R | | #5

      > if I'm going to make certain the sheathing doesn't form condensation

      It's appropriate to use the average of the 3 coldest months - the occasional sorption/condensation that will occur with no foam won't be a problem. But if you want to build to the unusual standard of "never ever reaches condensation temp", then you probably need more than 1 inch of foam (depends on amount of fiberglass insulation, interior dew point, etc).

    2. Expert Member
      Dana Dorsett | | #6

      >"So to confirm you feel 1-inch polyiso in my northern california location would be enough for dew point control if I use a color other than white? I really don't want to add additional insulation, the house seems fine as it is, never to hot or too cold. I believe I have 6 inch fiberglass insulation up there no however, I won't really know until the roof is torn off (I can see it through the fascia/soffit vent)."

      Without knowing the R value of what's below the roof deck it's hard to make a guess as to how much polyiso it's going to take, but at just 6" of fluff AND a vented air space it's enough. But 6" of fluff isn't even close to IRC 2018 code min R38 for climate zone 3, let alone Title 24 performance. Re-roofing time is an opportunity moment to bring it up to code without affecting the interior finishes, and blowing even 5-6" of cellulose over the fiberglass batts to fill up the space is remarkably cheap & effective. (What is NOT really effective is soffit venting on a low-slope roof- the vents and gap is not really doing anything for you since there isn't sufficient convection drive over such a low rise.)

      For the IRC minimums seeTABLE N1102.1.2:

      https://up.codes/viewer/wyoming/irc-2015/chapter/11/re-energy-efficiency#N1102.1.2

      From an IRC compliance point of view the R-value of the polyiso layer only counts if the fluff in the cavity is snugged up to the roof deck. If there is a vent gap it doesn't count at all.

      But for a CA Title 24 compliance point of view it does, as part of the U-factor calculation. See "TABLE 140.3 ROOF/CEILING INSULATION TRADEOFF FOR AGED SOLAR REFLECTANCE" on page 175 (p.187 in PDF pagination:

      https://www.energy.ca.gov/2015publications/CEC-400-2015-037/CEC-400-2015-037-CMF.pdf

      What is your CA Title 24 climate zone, and the spacing between the rafters?

      Figure out the total depth of the space between the ceiling gypsum and roof deck, using a stiff wire (a coat hanger or bicycle spoke will do) as a probe. You may be able use the gap at the edge of a can light fixture or drill an easy to repair 1/8" hole. Since the roof has some slope, if the ceiling is level it's not the same depth everywhere, to measure it at both the soffit and the deeper end. To be able to estimate the U-factor measure the rafter spacing too.

      Assuming a 2 x 10 rafters there would be 9.25" of nominal depth, maybe 10" if there is a 1x strapping layer. Filling over the batts with cellulose would R34-ish, if 10.0" it would be about R37-ish. With a high SRI PVC roof adding 1" of polyiso would bring the 9.25" deep cavity up to IRC code min, but it could be marginal for dew point control due to the cooler average roof temperature- 1.5" might be enough, but 2" would be better. (Note: 2" roofing polyiso is a very common thickness, and used roofing polyiso can often be had for dirt-cheap on the reclaimed materials market.)

      If they are 2x8 rafters there is 7.25" of depth, and filling any remaining gap with cellulose would yield R27, and it would take 2" of 2lb roofing polyiso (R11-ish) to bring that up to IRC code min. With only R27 in the cavity and R11 above the roof deck you would have huge dew point margin at the roof deck.

  5. SJDG | | #7

    I believe my CA Climate zone is 3B (Zip 94564) and my average 3 cold months are 45 degrees but some do drop into the 30's and on rare occasions into the 20's (but the average is 45 degrees). I talked extensively to IB today and here's what they said: all low sloped roofs that had condensation problems didn't have 'any' insulation in between rafters or areas where insulation was missing and/or no ventilation. They said the fact that I have fascia ventilation that exhausts into pitched roof and fiberglass insulation in-between rafters is all good for controlling dew point. They said they haven't seen low-sloped roofs with condensation issues in their other colors, only with white. But white has the other benefits of limiting heat transfer and keeping the roof itself cool. They said adding 1" of polyiso is more than I need for Zone 3 when installing a White PVC roof as the 1/4" Densdeck will also help a little too with insulating the sheathing. They said the recommendation for zone 3 (my zone) is R-5 which is 1" polyiso and that recommendation is over estimated so I should be good with 1".

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