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How to vent a flat roof

jcrarch | Posted in General Questions on

Designing a home in Santa Fe, NM. Roof will be TPO over open web trusses. Pitch is 1/4″ / ft.
Parapets all around. Insulation will be blown-in cellulose. R value to be 50. How do I vent the roof cavity above the insulation? I could use mushroom caps. I’ve done the calculations and I need quite a few. How much air space is required above the insulation? 2″, 3″ or more? Also planning on installing a radiant barrier between the tops of the trusses and the sheathing. When I build crickets over the sheathing will I need to provide air ventilation for the cavity? Thanks!

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  1. iLikeDirt | | #1

    Honestly I would ask local energy nerds. Most of the expertise here pertains to the cold cold northeast, and there is little personal and professional familiarity with climates that have dry weather, freezing snowy winters, hot hot summers, huge diurnal temperature swings, sun year-round, little rain, and termites. I live down in the Albuquerque area and a good deal of the advice thrown around here is either inapplicable or just the starting point, totally incomplete without adaptations for our unique climate.

    Irrespective of anything else, make sure the roofing is bright white! If you build the house right, you should have zero cooling load and a heating load that is nearly as low. Carefully consider factors such as shading, earth coupling, window placement, and tree placement. Radiant barriers in at least your western wall will probably be a wise investment, and you're right to specify one in the roof as well. Just remember that they need to be facing an air gap to work, and any radiant barrier facing up will eventually gather dust and be useless.

  2. GBA Editor
    Martin Holladay | | #2

    Everything you need to know is in this article: Insulating Low-Slope Residential Roofs.

    Nate's advice -- "make sure the roofing is bright white" -- is exactly backwards. In the Southwestern U.S., white membrane roofing is associated with more moisture problems in this type of roof assembly than dark-colored roofing. (Due to nighttime radiational cooling, white membrane roofing gets cold enough to become a condensing surface for moisture, and the membrane and roof sheathing never warm up enough during the daytime to dry out the moisture. Dark roofing gets warm enough during the daytime to drive out the accumulated moisture and keep the assembly safer. More details in my article.)

  3. jcrarch | | #3

    Thank you both for your responses. Interesting note about the roof color. I thought it would be more important to use a light color to prevent heat build up during the days, especially summer. We get a lot of sun in Santa Fe. However, a TPO roofing membrane with a radiant barrier cannot function as a heat sink so I don't understand how it would dry out any accumulated moisture even if some of the heat is retained in the sheathing. I'll definitely give this some thought. Thank you both.

  4. jcrarch | | #4

    When I build 2x crickets over the sheathing do I need to ventilate that space? If so, maybe the crickets are fastened to the trusses using structural clips with the radiant barrier draped over the crickets/trusses. Sorry to ask so many questions. There seem to be a lot of factors involved and I want to get it right. Is there a building expert that would act as a consultant for a fee on this project?

  5. Dana1 | | #5

    Radiant barriers in walls are next to useless unless the wall is uninsulated AND low mass.

    Radiant cooling to the clear night sky is real- it'll drop the temperature of the roofing to more than 10F cooler than the outdoor air temperature, sometimes more than 20F cooler in high-dry Santa Fe. If the roof deck is above the insulation that's a potential moisture problem for the roof deck, since the average temperature of the roof deck in winter will be below the dew point of the conditioned space air, and becomes a moisture sink for any air or diffusion transported moisture that isn't purged by the venting schem. If all of the insulation is above the roof deck and unvente, it's not a problem, since the roof deck is the same

    Venting flat roofs is a PITA, and puts more penetrations in your membrane roof, making it more leak prone. Go unvented. That can be done nearly as cheaply as blown cellulose + venting if you're creative.

    Santa Fe is in US climate zone 5B. In zones 5A & 5B in an unvented assembly, if you have at least 40% of the total R above the roof deck the roof deck would be moisture safe, UNLESS you used a "cool roof" with a high solar reflective index (SRI). When using a high SRI roofing that would have to be bumped to 50% or even higher.

    If you can find a supply for used roofing foam, it's often as cheap or cheaper than just the cellulose not even counting the cost savings of not having to deal with the venting. With 8" of continuous polyiso under a TPO roof you would beat code min with about 15% of margin on a U-factor basis. If you used EPS instead it would need to be about a foot.

    Using virgin-stock goods it would run about $5 per square foot for R50-ish rigid foam (EPS or polyisocyanurate). With reclaimed/used goods it would run about a buck-fifty.

    If you want to split the R due to height restrictions or cost, you can cut the foam numbers in half, and put 5.5" / R23 rock wool or 6" of damp sprayed cellulose on the underside of the roof deck, which would be about 50% of the center-cavity R, but it would be slightly lower performance than an all-foam above the roof deck solution due to the thermal bridging of the rafters. At 50% or more above the roof deck the risk to the deck is very low, even with high SRI roofs, but at 100% above the roof deck the risk really is zero.

    If you can't find a local source for used foam, Nationwide Foam will ship anywhere in the lower 48 (for a price), and they have multiple regional depots to ship from. (I'm not sure where the nearest one would be to Santa Fe.) See:

  6. Regulajo | | #6

    Just a quick question on this same note: CZ6 build about to start with a 1/12 pitch roof. I have 12-14" Cellulose below the roof deck and 5" polyiso above the roof, covered with 1/4" densedeck and a membrane roof. Does this work and does color make a big difference? I also though a lighter colored roof was good for some reflective value.

  7. GBA Editor
    Martin Holladay | | #7

    If you plan to drape a radiant barrier over your rafters before installing the roof sheathing, the radiant barrier might limit the flow of moisture from the interior to the cold sheathing. But that's not a dependable way to avoid damp roof sheathing, because of possible air movement through the radiant barrier seams.

    In any case, installing radiant barrier sheathing is easier than installing a separate radiant barrier.

    All of this is irrelevant, however, if you build the roof the right way -- as an unvented low-slope roof. You are in Climate Zone 5, so you need to put at least R-20 of rigid foam above your roof sheathing -- then you don't have to worry about moisture problems or venting. Here are links to two article that explain what you need to know:

    How to Install Rigid Foam On Top of Roof Sheathing

    Combining Exterior Rigid Foam With Fluffy Insulation

  8. GBA Editor
    Martin Holladay | | #8

    Don't worry about condensation on your roof sheathing -- if you have that much rigid foam above your roof sheathing, then the roof sheathing won't get cold enough for condensation or moisture accumulation, no matter what color of membrane roofing you choose.

  9. Dana1 | | #9

    Steve: Martin may have missed your location. In zone 6 you need at least half the center-cavity R to be above the roof deck just to meet code independent of roof color. If you de-rate the polyiso to R5/inch, the 5" is good for R25, and ~R45-R50 below the roof deck. Even assuming R6.5/inch (a performance it will NOT achieve) you're still less than R35 above the roof deck, with well over R40 below. This would almost make it in zone 5, but not zone 6.

    The R25 prescriptive in the IRC presumes R49 code min, and you will have well over R49 total. For the temperature at the roof deck to be the same at higher R, the ratio has to be preserved. At your ratio the average temp at the roof deck in winter will be below the dew point of the conditioned space air, and take on moisture.

    You can either dial back the cellulose to R25-ish, or bump up the exterior foam-R to match or exceed that of the cellulose, or you can install an air tight class-II vapor retarder or smart vapor retarder on the conditioned space side of the cellulose.

  10. GBA Editor
    Martin Holladay | | #10

    Good catch. I missed the fact that he is installing so much cellulose on the interior side of the roof sheathing.

    Steve -- Dana is right.

  11. Regulajo | | #11

    Thank you for clarifying.
    So does painted drywall, or taped OSB with T&G nailed to it work as the vapor retarder or does it have to be a membrain type product?

  12. jcrarch | | #12

    Thanks Martin for the reference. After considering all the feedback on my post we will be doing an unvented roof assembly:
    TPO roof membrane white
    o/ 3" unfaced tapered closed cell polyisocyanurate 1/4"/ft slope
    o/ "ZIP" roof sheathing
    o/ 6" min. open cell polyurethane spray foam
    w/ TJI's
    Great to have this resource for Building Science!
    Is there anything I'm missing?

  13. GBA Editor
    Martin Holladay | | #13

    A few points:

    1. You mentioned installing tapered polyiso above the roof sheathing. Is 3 inches the maximum thickness or the minimum thickness?

    2. If 3 inches is the minimum thickness, then the R-value of your planned roof assembly is: R-17 (polyiso) + R-22 (open-cell spray foam) = R-39. That's less than the minimum code requirement in Climate Zone 6.

    3. A better approach would be to increase the thickness of the rigid foam insulation above the roof sheathing. In my article, I suggest that builders in Zone 5 include at least R-20 of rigid foam above the sheathing -- so 4 inches of polyiso would be good. Then install at least R-29 or R-30 of vapor-permeable insulation under (and in direct contact with) the roof sheathing. Some type of fibrous insulation (cellulose, fiberglass, or mineral wool) would be cheaper than open-cell spray foam.

  14. jcrarch | | #14

    Thank you. I'll use 4" min. polyiso. How do you install cellulose, fiberglass or mineral wool so that it is in direct contact with the roof sheathing? That is why I thought I needed open cell spray foam.

  15. GBA Editor
    Martin Holladay | | #15

    The easiest way is to fill the rafter bays completely with insulation. The interior side of the rafter bays need to be enclosed with drywall, OSB, or InsulWeb netting to retain the insulation.

  16. jcrarch | | #16

    Thank you for this great resource. I think I've got it all figured out now. Couldn't have done it without green building!

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