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Community and Q&A

Unventilated, highly insulated roof — replacing shingles

Sharon Secrist | Posted in General Questions on

We are replacing the shingles on a cathedral roof.

The roof has been insulated from the inside. 8 inches of expanded polystyrene below the roof deck including cut and cobble between the rafters, and 4+ inches of additional eps and mineral wool below that.

The roof is not ventilated.

The shingles will be asphalt.

From what I have read, asphalt shingle roofs are not water or vapor permeable, so using a permeable underlayment will not provide a benefit, because vapor will still not be able to dry through the shingles.

Two questions.

1) Is the best underlayment in this case, one of the impermeables listed on the fourth page, such as Owens Corning Deck Defense?

2) Is there any other step to be taken? Such as OC Ice and Water barrier near the edge?

Thank you.

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  1. Sharon Secrist | | #1

    Now I see manufacturers for synthetic only allow for use in ventilated roofs.

    So, 30# felt seems the best option for unventilated. I will also ask about adding ventilation.

  2. GBA Editor
    Martin Holladay | | #2

    Unfortunately, your roof is insulated with a method that is associated with moisture problems and rot. The method you have used -- the cut-and-cobble method -- is not recommended for unvented cathedral ceilings. Here is a link to an article with more information on the issue: Cut-and-Cobble Insulation.

    At this point, you probably don't want to start from scratch. The best way to salvage the situation, in my opinion, is to install a ventilation gap above the existing roof sheathing. That way, if the roof sheathing ever gets wet, it will be able to dry upward.

    If you follow my advice, you'll need to select a roofing underlayment that is somewhat vapor-permeable -- for example, #15 asphalt felt, VaproShield SlopeShield, Perma R Products PermaFelt, GAF Deck-Armor, or Cosella-Dörken Vent S.

    Then you will install 2x4 sleepers, installed on the flat, either 24 inches o.c. or 16 inches o.c. (depending on your rafter spacing), running from eaves to ridge, with one 2x4 above each rafter. These 2x4s will create ventilation channels that are 1 1/2 inch deep.

    Next you will install another layer of OSB or plywood roof sheathing, followed by the roofing underlayment of your choice, and new asphalt shingles.

    Needless to say, you'll need vent openings at the bottom and the top of these new ventilation channels.

  3. Sharon Secrist | | #3


  4. Expert Member
    Dana Dorsett | | #4

    An above-the-deck venting approach is nowhere near as effective as under-deck venting, since interior moisture drives that find it's way in has to pass through the vapor-retardent deck itself to dry. But it's way better than nothing. Above-deck venting has a better chance of being effective in zones 4A/4B or lower than in zone 4C or zones 5 & up.

  5. Sharon Secrist | | #5

    I think I understand too much moisture can be a problem, though it all does get confusing for me.

    We have been able to remove the central air and central gas furnace. Which is a benefit in itself, opening up space, including removal of the furnace vent.

    This summer, we cooled with a 5,000 BTU window unit. It is a 1,600 square foot house in the central midwest, zone 4/5. Hot humid summer, kept it at mid-60s F and used a cumulative 400 kwh.

    If moisture is a concern, I will crank it up and am willing to spend an extra $12 per summer, even if it requires a sweater.

    So, as far as I can tell, the risk of interior moisture can be mitigated, but I may be wrong.

  6. Expert Member
    Dana Dorsett | | #6

    I can't imagine cooling to mid-60s indoor temps in summer!

    Cranking the AC in summer doesn't really mitigate a wintertime moisture accumulation problem. The 8" EPS is fairly substantial vapor retarder, and you can't guarantee that convecting air leaks creating a damp spot on the roof decking will reverse them selves in the cooling season. The risk of mold/rot getting going is really in the springtime shoulder season, when a potentially damp roof deck is warm enough to promote rapid fungus growth. By summer it will have already dried into the above-deck vent channel, if you build one.

    So in short, don't go out of your way to over air condition the space- it isn't really buying you any additional resilience.

  7. GBA Editor
    Martin Holladay | | #7

    You can't necessarily keep your roof sheathing from getting damp (or from rotting) by controlling your indoor relative humidity levels.

    Unfortunately, rafters move -- due to changes in humidity and temperature, and due to settlement and snow loading. When rafters move, cracks can open up between the rigid foam and the rafters in a house with a cut-and-cobble insulation job. These cracks allow warm, humid interior air -- and interior air is always relatively humid in winter, regardless of your attempts to keep indoor air dry -- to contact cold roof sheathing.

    That's where the trouble starts: condensation or moisture accumulation in the roof sheathing.

  8. Sharon Secrist | | #8

    Mid 60s is colder than I would normally keep a house, but from what I have read it seems a common situation in insulated homes. The air conditioner is run more to get rid of humidity than to cool the house, especially on the second floor, resulting in a colder house.

    Below / Covering the rafters there are 4-6 inches of expanded polystyrene, so hopefully that will keep the moisture out. The cut and cobbled foam is tight enough for a friction fit but also sealed with caulk and Great Stuff spray foam. It is a tight fit. The house is 60 years old, so maybe has settled, and that plus the foam, maybe the rafters won't move much.

    Thanks for the comments. Just wish there was a way to know what is going on at the roof deck.

  9. GBA Editor
    Martin Holladay | | #9

    If I were you, I'd invest in the above-sheathing ventilation channels. You'll sleep better if you do.

  10. Expert Member
    Dana Dorsett | | #10

    +1: What Martin said.

    BTW: A room dehumidier would be a more comfortable and more efficient way of controlling moisture than cooling it another 5-10F. A half-ton Mitsubishi mini-split in "Dry" mode might do it too, but that's a much more expensive propopsition.

  11. Sharon Secrist | | #11

    A comment on a GBA thread mentioned a synthetic underlayment that the manufacturer says can be used on an unventilated roof with spray foam. Titanium UDL. I do wonder what, if anything, makes it unique from other synthetics.

    Also, here is a detailed publication from the EPA "Moisture Control Guidance for Building Design, Construction and Maintenance" much of which is covered at GBA but may help others trying to understand.

    Q. Can Titanium UDL be used with foam insulation?

    A. Blown foam insulation by its very nature can make a roof attic not well ventilated. Titanium installation instructions state to use "above a properly ventilated areas, as it is considered a vapor barrier". Titanium UDL underlayment can be used with a blown foam system if the attic area is part of a closed loop, meaning it is part of the HVAC system. Therefore the attic air temperature and moisture is being controlled by the HVAC mechanicals.

  12. GBA Editor
    Martin Holladay | | #12

    Whether a manufacturer of synthetic roofing underlayment permits its product to be installed on an unvented roof has more do do with marketing than building science.

    If an unvented roof is properly designed, the roof sheathing won't rot. If it is poorly desgined, the roof sheathing can rot.

    Of course, it's also possible to have a poorly designed vented roof.

    Sheathing rot rarely has anything to do with the roofing underlayment, since most roofs can't dry outward anyway.

    Manufacturers of synthetic roofing underlayment don't like to be dragged into arguments that blame their products for sheathing failure: "My roof sheathing is rotten, and it must be the fault of the underlayment!" So many -- but not all -- of these manufacturers just make a blanket statement that their products can't be used on unvented roofs.

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