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2/3 1/3 insulation rule in roof

Old build with flat wood deck on 2x13s. 4 inches Batt in joist space.(R12) 2 inch air space above batt insulation under wood deck No external vents into air space Adding polyiso on top of deck R 28. Before adding the polyiso can/should we put a separator sheet or vapour barrier on top of the wood as there is more than 2/3 the insulating value above the deck There is a poly vb above the drywall ceiling below e joists.

Asked by UBSC
Posted Apr 19, 2017 2:24 PM ET


7 Answers

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You don't need a vapor barrier on top of the existing wood sheathing, but you do need an air barrier. (The R-28 of polyiso is already a vapor barrier.)

If the existing sheathing is plywood, you can tape the seams with a high-quality European tape. Otherwise, you'll need an air barrier membrane like Solitex Mento or Ice & Water Shield.

For more information on this type of roof, see Insulating Low-Slope Residential Roofs.

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 19, 2017 2:49 PM ET


I really wish folks STOP using rule of thumbs, for insulation/outsulation, A/C systems, framing or anything else. For a roof, 1/3 outsulation works in CZ4A&B for roof ONLY, for other climate zones, it would be either a big mistake or an over-expense.
Wall systems are the same story, which is mostly depending on thickness of framing members and type of insulation/outsulation used. Where I live, in CZ3, we need 1/2" outsulation (R3) on a 2x4 or 2x6 wall; and if I were to use the 1/3 rule, I would have to install 1.5", at an additional expense to the client, not only in outsulation costs, but in foundation costs, since would need 7.25 brick ledges vs 5.5".
In addition, there are many places in the country where a single county may have one CZ but a municipality may have a different CZ, specially if if its near a mountain region. To be sure, one should perform a dew point analysis based on your specific location.

Answered by Armando Cobo
Posted Apr 19, 2017 6:54 PM ET
Edited Apr 19, 2017 6:56 PM ET.


In the system mentioned, If I put the ice and water shield on the top of plywood would I not get condensation on the underside if moist air leaked into the Fiberglas from improperly sealed poly above the drywall

Answered by UBSC
Posted Apr 19, 2017 7:22 PM ET


The 2" unvented air space between the batts and under side of the roof deck don't buy anything, but provides a potential thermal bypass around the insulation. Snug the batts right up to the roof deck.

With 2/3 of the R value above the roof deck there is no need for an interior side vapor retarder for any climate warmer than US Climate zone 8, which is basically the subarctic/arctic zone on this map:


South of that zone don't put any poly sheeting behind the drywall.

Answered by Dana Dorsett
Posted Apr 20, 2017 10:29 AM ET


Thanks. But this is an not a new building and the poly vb is already between the ceiling drywall and the joists. Therefore is the new peel and stick suggested above the roof deck below the new insulation going to be a benefit or a place for condensation to collect on the underside. i.e. Should we leave it out or add it when we place the R28 above the deck? The building is in Calgary Alberta Canada

Answered by UBSC
Posted Apr 21, 2017 9:49 AM ET


As I wrote in my first response, the rigid foam that you plan to install above your roof sheathing is already a vapor barrier. Installing Ice & Water Shield adjacent to one side of your R-28 rigid foam will not make this any more or less of a vapor barrier.

It's a vapor barrier -- with or without the Ice & Water Shield. As long as your rigid foam is thick enough -- and, in Climate Zone 6, your R-28 of foam will work (as long as you don't install more than R-27 or R-28 of fluffy insulation in the interior side of the roof sheathing) -- you don't have to worry about condensation on the interior side of the rigid foam.

For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation.

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 21, 2017 10:51 AM ET
Edited Apr 21, 2017 10:52 AM ET.


What Martin said. Unvented roofs don't dry to the exterior even without the foam, even a #30 felt + asphalt shingle stackup is a vapour barrier (as defined by Canadian code). But so is R28 of any type of foam.

The key issue for condensation or long term moisture accumulation in the roof deck from interior moisture drives is the average temperature at the roof deck. With 2/3 of the R-value above the roof deck it's warm enough be be protected in most (but not all) of Canada. In Calgary it's more than enough, but in Ft. MacMurray it could be cutting it close.

The polyethylene sheeting obstructs the drying path toward the interior, but as long as the exterior layers are air & moisture tight it's a pretty safe assembly, even with the poly sheeting left in place. Self-healing peel & stick membranes such as Ice & Water Shield are pretty good at maintaining water tightness, whether it's above or below the foam layers. While getting rid of the polyethylene sheet would give the assembly a bit more resilience, it's not necessary, and it's fine to leave it in.

You DO want to fill the old 2" gap with fiber insulation for higher air retardency, eliminating the thermal bypass potential. Filling it with blown cellulose or fiberglass drilling from the exterior would be pretty easy even with a rental blower, and it need not be "dense packed" with that stackup. That would raise the cavity to R18- R20 performance, but not more. In Calgary's climate a bare 50% of the insulation above the roof deck is adequate, so with R28 above there is still reasonable margin.

Answered by Dana Dorsett
Posted Apr 21, 2017 11:48 AM ET
Edited Apr 21, 2017 11:54 AM ET.

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