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Question DetailsAsked on 8/7/2017

use of a self adhering underlayment vs felt on a 4/12 single assembly roof deck using shingles

I have a 2x8 single assembly framed greatroom with R-19 batt insulation installed and with eave vents and
GAF Cobra 2 ridge vent. Getting ready to reroof the whole roof and wanted to use a self adhering underlayment or a product like GAF Deck Armor. Adviseable or not? Total room system is 4/12 but rest of area is open attic with R-19 blown over R-19 batts.

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Unless you have a high-moisture level attic (in which DeckArmor would be better because of higher moisture venting permeability), the type of ceiling/attic underneath does not make a lot of difference in which shingle underlayment you choose.

If you could get the true asphalt roofing paper from many decades ago I would prefer that for a normal ventilated attic situation - but the current products are a far cry from that. Personally, if you decide to go with roofing felt I would use "30#" - which is more like about 12# in the old standard. But with modern materials, and especially if in an area where the felt might get rained on before the shingles are applied (because it buckles and wrinkles) I would recommend DeckArmor - as far as I have seen a good material - just make sure that if the contractor is going to use the pre-printed gridlines for shingle placement that it is ACCURATELY laid down, because curved or slanted gridlines makes for a terrible looking roof when they follow them with the shingles. Personally I prefer ignoring the gridlines and using accurately snapped chalklines.

You say R-19 blown over R-19 batts - if you do not have a consistent 3-4" gap (or more) above the insulation (usually easily seen from eaves in daylight, with backlighting from ridge vent) under the roof sheathing (actually should normally leave the rafters/truss top chords fully exposed if done right), then if it is tight up against the sheathing I would look into getting enough removed to provide a continuous ventilation path under the sheathing - which in some cases means removing at least some sheathing (commonly a strip across the middle as well as the ridge vent, to access the blown-in insulation from both above and below to vacuum some out, using a piece of rigid pipe or several rigid wand extension pieces as a vacuum hose extension.

If having clearance problems I have also seen (though compacts the insulation a bit) pieces of bell-joint PVC pipe slipped in just under the shething from the eaves as a support, then sliding in continuous plastic wavy or W-shaped cross-section baffles to minimize buckling and to provide a permanent clear ventilation space under the sheathing - should be the type with minimal contact with the sheathing, not the box-grooved jokes where about half the baffles is in direct contact with the sheathing. There are a couple of brands which come in long lengths (10 or 12 feet as I recall) which make it a lot easier.

Note if using continuous baffles to maintain an air gap over the insulation, they have to be ventilated type so they do not act as a vapor barrier and trap moisture ffrom the house in the insulation.

Answered 3 years ago by LCD

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