Felt (semi-permeable) vs synthetic (impermeable) underlaymen


I am getting a new roof for my 90+ y/o house in Western New York (cold winters with some snow; hot, humid summers). Approx. 17 sq. (with porch). 7:12 slope. Full length ridge vents. No valleys. Plan to use architectural shingles; ice & water shield. My question is about synthetic vs. felt underlayment.

Local roofers are experienced but not terribly knowledgeable beyond their personal experience. Mostly, they just do what they do. Most prefer synthetic underlayment for all the benefits that it provides the roofer–and it appears to be very strong, so they think it must be good for the roof, too. They mostly think in 10 year time frames, not 20-30 yrs (the nominal life of the roof).

My concern is that synthetic (e.g., Titanium 25 UDL) is vapor impermeable, unlike felt, which is semi-permeable to vapor. I fear that moisture will be trapped between the tongue-in-groove roof deck and the underlayment. (Additionally, I am concerned that a crew used to using synthetic may take shortcuts when prepping the deck, nailing the felt, etc., resulting in compromised felt installation.)

Internet searches for info have led me primarily to contractors who love synthetic, a number who have stuck with felt, no one who has switched back from synthetic to felt, and very little actual information about this issue, except for a few people who raise the question without answering it. These older buildings may have lasted as well as they have because they were built with lots of “natural ventilation” (i.e., air leaks). Now that we are sealing houses up, paying more attention to venting attics, etc., the whole physics may be changing–with lots of unintended consequences. Roofing old houses with new materials and techniques does not seem to be getting much scrutiny. There’s a part of me that wants to stick with what’s worked, but I know that felt only works when it’s used carefully and correctly. Synthetic is a lot more forgiving.

Can anyone offer an informed opinion on this subject or direct me to a reliable resource? Am I making a mountain out of a molehill? I have questions about proper roof ventilation dynamics that I will post separately (and would welcome feedback on that as well).

I am very grateful for any feedback anyone might provide.



Yeah I feel you are worrying too much about nothing to be honest. If your roof is properly ventilated you won’t have an issue with either. I prefer synthetics because they install quicker and are you have better traction but you can still have a quality install with regular felt.

Installed shingles are a class 1 vapor retarder. Felt/synthetic makes no difference once the roofs on.

You can get a vapour permeable synthetic underlay, however, there isn’t much point. Patchap is right when he says that shingles as an assembly act as a vapour barrier (which is why it’s so important to have proper ventilation).
You can read more about it here
rci-online.org/interface/201 … assman.pdf

I agree with island roofing, if your attic is properly ventilated there will be no issue.

Thank you, all, for your input, and in particular for the link to the paper. Although the paper was written primarily by employees of Owens-Corning, it provided some objective data/specs and issues to consider.

You are probably right that I am worrying about nothing. If the authors (and you helpful respondents) are right, the impermeable underlayment (vs. felt) is irrelevant because the shingles themselves are semi-impermeable (i.e., between 0.1 - 1.0 perms), so the moisture won’t escape anyway. (Unless “semi-impermeable” is enough permeability to let this amount of moisture out. the paper’s authors don’t specifically address this.) The moisture is controlled by adequate ventilation, and not conductance through the roofing.

I wish I could shake the image of tongue-in-groove decking soaking up moist summer air, then sweating next to the underlayment and stewing. If 30# felt (semipermeable at 1.7 perms) had the effect of wicking the moisture away from the wood deck, the moisture would at least sit between the felt and the shingles, allowing the deck to dry. I say “if” because I have no worldly idea if it acts that way.

In your experience, will adequate venting allow the wood deck to dry completely through to the top? Or is my image of moisture collecting at the wood/underlayment interface a real possibility (say, as the hot, humid day yields to a cooler night, with the roof cooling faster than the interior–promoting condensation on the underside)?

I think my fussiness is in part prompted by both great respect for the lasting construction of a number of old buildings I have worked on, combined with dismay at the unintended consequences of innovations that did not take a sufficiently long or systems view.

Again, thank you for your input. If you have any thoughts about adequate venting, please see my other post on 10/21/13 regarding too much ventilation.


No, if your roof deck is getting that wet from condensation you are not gonna be saved by all that moisture going through the felt and getting trapped between it and the shingles. If there is that much moisture in your attic there is a much bigger issue going on that breathable underlay will do nothing to solve.

However, if it will really make you sleep better at night just offer them a little extra $(for the extra labor) to install regular felt. I doubt they will have an issue with that. You said your roof already lasted 90 years so I really doubt you have any major issues.

Thank you, IslandRoofing.

I expect I will go ahead with the Titanium 25 UDL. They know how to lay that down and with the weather turning cold the felt will be just that much harder to handle. I think poorly laid felt (e.g., because of inadequately prepped deck) is way more likely to cause problems down the road than well laid synthetic!

Thanks, too, to the others who replied.

Any ideas about having too much ventilation? The ridge vents they will be using, coupled with proportionate intake vents will be, like, three or four times more ventilation than warranted. (I posted on that subject but no one replied.)

Good luck in your endeavors,


The 1:300 stated in most codes is a minimum. More ventilation the better, as long as it is balanced.

I would think the only time the permeability of the felt would matter is in a spray foamed attic with no attic ventilation, and even then if the water vapor passed the felt vapor barrier it would be trapped under the shingles. This winter a contractor i worked for built a large home between rains. I came out and papered each section of roof he finished as he went. He chose a high end non permeable synthetic felt because of the longevity of the build thinking it would hold up better. Once the structure was completely under paper the subfloor and basement started drying rather quickly. During the process of shingling i had read a research paper from Owens Corning on permeable vs non permeable synthetics. So the next day I cut a big U shaped slice in the felt to inspect the papers theory and the OSB was absolutely soaked with water droplets built up between the OSB and felt even though the felt was completely dry on the exterior and the OSB was dry on the underside except a touch of condensate at each seam. Now mind you the ridges were covered in felt until i was ready for ridge vent. I had the contractor sign a release of liability if this caused future concerns as he picked out all the materials. Im going back within the next 2 weeks to shingle an added porch roof and im going to inspect the decking. Needless to say I still use synthetic but ive dropped down to a lower end permeable product