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Ice & Water Shield on whole roof

HenryS's picture

I work on old houses as a carpenter-designer-builder and GC. More and more I see on other people's jobs and on mine as well, roofers putting Ice & Water Shield (Bituthane etc.) on the whole roof deck before applying slate or wood shingles. It is handy when a roof is complicated, has a low pitch, will be uncovered for a long time in winter and so forth, but is it a good idea? Is it not a vapor barrier on the wrong side of the insulation? Particularly if there is blown-in cellulose insulation, installed above existing plaster and therefore no interior vapor barrier, it seems to me that there is a possiblity of condensation on the backside of the roof deck. If it were possible to install soffit vents and ridge vents that might address my concern, but on many historic houses that is not possible, or on hip roofs or other awkward shapes. My roofer points out the 30# felt, which we would otherwise use, is also a vapor barrier. However I assume that punching it full of nail holes makes it permeable after roofing installation.

I am going to be installing a new wood roof on a house in Cambridge, Mass. that is a national historic and architectural landmark and am very concerned that I do nothing to jeopardize its future. Of course, installing a roof that does not leak or fail prematurely is part of this concern, as well as vapor worries. Any opinions, studies etc. that could help me with this?


(post #98816, reply #1 of 24)

I grew up in the upper Midwest and we always use ice and water shield from the edge of the roof eaves to about 2-4 feet inside the top plate. We also use it on all valleys and around all perforations in the roof. I still like the method and still use it on roofs in California, although it is probably overkill here.

I would not hesitate at all using it all over the roof. I'm not sure my customer's would want to pay for it, but for your own home, I would go for it.


"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934

Regards, Boris "Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934

(post #98816, reply #2 of 24)

Henry, I am but a mere homeowner, but I live in a 1872 Mansard that sits in a historic district. We used ice&water shield everywhere. However, we also used either 6" of Corbond or 10" of Icynene everywhere. Thus, we have no concern about water vapor reaching the roof membrane. Plus, with the attic in the controlled space of the structure, we have no temp variations, etc. like normal attics that could precipetate condensation.

Perhaps you can contact RayMoore, he's got a lot of good information to share.

(post #98816, reply #8 of 24)

I also have done jobs where the interior was gutted and we were able to spary in Icenene. With a good air barrier like that, I am comfortable using I&W S over all.

(post #98816, reply #3 of 24)

Becoming very common around here, even for asphalt shingles.  I've asked a couple of contractors about it, and their argument is "hey, what's another $400 for that kind of insurance".

Guess it's a cost/benefit argument.

I would not worry about the moisture.  If moisture is going to condense on the backside of the roof sheathing, there is another problem that needs to be solved.  Roofing materials are not particularly permeable as it is.  Think about all of the membrane roofs in the world.

There are other 'halfway' options these days as well.  Several non-felt underlayment products that are tougher than felt are on the market.

(post #98816, reply #9 of 24)

Hi. I'd be interested in what these "non-felt" options are.

(post #98816, reply #11 of 24)

"Hi. I'd be interested in what these "non-felt" options are."

There are many, but they tend to be similar in concept.  Basically a woven sheet (often polypropylene) saturated with some sort of synthetic binder.  Very difficult (or impossible) to tear.  Thinner, lighter, and more expensive than felt.

(post #98816, reply #12 of 24)

Thanks for the info. Sounds like Tyvek with a waterproofing treatment...Henry

(post #98816, reply #13 of 24)

"Sounds like Tyvek with a waterproofing treatment"

Sort of, though Tyvek is polyethylene and spun.

Most of these new so-called "synthetic" underlayments are polypropylene and woven.  Though a few do appear to be spun.

(post #98816, reply #15 of 24)

The perm rating of Tri-flex 30 is less than 1/2 perm and titanium is even less. If you have an issue with I&W then you would have an issue with these as well.

In your climate, as others have said, foam it or ventilate it. In my opinion, felt paper is not a substitute for ventilation. Even if you use felt, you should ventilate the attic or spray foam. If you spray foam, you must do a very good job of it. Large voids that are left due to poor access, can cause problems in those areas.

(post #98816, reply #16 of 24)

Hi Ray; As I said in an earlier reply, my problem is that the top floor of this house is fully finished up into the roof with no possibilty of foaming,installing a vapor barrier or venting. Any vapor that gets up to the roof deck can only get out through the deck and the roofing materials. I guess I'm confirming, through this discussion, what I already suspected; a full covering of I&W S is a bad idea. The next question, given that, would be: what is the best material to go under the shingle breather and shingles (if not 30# felt)?

(post #98816, reply #19 of 24)

You should check these guys out
It's totally waterproof but completely breathable. Like gortex for your house. It has a perm rating of 212. Runs abought .47c a sq ft.
Used successfully all over Europe and know being marketed in the US.

(post #98816, reply #21 of 24)

Hey thanks; This looks like the kind of product I am looking for. Henry

(post #98816, reply #23 of 24)

BTW I think Beacon Sales in Somerville is a distributor.

(post #98816, reply #24 of 24)


(post #98816, reply #4 of 24)


I disagree with your roofer that #30 felt is a moisture barrior. I uses it above the ice and water  and install it with roof tins ( which I also use as markers for the rafters so I know where to nail the roof brackets as I go up) . The thing is if I get back early in the morning I find the morning dew has buckled the #30 paper a bit- then it goes back to flat as the air warms up with a bit of sun. This tells me the paper has the ability to take on a little moisture then give it back off- something a vapor barrior like poly sheeting wouldn't do. 

I've stripped roofs without the paper and seen surface dryrot so I won't go without it.

(post #98816, reply #5 of 24)

"Is it not a vapor barrier on the wrong side of the insulation? "


Yes it is.

I've only done it where the special circumstances call for it. And extra venting was required those few times.

Don't rely on roofers for whole house venting info!

and ... felt ain't a vapor barrier ... no matter what the weight ... that's the beauty of felt ... it lets any moisture that snuck behind dry out over time.

ask a coupla shingle and insulation manufacturers ... or local code officials ...

bet they'll say more ventilation is needed with a full "shielded" deck.

the ones I've talked to have ....


    Buck Construction

 Artistry In Carpentry

     Pittsburgh Pa

    Buck Construction

 Artistry In Carpentry

     Pittsburgh Pa

(post #98816, reply #6 of 24)

If the place has adequate venting, the I&W won't be a problem. I rarely use it for the whole roof surface. There are times when it is berneficial. For insatance when doing a tile roof which takes a long time, and has a grid of 2x2 nailers all over that can impede the runnoff of rain water, and which, like slate, is a lifetinme roof and needs a long lasting underlayment. Slate typically requires a double ply of good thirty pound felt under it anyway.

But the concern I would have that has not been mentioned here is that Cedars should not be laid in direct contact with either type of underlayment. They should be given a breathing space by installing over skip sheathing or over the new product Cedar Breather. That way, when wetted by weather, the cedar can dry more equally to both sides and be less likely to warp, cup, split, and deteriorate.
IMO< the steeper the roof, the less of a concern this is, but for the lonst life and the most historically correct application, you definitely need to let that wood breathe.



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Oh Well,

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(post #98816, reply #7 of 24)

Thanks. The reason we are replacing this cedar roof is that it was installed 16 years ago without breather!

(post #98816, reply #14 of 24)

"If the place has adequate venting, the I&W won't be a problem. I rarely use it for the whole roof surface."


I don't have much experience with the interaction of some roofing products and I'll be re-roofing my house after a small addition.  The house is a split level style house with the back half of the house vertically in between the first and second floor of the front half of the house.  Well, a large portion of the back half is a cathedral ceiling that  runs up into the roof of the front half of the house, and do to winter sun exposure and no existing venting on the back half of the house, I was planning on spraying incyclene on the underside of the rafters and I&W the whole roof (I'll be roofing over a few weeks and don't want to tarp it all the time.)

Now you have me thinking a bit.  If I am spraying on incyclene under the rafters and doing an unvented attic should I be concerned about I&W the whole roof?  The house is 40 yrs old, no moisture problems and incyclene is vapor permeable.  Will the incyclene let the roof sheathing breathe enough?  After the addition, I'll have a few valleys 15' long that will terminate halfway up the cathedral roof in the back. 

Any thoughts would be helpful.  Thanks.


(post #98816, reply #18 of 24)

A good dose of finwer insulation like Icy, urethyne, denspaks, etc, eliminates or severly reduces the need for venting.

The purpose of a vented attic system is that in a normal house ( is there such a thing?) warm air rising to the cieling and beyond will carry moisture from many sources up until it finds a cooler surface to condense on and cause problems. by providing an escape egress, the vents let the moisture be carried out before It can do much harm.

With tight insulations such as you are considering, the moisture has a harder time penetrating it in the first place. Convection currents are eliminated so no moving air currents carry moisture to a point of contact with a condensing surface, urethene foam further is a VBG on it's own.

But that means that the stale air and moisture stays within the house. The industry is learning that such tight houses need HV recovery systems. - air exchangers that save the heat energy mechanically.



Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!



Oh Well,

We did the best we could...

(post #98816, reply #22 of 24)

Thank you for the reassurance.  The only incyclene will be on the underside of the roof, so it shouldn’t be super tight but when we put in the AC air handler I’m going to add the air exchanger.  Its only a few hundred more and with all the gases coming off various household products, etc and houses being closed up for either air conditioning or heat, I figure its much better to err on the safer side.


Thanks again.

(post #98816, reply #10 of 24)

I know at least one of the Ice and Water Shield products states on the box that if you cover the entire roof proper ventilation must be present because the product is a vapor barrier.

Rich Beckman

Another day, another tool.

(post #98816, reply #17 of 24)

In my new construction, I completely sheathed my 3.5 and 3 pitch roof with Water/Ice. To solve the potential vapor barrier problem, I designed a 2" space under the roof deck between the 2X12 rafters for air to travel from the soffit up to the ridge vent.  Now, I'm on the Oregon Coast where it doesn't freeze, it rains a lot but during the summer it can get quite warm with single digit humidity. The inspector seemed to think my approach well thought out, and doubted I'd have a moisture problem.


(post #98816, reply #20 of 24)

Hi Bruce; That is standard procedure in new construction, but unfortunately I haven't got that option in this old house. Henry