Synthetic Underlayment as HouseWrap?
hotwheels
| Posted in Energy, Heating & Insulation on
I have some left over Sharkskin synthetic roof underlayment. I would like to use this leftover as housewrap for some patching … any reason that synthetic roofing undelayment cannot be used as housewrap, other than cost issues?
Replies
Generally a roof underlayment isn't going to be designed to transpire moisture.
How big of a patch?
Rebuilding my home in Cypress, CA
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What is the perm rating of it?
Siding wraps allow moisture to transpire. Too low of a permeability rating will trap moisture in the wall
But I do use underlay sometimes under window sills and on down form there occasionally
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Roof memrane isn't designed to pass water vapor the way housewrap is, so check the perm rating. Tyvek has a perm rating of around 50, stuff like the blue Dow Weathermate wrap is down around 6 perms...so I guess it's a crap shoot even when you use housewrap. Point is, the roof membrane might have a perm rating of .05 - which would be a bad choice.
I've seen roof membrane used on sidewalls once in a high wind, very cold area - but I'm not convinced it was a smart idea, even there. If it's just a small area to be patched, I'd just buy a roll of #15 felt paper, or a small roll of housewrap.
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I checked with the maker and although Sharkskin has a fairly low perm rating, given the California environment, they suggested it was fine. I installed the SS as wrap on the gable ends of the outbuilding under construction and I am moving on ... thanks for the help.
"although Sharkskin has a fairly low perm rating, given the California environment, they suggested it was fine"Just got me scratching my head to know what in the devil that means?Fairly low- they know exactly what the perm testing for the product is. What is that in numbers?Given the CA... - again, how does that mean anything? Siding specs have a require perm range. Your building code might also. it is specified in numbers.Do the numbers compare?
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Probably more significant than anything is the fact that this is an "outbuilding". In most cases that would suggest a building with low internal humidity levels and hence not at risk for "trapped" moisture.
Too much sanity may be madness. And maddest of all, to see life as it is and not as it should be! --Miguel de Cervantes
I must have missed the part where he said that. Does he have another thread running?
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#6.
Too much sanity may be madness. And maddest of all, to see life as it is and not as it should be! --Miguel de Cervantes
I CI was too focused on the other confusing language.
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credit to Dan for sleuthing out that the building under construction is an outbuilding. I am constructing it as if it were a small house, although there is no heating or air conditioning, it is important to me that the structure is insulated and that I learn about all the aspects of building using this "outbuilding" as a means to do so. Sorry for any confusion created by the question. The Sharkskin technical person ... very helpful and knowledgeable ... did give me the perm number ... but it was less important than his explanation of the other possible routes for air infiltration and the ratio of the Sharkskin to the Typar on the rest of the structure. It seemed to me that others may also be in similar situations where you run out of one material such as housewrap and have leftover roof underlayment ... This is an oportunity to learn more about the principles of energy efficiency and the building science of insulation.Thanks again and sorry for any confusion in the question ....
Not picking bones, or nits, but since you are doing this to learn, keep a couple things in mind.I am always suspicious when a rep promotes his product for something that it is not desined and intended for and doesn't have company literatur to back him up.I have seen humndrds of cases in my life of reps and subcontrators who do something like that with out being able to see the big picture - the overall tapestry that is a home. So they will each make a recommendation that fits them in their own narrow view of what is going on.For instance your statement from him that there is other infiltration going on in other places.the question with perms where you are using it is not whether or not there is infiltration, it is ex transpiration.
If moisture comes INTO the building in other locations via that infiltration, is warmed and rises, it MUST be able to transpire without being trapped behind a membrane that is too tight because of low perms.As long as this is an outbuilding with no heat and you don't put a VB on the inside creating a double vapour trap, you are fine. But if you did something like place one of those Kerosun oil space heaters in there, it would drive a lot of warm moist air into the wall assembly behind that membrane with the possibility of fast mold growth
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A vapor barrier inside shouldn't be a problem, though it's unneeded. In an outbuilding no one is taking showers, cooking, or doing other activities that put a lot of moisture in the air.Moisture coming into a building via infiltration is no problem since it will be at equilibrium with the outside and will cause no moisture "drive" through the walls.So long as the roof doesn't leak, you can basically forget about water vapor issues -- have none, have five of them, no difference.
Too much sanity may be madness. And maddest of all, to see life as it is and not as it should be! --Miguel de Cervantes
first - there should NEVER be a VB on both sides of any wall. any moisture that penetrates will never escape before it does damage.Second though, your assumption, "In an outbuilding no one is taking showers, cooking, or doing other activities that put a lot of moisture in the air." is totally wrong. There are a number of activites done in outbuildings that do create MORE than normal moisture levels than a home. I gave just one of them above.
Storing green firewood is another where I have seen damage from condensation caused by all the gallons of water transpired.
Some people put saunas in outbuildings.ALL these things should be considered and the design and venting be executed accordingly with out making assumptions.
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