“Crinkled housewrap” or tar paper under exterior foam?
Ry38men8ZA
| Posted in Energy Efficiency and Durability on
In the “Mind the Gap” article at http://www.buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh — If I understand correctly, when covering a plywood or OSB sheathed wall with foam, Lstiburek recommends a “crinkled housewrap” between the foam and the sheathing for diffusion redistribution. Would tar paper/building felt work in this situation?
Thanks.
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The point of the crinkled wrap is to act as a drainage plane. In this case the tar paper won't work as well as the crinkled wrap.
Many of the tapes that are made to stick to housewrap won't stick to tarpaper. I've been covering my wrinkle wrap with 15 pound tarpaper recently cause it's cheap insurance. and it was pretty sobering to see the data on how much the impact really is of siding nails in housewrap at this years ResNet conference.
I've seen some emulsification kind of issues going on when cedar and concrete board hold moisture against the face of Tyvek. Probably the better word is surfactant issue, anyway it's pretty easy to throw on a bit of felt and feel a little more secure. But I wouldn't skip the wrinkle wrap. That tape is critical, especially at the windows and doors. .
This is a classic case of a "solution" creating the next order of problem - or what is known as the Law of Unintended Consequences.
We have used up most of the fossil fuels and turned the earth into a hot house, driving the price of heating fuels skyward. So we demand (and now codes require) more insulation to minimize the heating load. We want more R-value but we want to get it in the quickest and easiest way possible, so we add exterior foam board (wrong side vapor retarder), and we typically install this over highly moisture-vulnerable OSB sheathing.
Then we can't place siding directly over the foam because it can't breathe (and we better make sure that the interior of the wall assembly has some potential for inward drying, even though the dominant drying direction in a heating climate is to the outside). So we have to use a crinkly plastic housewrap (with all the inherent problems of yet another petrochemcial product that's vulnerable to natural surfactants and can trap moisture on the back side - or we have to use a rainscreen.
We want to use the housewrap as an air barrier also but the tapes don't stick or seal well to it and it's going to leak when we penetrate it with hundreds of siding nails, and that leaves the problem of sealing the window flanges to the WRB as well. The rainscreen makes it even more difficult to create a continuous drainage plane because the window flanges are proud of the WRB, and that makes flashing more problematic as well.
Of course the primary reason for an air barrier is to prevent inside warm, moist air from getting into the thermal envelope, so an interior air barrier is much more sensible, but we might have to consider that in the design and framing stages since it doesn't work well as an afterthought or when several different subcontractors are involved and no one is coordinating the work from start to finish.
And we've forgotten that the more breatheable the exterior sheathing and cladding the more resilient the house is to moisture flows, and the more the materials resemble real wood (rather than processed wood materials) the more resilient it is as well. And we've forgotten that the ancient technique of shingling layers allows gravity drainage of liquid water, which makes lapped layers of 3' wide felt the best WRB and the one that is the prescriptive standard in all building codes.
And, with a lapped WRB and the windows in the plane of the sheathing, there is no need for tapes of any kind, just good old fashioned felt gasket strips as AAMA window installation method A demonstrates.
Some builders will go so far as using expensive crinkle wrap and topping it with inexpensive and reliable felt because they don't trust the high-tech product. And they'll try all the high-tech tapes, even though a taped joint is not likely to be durable and reliable over the life of a house, while gravity can always be relied upon to drain water downward.
What's the world coming to? Aiming for green but getting browner day by day!
"And, with a lapped WRB and the windows in the plane of the sheathing, there is no need for tapes of any kind, just good old fashioned felt gasket strips as AAMA window installation method A demonstrates."
Robert, could you describe this assembly in greater detail? Thanks
Robert,
Would you regard a properly-detailed combination of lapped WRB and pan flashing as equivalent to what you recommend - for those of us with inset windows?
Robert,
You have recommended Typar in past posts.
Why the change to felt?
Also, felt does nothing for hydrostatic pressure between the sheathing and exterior finish material.
Would you agree that the crinkled housewrap provides a better drainage plane?
Robert,
You have recommended Typar in past posts.
Why the change to felt?
Also, felt does nothing for hydrostatic pressure between the sheathing and exterior finish material.
Would you agree that the crinkled housewrap provides a better drainage plane?
ANSWERED BY BRETT MOYER - Nov 5 10
I prefer Typar to Tyvek, since it's moderate perm rating is far preferable to Tyvek's extremely high perm. 5 perm is all that is necessary for outward breathing and a higher perm WRB can allow too much inward vapor diffusion in warm or sunny weather.
But Paul Fissette and other building scientists prefer felt, not only because it is shingle-lapped for drainage but because it does not trap liquid water on the backside as all plastic WRBs do. Felt will allow liquid water to diffuse through over time, and hence dry in either direction.
Robert,
Would you regard a properly-detailed combination of lapped WRB and pan flashing as equivalent to what you recommend - for those of us with inset windows?
ANSWERED BY INTERESTED ONLOOKER - Nov 5 10
With "innie" windows, pan flashing is a necessity. Though it has now become required by code and AAMA standards, I do not believe pan flashings are necessary with "outie" windows as long as there is a good drainage path and the thermal and structural envelope are made from durable and water tolerant, breatheable materials (such as real wood and cellulose insulation).
"And, with a lapped WRB and the windows in the plane of the sheathing, there is no need for tapes of any kind, just good old fashioned felt gasket strips as AAMA window installation method A demonstrates."
Robert, could you describe this assembly in greater detail? Thanks
When windows came with attached brickmold or flat exterior casing, it was conventional to install felt gasket strips, from the bottom up, under the sill, the side casings and the head casing. Then the field felt was installed to lap over the gaskets protruding beyond the casings. These gaskets both created an air and water seal between the casing and the sheathing and also a lapped drainage path should any water get past the siding/trim interface.
With shingle-lapped WRB, there is no need to cut that annoying flap in the WRB above each door or window, which then has to be sealed with tape (which typically fails in pressure tests).
The accepted procedures for installing finned windows is similar (though caulk and sill pans are now considered necessary, except by me):
http://pro.milgard.com/_doc/products/installation/pdf/aama-2400-02.pdf
Brett,
I failed to respond to your last two questions.
There should be no hydrostatic pressure between siding and sheathing. There is always the potential for leakage or wind-driven moisture, but that is what a WRB is designed to respond to. Felt works very well for this, except when rigid foam interrupts the natural movement of moisture.
In that case, crinkle wrap may be an improvement - but the problem is not with the felt (which is still the code standard against which all other WRBs are measured), the problem is the exterior foam.
So, as I explained before, the expensive crinkle wrap is a "solution" only to an unnecessary problem. And this is mostly how we build today: creating unintended consequences and then purchasing additional "solutions" which almost always create another set of unintended consequences. I guess it keeps the economy going (such as it is), but it doesn't seem like an intelligent way to build and certainly not a sustainable way to build.
As you have pointed out in past posts, foam board does not belong in above grade applications-- In new construction, I whole heartedly agree with this.
However, logic tells me that with certain exterior finishes (think Hardie board and batten), the finish material is fastened tightly against the sheathing, and the drainage path is interrupted. This is why I specify Tyvek Drainwrap behind basically all finishes. Behind cedar, stucco, stone veneer, I specify the Drainwrap and another layer of felt as a bond break.
I agree that felt allows for drying in both directions. It can also "wick" moisture away from the sheathing which is a good thing. Two layers of felt is a decent WRB. Felt just tears easy, has a low exposure time, and is difficult to integrate with flashing/tapes.