Building felt vs. house wrap – white cedar sidewall
GBA Editor
| Posted in Green Building Techniques on
We have only been specifying building felt as the WRB under cedar shingles. Several years ago there were issues with some of the house wraps disintegrating after being behind cedar sidewall for a few years. Presumably had something to do with the moisture held by the cedar and also certain oils in the cedar.
Does anyone know if this issue has been completely resolved? Any particular house wraps work more effectively than others?
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Has anyone seen a workable detail for exterior rigid insulation with white cedar shingle siding? The only option I have come across is to sheath over the rigid w/ 1/2" plywood? I'm hoping to find a more economical solution, but assuming there may not be...
I've never seen any reliable evidence that polymeric housewraps disintegrated due to cedar extractives or any other cause other than UV light. When Tyvek was first introduced to the market, it was not UV stabilized and it was discovered that as little as three days exposure to direct sunlight began a process of material breakdown that continued even after it was covered.
However, it's true that any type of surfactant, including the tannins from cedar or detergents used in powerwashing before repainting, will increase the water porosity of plastic housewraps. Felt does not suffer from the surfactant effect and outperforms most housewraps in several other ways as well.
We've seen weakened house wrap under cedar shingles and lap siding so we've gone to wrapping and taping our homes and then putting up a layer of tar paper between the shingles and the house wrap (same for Hardi and Nichiha) it's not that expensive to do and feels like good insurance plus if you have any breaks in the shingles that line up you see a black line rather than a white one and it feels like good insurance. We tell the customers "we're already fixing problems other builders haven't hear about yet". (its all marketing and marketing is theater) With the wrinkle wrap Tyvek it helps keep the drainage plane from getting dammed by the siding too. Much cheaper than that cedar breather stuff.
Tyvek is the only brand of housewrap that has surfectant claims against it. I have not heard of Typar being affected.
We used Tyvek's "crinkle wrap" on a couple of cedar siding projects as a cost-saving alternative to Obdyke's Homeslicker, but we were only comfortable with it because we used the ZIP system sheathing under the Tyvek. We've also used the felt paper-over-housewrap scheme that Michael Chandler uses. We did it because the housewrap had been exposed for too long but I like the idea of turning it into a marketing opportunity.
Housewrap can be much easier than building felt to install, depending on the project, so we look at each project an determine which product is most appropriate. I generally ask for Homeslicker pre-adhered to Typar for cedar shingle sidewalls. It costs more than other options but it works and is well-accepted by the building community.
Paul Fisette, UMASS Amherst Building Technology Dept, tested several housewraps and found that all the plastic materials, particularly the perforated ones, passed more water when it was wetted with surfactants and cedar extractives. He found that the 2nd generation Typar (non-perforated) was effected less than Tyvek and R-Wrap, and #15 felt was not effected at all.
Surfactants don't change the properties of housewraps, they change the properties of water - effectively making water wetter by reducing surface tension and increasing filming and absorption.
Robert, do you have a source for Paul Fisette's study other than this: http://bct.nrc.umass.edu/index.php/publications/by-title/housewraps-felt-paper-and-weather-penetration-barriers/.?
That study is the one that has been repeated and summarized several times, including here http://www.finehomebuilding.com/how-to/articles/making-sense-of-housewraps.aspx.
Unless I'm missing something, it seems that initially Fisette only tested Tyvek, R-Wrap, felt for resistance to surfactants. Later, testing Typar as you mentioned, he found it to have "superior resistance" compared to Tyvek.
It's interesting to note that while surfactants did not affect felt, it still performed much worse than Tyvek in the water resistance tests.
Much worse? That wasn't Fisette's conclusion. In fact, he's repeatedly stated that he would still choose felt for his own house.
What that article did report was that, even in the very demanding AATCC 127 hydro-head test, in which the material is subjected to a 22-inch column of water – the same force exerted by a 200-mph wind – and must not leak a drop for 5 hours..."some researchers claim that felt has also passed, though inconsistently."
His lab testing showed some leakage through felt with the equivalent water pressure of a continuous 70 mph wind, but also that the results were inconsistent and "felt often held water for 30 minutes or more before leaking." A half hour of a 70 mph gale with no siding as the first line of defense is a pretty demanding test, which the felt often passed.
In a 2000 article (http://www.umass.edu/bmatwt/publications/articles/leaky_housewraps.html), in which Fisette reported on testing the same WRBs sandwiched against a piece of water-saturated cedar siding with blotter paper and plywood on top, he reported that "Tyvek, Rwrap and 15-pound felt showed no signs of liquid water leakage throughout the 2-day test period. However, the blotter paper on the Tyvek and Rwrap samples felt slightly damp to the touch by the end of the period, suggesting that there had been some transfer of water vapor by diffusion. Blotter paper disks on the 15-pound felt paper samples remained bone dry to the touch."
He also concluded that "I think that felt paper blocks the flow of liquid water under pressure (column) for a respectable length of time. When it leaks under pressure, it seams to leak very slowly. It blocks capillary flow for days. Felt is inexpensive. What is also appealing is that felt is forgiving. If water gets on the wrong side of a felt-wrapped wall, the felt can absorb the water and over time allow drying to the outside of the structure. Plastic housewraps don't move water this way. Plastic wraps are non absorbent. They rely on vapor diffusion to move water that gets on the wrong side of the wrap. Diffusion is a slow and weak force."
What's sold as #15 felt today can be any of three products. Unrated felt can vary from 7.6 – 8.8 lbs/square saturated weight. ASTM D4869 #15 felt can be 8.0 – 9.7 lbs/square. And ASTM D226 #15 felt must be 11.5 – 12.5 lbs/square (which is the code standard for WRBs).
Asphalt-saturated rag felt has proven itself effective as a weather barrier since 1844. It remains to be seen how long plastics - which invariably lose their plasticizers over time - will remain protective hidden within our walls.
How does #15 felt perform as an air barrier as compared to plastic housewraps?
Lucas,
That depends on what you mean by "air barrier". If you mean the ability to resist wind-driven rain, then felt performs well. That's why the IRC (and earlier codes) uses #15 felt as the standard against which all other WRBs must be measured. But if you mean the very strict standards of the Air Barrier Association of America (< 0.004 CFM/SF @ 75 Pascals), then no it doesn't pass. But neither does EPS or unpainted concrete block or brick walls.
To perform as an effective air barrier, a material or assembly must have these qualities:
low air permeance
continuous, contiguous
inflexible
durable
And must be able to:
resist positive & negative pressures
transfer all wind loads to structure
resist displacement under pressure
be flexible in joints between dissimilar materials
resist seismic, thermal, swelling, shrinkage and creep movements
seal at all penetrations
separate spaces with differing temperature or humidity regimes
be continuous along all 6 sides of enclosure
But polymeric housewraps don't all meet even the minimum air resistance standard, either. None of the perforated housewraps do, and the non-perforated olefin materials barely do. And that also depends on the ability of the taped seams to hold over time and under pressure differentials and movement. Water pressure tests often show the taped seams to be the weak point.
Felt is advertised and sold as a roofing underlayment (that's been its function for more than 150 years). But builders have also relied on it as a siding and flooring substrate. The first plastic housewraps were advertised as energy-saving air barriers, but then marketed as WRBs, since that's what the codes required.
Best practice is to use the WRB for its intended purpose - as a secondary weather barrier and drainage plane - and to use more solid materials (such as plywood or drywall) as the air barrier. The ABAA uses drywall as the standard against which all other air barriers are measured.
Robert, in Fisette's 1998 tests using a 3.5" column of clean water, Tyvek showed no water leakage while felt lost 30% of its water on average. Using soapy water, Tyvek lost 10% of its water column while felt lost 30%. Using cedar extractives, Tyvek lost 3% of its water column while felt once again lost 30%. Those were the water resistance tests I was referring to.
In those tests, Fisette did find that felt often held water for 30 minutes before leaking, and he prefers its hygroscopic qualities to the non-absorbent plastic wraps. I'm a fan of felt paper myself, and the follow-up study you linked to probably does reflect real-world situations more accurately. But in testing water resistance, using a 3.5" water column to simulate 70mph wind-driven rain, you can't tell me that 30% leakage is better than 3% or even 10% leakage.
Robert,
Thanks very much for your thourough reply. I have been under the impression that it is important to encapsulate insulation within the walls. I assumed this meant that it is desirable to have an air-barrier installed on both sides of the wall assembly with plastic house wrap being the standard choice for this purpose.
Is my assumption incorrect? If the primary air-barrier exists on the interior side of the walI, is it sufficient for the WRB to act only as a secondary weather barrier and drainage plane (and not a secondary air-barrier)? In other words, will air infiltration (from the outside) through #15 felt compromise the effectiveness of the insulation?
Lucas,
The only insulation that really needs complete encapsulation is fiberglass (batts or loosefill), because it's so permeable to air. Any dense-packed fibrous or foam insulation is reasonably impervious to convection.
As long as there are no open air passages internal to the thermal envelope (plumbing & wiring holes, e.g.), then air cannot enter a space if it can't exit somewhere else. So an air barrier can be applied on either side of the thermal boundary.
Plywood and OSB and exterior gypsum sheathing or interior gypsum (if the seams are taped or otherwise sealed) creates an air barrier. Tape can be used on exterior sheathings, but I don't believe tape is a reliable long-term component to an air barrier assembly. Taped and spackled drywall seams, however, are long-term components of an air barrier assembly, and are visible for inspection and repair.
Robert,
I think you just answered my question as well with the above. If I understand correctly, I can drop the rock regarding my reliance on housewrap as my principal exterior air seal. Instead, my 1/2" cdx (and caulking/foaming of all rough-in air intrusions) is the principal exterior air barrier. I can then use either 15 lb felt or 60 min paper under my cedar planks and shingles as the WRB for my SFH Pac NW (which do you prefer?).
My air sealing efforts (for the purpose of low ach50) are better focused on a combination of Airtight Drywall Approach (using EPDM gaskets), simple caulk and seal, and...."kids, Close the Door!, what were you born in a barn?!." thank you
Frank,
"drop the rock"? Rock as in sheet rock? Pet rock? Hot rock?
"SFH Pac NW"? LOL, but acronyms and abbreviations don't often make for clear communication.
And I suspect you're referring to a wall assembly you've described elsewhere on this forum but didn't link to or re-describe so I (and others) can remember what you'e talking about.
Sorry, but I need more details and more clarity in order to understand what you're saying and what you're asking.
Robert
I'm sorry for the confusion - drop the rock is a term I use which to me means I can stop thinking about something and move on (probably inappropriate for this forum).
In 13 above, I'm referring to my Single Family Home project here in the Pacific Northwest where I am applying cedar shingles and planks to 1/2 " cdx exterior sheathing (standard framing, 2x6 walls). I was attracted to Typar or Wrapshield for their air barrier claims. But from your comments above, I gather I would be better served with 15 lb felt OR 60 min building paper for my weather barrier wrap along with caulking/sealing all rough-in air infiltration holes, and concentrate my air sealing efforts, for the purpose of low air changes per hour, to the interior with an airtight drywall and simple caulk and seal approach.
Thank you for the feedback and I think I'll leave my texting verbage for my phone,
Frank
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Frank,
I prefer to think of exterior layers as weather barrier layers, with the siding/cladding/flashing as primary and a secondary wind/rain barrier and drainage plane tightly pinned behind it or spaced only by the texture of the material. I think polymeric housewraps with taped seams work reasonably well as weather barriers unless they are behind the furring strips of a rainscreen which leaves them vulnerable to pressure differentials.
It's certainly easier to design a continuous air barrier on the exterior, as in REMOTE/PERSIST or the use of air/vapor barriers in the 1/3-2/3 double framing approach, but the primary air movement that must be stopped to prevent moisture damage within the structure is from moist heated indoor air moving from low to high through penetrations in the structural/thermal envelope, except in AC dominated climates in which infiltration or humid air to colder surfaces is problematic.
That's why I prefer the air-tight drywall approach (not the "simple" caulk and seal"), because it seals every framing assembly to the next and bridges the gaps with sealed drywall, which is easy to inspect and repair as necessary. While it may be difficult to reach PassivHaus standards (which I believe are excessive and unhealthy) with this approach, it's not difficult to achieve 2 ACH50 or less which is more than tight enough for an efficient and healthy house. But it does require an attention to detail and sometimes a sufficiently unconventional construction sequencing that makes most builders reluctant to use it.
Robert,
Could you let us know where we might learn more about the "air-tight drywall" approach? What trade typically handles this type of detailing?
One of the best descriptions is from the man who invented it, Joe Lstiburek, now of BuildingScience.com:
http://www.buildingscience.com/documents/information-sheets/4-air-barriers/air-barriers2014airtight-drywall-approach/
There is no "trade" which is solely responsible for the many details of this system, so you need a competent designer/builder who has experience with ADA and can orchestrate the various trades to make this happen.
Not sure if I can find the document now but 10 years or so I got an installation manual from one of the cedar shake manufacturers and it instructed us to install a layer of 30# felt on each course as we installed the shakes. We've always installed a layer of 15# felt at the framing stage and would then cut the 30# felt into 12" strips or so and install it over the upper half of each course as we installed it. The top course would of course overlap the lower course.
Danny,
That's the traditional method of installing cedar shingle or shake roofing on skip sheathing, which allows the roof to breathe and sheds water well. Skip sheathing (strapping) is not amenable to continuous underlayment.
On a sidewall installation with cedar shingles (sawn, not split like shakes) over solid sheathing, interwoven felt is unnecessary as long as joints are offset at least one inch laterally. A continuous felt (not plastic) WRB is all that's required.
I found this conversation useful. I have a question regarding what other peoples experience is with Jumbo Tex (asphalt impregnated kraft paper) vs. 15# felt. The Jumbo Tex is much less likely to tear and I find it easier to apply. Any thoughts?