Fiber Cement over old T1-11

[simons]

I have a client who wants me to install fiber cement over his existing T1-11. I'm concerned about rot issues that would come with this. I guess another contractor suggested just cutting the bottom off where its starting to swell and replacing that portion them just cover up. Is this a good Idea. Is there typically already a vapor barrier behind the T1-11, and would putting another layer of house wrap cause problems. Any advice is much appreciated.

[Dick Seibert]

Jonny:

T1-11 was originally approved without a WRB behind it, only 4" strips were required behind the joints, better open it up and find out what's behind there.

[Riversong]

I have a client who wants me to install fiber cement over his existing T1-11. I'm concerned about rot issues that would come with this.

Why? T-111 is no different than any plywood sheathing, except it typically has vertical grooves for better drainage.

I guess another contractor suggested just cutting the bottom off where its starting to swell and replacing that portion them just cover up. Is this a good Idea.

Of course, remove any swollen or rotted section and replace with CDX.

Is there typically already a vapor barrier behind the T1-11, and would putting another layer of house wrap cause problems.

If there's a vapor barrier, it would be on the inside behind the drywall. There won't be anything behind the T-111, so install housewrap over it and then siding.

Multiple layers of housewrap don't cause problems, since they are vapor permeable.

[David Meiland]

If there's a vapor barrier, it would be on the inside behind the drywall. There won't be anything behind the T-111, so install housewrap over it and then siding.

I have a couple of customers with T1-11 siding. In both cases there is no sheathing, but there is Tyvek over the studs. Seems like you'd have a very drafty house without it... not that you have a perfectly tight house with it, of course.

[davenorthup]

Yep - I got the OK from a manufacturer to double up WRB's before - no issues. Might look at a rain screen or at the very least a drain wrap.

[gburnet]

We've successfully gone over T-1-11 with other lap sidings in the past. I approach the job as if the T-1-11 is bare wall sheathing: I verify that its condition is good enough to hold nails & replace any questionable areas with new plywood. Then I repair or replace any damaged or missing flashings & install an appropriate WRB over the walls. Once this work is done, install your siding.

BTW - vapor barriers don't belong on the exterior of buildings in cold climates, as Robert has pointed out. Were you confusing the term with a weather resistant barrier (WRB) such as housewrap or felt paper?

[JCadorin]

I concur with gburnet. In numerous occurances, we have installed cement lap siding with success. We always remove the rotten and questionable areas.

In our area, a building permit is required with an inspection prior to house wrap(yes we always install new house wrap over the old siding). The inspector checks that the substrate being covered is not rotten and is structurally sound.

Experience has taught us to look for waves and other inconsistancies in the wall, especially on long walls. They get magnified when looking at the wall with new horizontal lines (lap siding). Shim them out if possible, similar to how a drywall hanger shims his boards out. But use felt paper shims instead of cardboard.

[Mark Parlee]

Jonny

no more to add than what has been said; Go Forward

[S.Joisey]

I'll second the notion of taking a look before you proceed. I've seen a number of different T1-11 installations. No wrb, felt over the studs behind the T1-11, a layer of celotex (fiberboard with a felt paper-like face) behind the T1-11. You get the idea. Best to look first.

[simons]

Thank you for all your replies. It's good to know that you can double up on the WRB's (yes I was confusing terms).
Thank you also for the tip on checking for wavy walls, I don't imagine it was fun learning that one the hard way.

[Dick Seibert]

I wouldn't double up WRBs without checking first, people confuse water permeable, air permeable, and hydrostatic pressure permeable. The big factor is the water holdout time, if you've got a plastic wrap behind there no problem, if you've got asphalt impregnated felt paper or certain Kraft papers there could be a problem trapping water between the WRBs. check the attachment below, should you have a Two Ply super Jumbo Tex 60 Minute WRB you've got a 150-Minute Water Holdout time with a Vapor Permeability of 10 perms that will trap water, if you've got a 15 # asphalt felt you've got 20 minutes, a plastic wrap only 10 minutes with 50 perms, and a perforated plastic wrap virtually no water holdout or permeability. It won't take long to check and see what you have.

[Riversong]

I wouldn't double up WRBs without checking first, people confuse water permeable, air permeable, and hydrostatic pressure permeable.

Since WRBs, by definition, are water impermeable and air impermeable, and not hydrostatic pressure impermeable, there should be no confusion (at least this side of the left coast).

The big factor is the water holdout time

No, the only pertinent factor is water vapor permeability.

The building papers in your chart are for stucco cladding, are not applicable to other sidings, and are not what the IRC refers to as a WRB (which is #15 felt or equivalent). And even with stucco, it is recommended to install a double layer of grade D paper.

[Dick Seibert]

Jonny is located in Washington State, in all probability the building he is talking about was built under the UBC (the IRC didn't come out until 2000 and I don't know what date after that Washington adopted it), the UBC allowed either No. 15 asphalt rag felt or approved kraft paper, UBC §1402.1 in the 1997 UBC stated: "Such weather resistive barrier shall be equal to that provided for in UBC Standard 14-1 for kraft waterproof building paper or asphalt saturated rag felt." . The only applicable test at that time under the UBC was the "Boat Test" (water holdout) ASTM D779 based on Federal Specification UU-P-31b (incorporated into UU-B-790a). The Hydrostatic pressure Test specifically designed so polymeric materials could pass wasn't devised until later (they never were able to pass the Boat Test).

The fact is that if he has a WRB approved under the UBC it would comply with the Boat Test, unless the AHJ specifically approved a polymeric material under the alternate methods and materials code section, so the applicable standard would not be vapor permeability but water holdout. depending upon the age of the building there is a possibility that it has a plastic wrap, that's why I suggested that he check and see first, as I've said before T1-11 was originally approved with no WRB, just 4" strips behind the joints, if that's the case he has no problem, but that approved installation was not in the UBC and would had to have been approved by the AHJ under the alternate materials and methods section.

[Riversong]

The only applicable test at that time under the UBC was the "Boat Test" (water holdout)

Which was a meaningless test, since WRBs are not intended to float on water without absorption, but to allow liquid water to drain vertically and vaporous water to permeate.

The Hydrostatic pressure Test specifically designed so polymeric materials could pass wasn't devised until later (they never were able to pass the Boat Test).

That test was adopted because it was more relevant to a weather barrier intended to resist wind-driven rain.

so the applicable standard would not be vapor permeability but water holdout.

The issue here was whether a double WRB would create a moisture problem, so the only relevant quality is permeance - whether the wall system can still breathe and dry.

[Dick Seibert]

Robert:

The function or a WRB is, as it's name implies, to make the structure weather resistant, that means to hold water out as long as possible while still allowing the structure to breathe. Water resistance and vapor permeability are two different issues, issues argued about ad infinitem by the differing industries manufacturing the different products.

Here is a good ASTM article on it by Tom Butt, FAIA, and a presentation by Tom.

California refused to approve plastic wraps for may years because they didn't hold-out water, here is a quote from one of DuPont's desperate attempts to get California to approve Tyvek:

As a result, they are equivalent to a Grade D building paper having a 60-minute waterresistance rating as described in UBC Standard 14-1 Typical Grade D building paper has a minimum perm rating of 5. The Commission requirement for an air retarding wrap with a higher perm rating than required of building paper, which has been used successfully for years in North America, is unnecessary restrictive. It prevents air retarding wraps which meet ASTM E1677-95, Standard Specification for an Air Retarder (AR) Material or System for Low-Rise Framed Building Walls but have perm ratings of between 5-10 from claiming the default reduction in Specific Leakage Area (SLA) of 0.50 without diagnostic testing. There is no basis in building science to keep this value at 10 perms.¹

Here a response of your fellow Vermonter, Martin Holladay, to a question on these fora in October of 2005:

In the building science community, "permeance" refers to "water vapor permeance," not some other kind of permeance. Permenace is different from permeability. (The moisture transmission rate of a material is referred to as its permeability, stated in perm inches. This number is not dependent on the materials' thickness. Its permeance, however, is dependent on thickness, much like the R-value in heat transmission. Dividing the permeability of a material by its thickness gives the materials' permeance, stated in perms. Permeance is the number that should be used to compare various products in regard to moisture transmission resistance.)

ASTM has established testing procedures to determine a material's water vapor permeance, and the values obtained from of certain types of ASTM test procedures are used to compare the water vapor permeance of different materials. For example, a vapor retarder is defined by the 2000 IRC as a material having a permeance rating of 1.0 or less when tested in accordance with ASTM E96.

A material's permeance (that is, what you call its "perm rating") has nothing to do with its resistance to air or liquid water. Unpainted drywall is fairly vapor permeable -- it has a permeance of 50 perms -- but it is an excellent air barrier. The best air barriers are rigid and durable, and have edge or seam details that maintain the integrity of the air barrier. Good air barriers include gypsum wallboard, concrete, metal flashing, and glass.

I am less familiar with testing procedures to determine resistance to liquid water than those to determine permeance. The good thing about testing resistance to liquid water, though, is anyone can do it in their backyard or on the job site. Want to test a window assembly for reistance to liquid water? Use a garden hose. As you probably know, brick veneer is a fairly good air barrier, but a lousy barrier to liquid water. Testing procedures to determine resistance to liquid water can be misleading because of the "tent effect." (You know, you are camping in a tent without a rain fly; it's raining; the tent is waterproof until you touch it. Now it starts to leak.) Some plastic housewraps resist liquid water if nothing is touching them. But if they are pinched between other materials, they leak readily.

This whole area of water barriers, air barriers, and vapor diffusion barrers got confusing because many builders, including me, were led astray during the 1970s, when a lot of misinformation about vapor diffusion was spread. "You've got to have a vapor barrier to keep moisture out of your wall." As it turns out, vapor diffusion plays a very minor role in most wall failures. Far more important are air transport (because that's how most interior moisture gets into walls -- by piggybacking on exfiltrating air) and plain old-fashioned rain penetration. It's much more important to keep out rain and to establish an air barrier than it is to control diffusion.²

¹ http://www.energy.ca.gov/title24/200...em_various.pdf
² http://forums.jlconline.com/forums/a...p/t-28288.html