Siding over existing T1-11


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Old 07-07-09, 05:56 PM
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Siding over existing T1-11

There is a 17-year old 2-story wall on my house where the plywood siding seems to be having trouble holding paint the paint is starting to show vertical cracks after 1 year. This is under a gable end that gets sheeting rain and big sun exposure. I'm thinking that rather than paint it again and before it really starts to deteriorate bad I might consider options for installing something over it like vinyl lap siding. The plywood is installed over builder's paper directly over the wall studs (it's both sheathing and sidiing which meets code here in S. Florida).

Does the existing siding need to be removed or can it serve the purpose of sheathing under new siding? If new siding can go over it, would a new layer of builder's felt go between the old and new siding?
 
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Old 07-07-09, 06:36 PM
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Are you planning on just doing from the drip line to the gable only, or the entire house. The only issue you may have is the depth of the door and window trim. If brick mold was use, you should be just fine in using siding to do the entire house without removing the T1-11. IMO, a second vapor barrier will increase the moisture holding of the T1-11 should any get back there. I would opt to let it breathe. You could install Tyvek, but not building felt, as Tyvek breathes.
 
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Old 07-07-09, 09:31 PM
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The siding is your shear sheathing, do not remove it.

I would use felt paper as any water past it could dry out, not so with Tyvek. Read Felt paper vs. Tyvek here: UMass Amherst: Building Materials and Wood Technology Housewraps, Felt Paper and Weather Penetration Barriers

Second choice would be Typar over Tyvek: http://www.typar.com/pdfs/Typar_vs_Tyvek.pdf

Be safe, G
 
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Old 07-08-09, 06:48 PM
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I have to disagree with using felt over and under the T1-11. It will never dry out should water get to it. Just thinking out loud.

Larry
 
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Old 07-08-09, 07:55 PM
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Since we are thinking out loud... if that is the case, Larry, why would anyone use felt paper on a roof under their shingles? But they do, and have for years. Felt is not a vapor retarder (perm rating of 5) it is semi-permeable, which, when you want something to dry out, is a good thing. So if felt gets wet, it will stop most of the water... but can absorb "some" of the water. The water that it does absorb would theoretically be able to dry out (either up, down, in, or out) since the perm rating of 5 means it does not trap water vapor like a vapor barrier (perm rating < 1) would.

Many would argue that Tyvek is a better choice at a reported 58 perms, meaning if water gets behind it, it will dry out much more quickly. So I understand your point. But even Tyvek is not foolproof... (See Mark Parlee's article in the 12/08 JLC... where cultured stone was installed tight to Tyvek with no drainage plane.)

But #30 felt has been used on buildings (roofs and walls) around here for over 100 yrs. Most of the ones I have worked on have no problems resulting from the use of felt. Not arguing, just thinking out loud.
 
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Old 07-08-09, 08:05 PM
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But shingles aren't rottable, like T1-11 is, and you don't put felt "under" your decking. My thoughts are, if you encapsulate the T1-11 under and over with felt, air can't get to it to dry it out quickly enough. And the permeability of felt is questionable.
 
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Old 07-08-09, 08:13 PM
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I guess we will have to agree to disagree. LOL I don't think the felt paper behind the T1-11 will hurt anything. But it would be something good for those umass people to do a study on. What is the effect of water on substrates that are encapsulated in felt paper?
 
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Old 07-09-09, 06:20 PM
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Hey, if they act fast, they could get stimulus money for it!!!! Everyone but us small businessmen, anyway.
 
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Old 07-09-09, 07:12 PM
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That is why I chose felt over Tyvek------Tyvek allows vapor only to pass from inside out, not liquid water, like in a tear from the staple. Yet felt gets saturated and absorbs the liquid, drying it to the outside. The years I've put Tyvek on, no matter how softly using the stapler, it tears at the fastener very easily.

The other thing against Tyvek is it's an air barrier, not letting the house breathe, as felt will. How many people actually use their whole-house fan as recommended, because of the air-tightness? Then we see rot and mold problems because most won't even run the bathroom fan for 20 minutes after a shower, let alone a house fan. People with allergies, respiratory difficulties, and colds that last a long time. I'd like to see a study on that as well as three layers of vapor retarders ( the siding has a perm. of .75).

Felt will seal around the siding nail at installation, Tyvek won't.
From the U-Mass.:
Thatís because I believe that under certain circumstances, felt outperforms housewrap. For example, an ice dam or roof leak may allow liquid water to get behind the felt or housewrap. Itís also possible for the sunís heat to drive water vapor through the housewrap from the outside, where it can condense on the sheathing. In either of these cases, you now have liquid water on the wrong side of the wrap. Under these conditions, the liquid water would be trapped by the housewrap, which is permeable only to water vapor. Felt, on the other hand, will absorb the water, and more quickly dry to the outside.

Wood is an absorbent material. It stores water. Since rain is sucked through butt-joints, seams and even upward past overlapping edges, it has access to the back surface. We usually paint the face of siding to reduce water absorption. But many builders leave the backside raw. You donít want to store water in a place that has direct contact with vapor permeable housewraps. The sunís heat can turn the stored liquid water into vapor. The vapor moves inward when the temperature of the siding face is warmer than the air behind the siding. And since housewraps are vapor permeable, they can allow vapor to pass into the building envelope from the outside. As the sun sets or moves to another side of the house, the temperature of the wall may drop below the dewpoint temperature, changing the vapor back to liquid. And guess what? The reconstituted liquid is on the wrong side of a water-resistant barrier! This set of conditions is suspected to have caused wet sheathing in several unusual cases.------- Granted, unusual conditions, yet logical. Tyvek sold everyone, including me, on their product when it first came out. And I used to tout it, but I have learned a lot more--- just read the comparison article I listed. The logic of it all is what sold me. Be safe, G
 
 

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