What to put under Hardy siding?

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Old 03-11-05, 10:48 AM
tsampsel
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What to put under Hardy siding?

I have a house on Galveston Bay (read humid). As part of a remodel job, the contractor replaced some of the 40 year old aluminum siding with Hardy board siding. The original wood siding underneath was in good shape. The contractor nailed up 1/2" thick or so foam panels before putting up the siding. We've now decided to do the rest of the house. We had another contractor out for a bid and he didn't recommend the foam panels. He said it could make the siding wavy. He recommended Tyvex house wrap. The original contractor (who was also bidding on the rest of the house) said Tyvex is used for dryer climates and not really recommended where we are. So who's right? What's the difference?
 
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Old 03-11-05, 11:55 AM
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Tyvek is a water/air barrier, the 4x8 foam is just for insulation.
 
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Old 03-11-05, 12:02 PM
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So Tyvek is a preferred way to go in humid climates? I'll also throw in that this is a real old house and there is no insulation in the walls.
 
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Old 03-11-05, 08:18 PM
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Tyvek is not a vapor barrier, so it would be ok to install on your exterior. 4x8 foam products would also work fine in my opinion, provided they are rigid. The Hardiplank web site comments:

"Hardiplank lap siding can also be installed over foam insulation up to 1" thick. Irregularities in framing, sheathing, and/or foam insulation can mirror through the finished application. "

Notice it does not single out foam insulation as the "culprit" of irregularities in the finished application. The foam will only follow the wall. If the wall is crooked, the foam will be too.

You're builder may be concerned that if the house has foam insulation on it, the nails would pull the siding in too tight. Since the Hardiplank website does not even mention this possibility, I would assume he is worried about nothing, or has had bad experience doing this in the past, with lighter types of foam board.
 
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Old 03-20-05, 03:24 PM
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I called a radio show handyman in Houston asking this question. He first asked why the contractor installed the foam. I didn't have an answer since I figured he knew what he was doing. ( He's a Gold member, if that really means anything). The radio guy said because Tyvek is a water/air barrier that it was recommended for homes on or near the water. He was also concerned that with potential high winds, the Hardie that's already been installed had the potential to blow off. I doubt it since it's nailed into the original wood siding. When you say it's not a vapor barrier, is that different than a water barrier?

Thanks for the info.
 
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Old 03-20-05, 05:21 PM
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Tyvek sheds liquid water. But it allows water vapor to pass through it, allowing your house to breathe. Thus, it is not a vapor barrier.

Regarding why your contractor installed the foam, it was almost certainly twofold: The primary reason was to provide a flat surface over your original wood siding so that the hardiplank could be nailed flat. Secondly, (and less importantly in Texas) to provide added insulation to the home, as Hellrazor mentioned previously.

Regarding your radio personality's assertion that the hardiplank could blow off if it's installed over foam, that's certainly a possibility. But it could also blow off even if no foam was installed, provided you have a strong enough wind. So that argument is kind of pointless.

When hardiplank is blind nailed (no exposed nails along the bottom siding edge), a tornado or hurricane might blow it off. But if that happens, you'll lose more than just your siding.
 
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Old 03-21-05, 08:34 AM
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Might I assume a breathing house is a good thing?
 
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Old 03-21-05, 02:08 PM
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Your house needs to breath just as much as you do. A totally air tight house would be a bad thing.
 
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Old 03-27-05, 08:36 AM
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An interesting document on the subject of weather resistive barriers can be found here.

It suggests spacing the siding out from the wall using 1/2" furring strips attached over the housewrap. This creates a thin cavity behind the siding to vent the outbound mousture from the wall. I talked to someone at JamesHardie about this, and he said that was the way he would build his house. He also suggested using siding vent strips at the top and bottom made by Cor-A-Vent (SV-3). So that's how I'm doing it.

My next question would be whether or not to paint/prime the sheathing before the housewrap goes on. On one hand it would help it resist moisture and termite damage from any leaks. On the other hand, it would make it harder for the wall to breathe. Any opinions?
 
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Old 03-27-05, 11:46 AM
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Regarding painting the sheathing, that sounds quite unusual, and I wonder what possible reason a person would have to do such a thing. This is not a common practice, nor is it recommended in any specifications I'm familiar with. The only thing I could think of is that in the humid south, air conditioned houses might get condensation on the sheathing "if" the sheathing temperature would be below the dew point. (which, in southern states, is often above 70F.) You'd have to keep your house pretty cold for this to be the case. - and, it's more likely this condensation would occur inside the wall, on the cold side of the sheathing, not on the exterior side of the sheathing, directly behind the building paper.

If a house is covered with building paper (such as tyvek) there should not be any water infiltration behind the building paper whatsoever, provided everything is installed properly- therefore, there should be no reason to paint the sheathing.

As I understand it, most latex paints are generally not vapor barriers either. One of the reasons modern latex paints are superior to alkyd oil based paints is that they allow moisture from within the walls to escape. So if you do paint your sheathing, unless you are using a moisture resistant oil or alkyd based paint, you should not have problems with trapping moisture.
 
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Old 03-27-05, 01:36 PM
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No idea, i've never heard of painting sheathing either.
 
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Old 03-28-05, 09:13 AM
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Thanks for the comments.

I'm aware that painting the sheathing is not regular practice among contractors. It was mentioned, oddly enough, on a DIY site that claimed "It can't hoit," and was an extra optional step a DIYer might take on his own project.

My everything-goes remodel of the lower story of an old two-story duplex was prompted by extensive termite damage (consumption?) of most of the studs and plates. The joists had already been sistered, so since I was replacing the rim joists and plates, I beamed everything up and replaced most of them. Since there are probably some drywood termites stiill lurking around, I'm using pressure-treated wood and anything else I can think of to discourage their migration into the new stuff -- like painted Douglas-fir sheathing. Even though I don't think termites like plywood very much.

Most of this damage was due to plumbing leaks and problems over the years, not weather penetration. So, no matter how tight you wrap the house, there's always the potential for accidents. I know it's probably overkill, but when you get p***ed off about something and you're Doing It Yourself, this is what can happen.
 
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Old 03-28-05, 01:41 PM
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You are right, its better to go overboard and do it 110% the first time. Might cost a little bit more now, but its cheaper then redoing something later.
 
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