Re-siding with Hardi over foam board?


Old 12-01-14, 02:24 PM
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Re-siding with Hardi over foam board?

My house is a simple 25' x 50' box that I'd like to reside at the same time we do a "bump-out" of a portion of the front wall. Right now the exterior cladding consists of vinyl, which, as far as I can tell is laid over a thin (maybe 1/2"?) layer of styrofoam (the white crumbly stuff) board, some sort of Homasote type fiberboard material, tar paper, and exterior plywood. The vinyl, styrofoam, and Homasote layer is about one inch thick I'd say.

I would like to strip the house down to the sheathing and start over completely, first by airsealing with foam around window and door openings, and any other cracks and crevices I can find, then Tyvek with taped seams, then 1" foam board with taped seams. I'd love to go with 2" foam board, but I'm afraid the sides of the house are going to foul me up. Unlike some homes, there is no eave on the side of the house, only front and back. The shingles extend maybe an inch or inch and a half over the rake board, which is probably 3/4", so the shingles sit maybe 1 and 3/4" proud of the sheathing. The way I see it, I have two options:

Do 1" foam board/Hardi-Plank all around, OR, 1" and Hardi on the sides and 2" on the front and rear. Would the extra R-value on 2 of 4 walls be worth it, or negated by the fact that the two sidewalls would be less?
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Old 12-01-14, 08:34 PM
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You probably should not install hardi over anything thicker than 1". Even then you will need to be very careful when nailing. The siding will need to be nailed to studs.

Using foam that has a high compression rating will be wise, otherwise your siding may want to wave in and out as you nail. It is often difficult to set the siding nails just right when going over foam.
Old 12-04-14, 11:30 PM
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Having worked on only 2 small Hardiplank installations, I'm speaking purely as an amateur--but I would go with just 1" of foam all around. I always had a dickens of a time getting the nails set "just right" (either too loose or too tight), and that was over Tyvec and plywood. I can't imagine the headaches trying to attach it over thick foam insulation.

For more bang for your buck, I'd suggest adding insulation in your attic. I blew cellulose and fiberglas into two NM homes we owned, and both experienced heating and cooling bills almost cut in half. The gas company at one of them changed the gas meter twice during the first winter, because they thought the meters were faulty (bill went from almost $200 a month to consistently less than $100 a month).
Old 12-05-14, 06:49 AM
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Thanks for the replies. I'm one step ahead on the cellulose-that was already planned. I'll be pulling up some of the old fiberglass batts, airsealing extensively underneath, putting them back, and then blowing cellulose on top for an R-50/R-60 attic. I really need it more for summer A/C performance than winter. We heat our home primarily with a wood furnace with oil as a backup. The wood furnace typically produces more heat than our small home needs so we end up cracking windows on 20F days sometimes. In the summer, however, the solar heat load on home through the attic is enormous. The back of the house faces directly south with very minimal tree cover to shade the roof. Attic fans help, but a thick blanket of insulation would be even better.
Old 12-05-14, 06:57 AM
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Do yourself a big favor, buy a real pneumatic siding nailer.
It shots coiled, small gauge, long ring shank nails.
They will come out flush with the siding every time with no over nailing.
I also use it for lots of other jobs like outside trim.

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