Insulated sheathing with hardiboard
harvester
| Posted in Construction Techniques on
Hi all, I am new to your discussion group
I am building a new home in the Hudson Valley of New York state, zone 5, and I am debating the use of insulated sheathing (extruded polystyrene) vs. the more conventional approach of sheathing with plywood. I am planning on siding the house with hardiboard and am concerned with long nails holding the somewhat heavy siding on should I use an inch or more of styrofoam. I have more questions but lets start there…
Replies
Welcome to the nut house. Good move on your part to start off telling us where you live. You'll get better answers that way.
Check with your local building permit office. You might be required to use plywood/osb at the corners for strength.
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
I would use plywood. Foamboard just doesn't make for a very solid structure, even with diagonal bracing or plywood corners.
You're not going to get all that much better r-value with foamboard. You'll do much better to make sure you are putting the most energy efficient windows and doors that you can.
Quote: "Foamboard just doesn't make for a very solid structure, even with diagonal bracing or plywood corners."
How strong do we have to make our homes? I feel they're overbuilt in the wood framing presently. Two years ago in Atlantic Canada, my 18 (now) year old, 2 storey house with full basement withstood a cat 2 hurricane. We were within 15 miles of the eye. The house is built all wrong according to many but survives well. It's not the house I'd build for myself but the price was very decent (distressed sale)
Fault #1: Its got a perfect vapour barrier on the exterior (sealed double foil faced polyurethane foam board sheathing just under the 8" exposure cedar clapboard).
Fault #2: The wall is 2x4 @ 16 with inlet 1x4 corner bracing....no wood panel or board sheathing. There were 2 small stress cracks that have been at bearing points supporting glu-lam beams since we bought; these are also on either side of double flue 26 ft fireplace/chimney with lots of wieght- so these would almost be expected.
-Keep working on the foam sheathing idea. Consider using the rainscreen principle of vertical strapping over the foam at studding. This will make the siding attchment easier also.
-Buy better windows- at least Low E/argon in the sealed glass panes. Price out double Low E/krypton with a warm edge spacer (center R of 8)
-Consider a bit of passive solar aspect in your design. It can supply up 30% of your heat just by having some larger windows on the southern exposures.
-Air seal to 2-3 air changes per hour at 50 pascals pressure
-Buy a better heat recovery ventilator with more options/controls such as timed exchange periods with recirculation periods, at rest periods with timed exchange, programmable timer
-Blow R50 cellulose in the attic.
Check the builder's guidelines for the Lung Association's "Health House" at: http://www.healthhouse.org .
Thanks for the input everyone...
As for windows I will purchase double pane, low e argon filled but my site is a very poor one with regard to passive solar heat gain. I have a hill to the south and tall trees, but fantastic views to the north. Designing the house myself, with the site and views formost in mind, I did design quite a bit of glazing to the north. But I do care about energy savings as well as maximizing my heating dollars. So...What to do...
According to the res check software I used, 40% of the houses heat loss will be through the windows and doors. 32% of the heat loss will be through the houses stud walls. If I add 1 inch of polystyrene at r5 I will only lose 27% of the houses heat through the walls. It seems like a small savings compared to the amount of heat I am going to lose through the windows.
I am going to try to air seal the construction as well as I can as I build and I am using radiant heat throughout with a 95% efficient water heater.
I guess I am leaning toward plywood sheathing but another concern of mine is moisture in the wall assembly. One potential advantage of insulated sheathing is the fact that it can raise the temperature of the first condensing surface (the interior side of the sheating) to above the temperature at which the moisture in the wall will condense. I'm learning about this from "the builders guide to cold climates" but am still not sure if in reality it would work. If the air seal is not perfect, then cold air would still get behind the sheating and cause moisture to condense. Now since the polystyrene is a better moisture barrier it could hold moisture in and cause more problems than a more moisture open assembly.
More confused than ever...
People don't seem to be seeing/understanding what's been going on in the "Building America" program. The houses are saving about 50% of the energy costs and the cost of the houses are about the same as regular housing.
How is this done?
-substitution of foam board for low R value material (sheathing); lower heat loss + savings = 5%; cost= usually a tradeoff = free savings
-Reduce unnecessary framing; less wood >>>more insulation= lower heat loss; cost= less wood/labour cost + more HVAC savings = bigger savings
-Use advanced airsealing techniques; house gets tighter = lower heat loss; cost= less labour cost +more HVAC savings = bigger savings
All the "lower heat loss" factors convert to a smaller HVAC system needed= less cost >>>>>so lets add another R10 to the attic (R40 -> R50) or bump up the windows to R6-8.
This is why there are now energy mortgages that let you borrow a bit more money as the mortgagor knows your energy costs will be lower therefore you can afford a bit bigger mortgage payment. You have the same cash flow each month but more of it is going to pay for the extras (more equity) in your home rather than the energy utliity (where you gain no equity). In the future, when you show puchasers of your home the low energy bills, it will be a big selling point!!!
If you're reading Joe Lstiburek's book, follow it. I first met him in 1985 at a large building conference titled "Avoiding Failures; Learning from Experience" and since have been following his developments and popularizing of building science. The last Gov't dept I worked for used to bring him in to speak every 2 years since 1984. This guy is the real thing, leading the field and garnering praise from such groups as the Energy-Effcient Buildings Association and then American Lung Association where co-chairs the Technical Committee for their Health House program.
He thinks "outside the box". For example, a large computer corporation had a building in Hartford that had winter condensation in walls due to higher humidity levels to reduce static electricity. The architects had a price (the cost of construction in Heaven) of about 5 million for a fix. Joe's fix -$450,000- he pressurized the walls slightly so moist air would not leak into them!!! The architects wanted to dismantle the whole wall and re-build that which they designed poorly.
Thousands of homes have been built with low perm exterior foam sheathings with no problems. They're safer for prevention of condensation than either ply or OSB which both have vapour barrier/retarder qualities due to the glues used. Don't worry about little air leaks; it's the big wet ones that cause the problems. If you have a properly maintained Heat Recovery Ventilation system, the air that leaks will be quite dry anyways. We don't need perfection in all these house systems, just good work and understanding of what they are designed to do!!
Thanks for all the advice!
I still am unsure what to do with the siding issue. In Lstiburek's book most references are to aluminum and vinyl siding. He does have some figures that show how he recommends detailing wood siding using a furring strips of 3/8" pressure treated plywood, wood lath, or 1x4. Should I just assume that I should use the 1x so that the nails have plenty of strength when holding up the hardiboard? Also should I screw the furring strips through the foam to the studs to prevent the failure of nails?
What other complications will arise by having thicker walls? How much work will it be to make custom extension jambs for all the windows and doors in the house?
Also should I screw the furring strips through the foam to the studs to prevent the failure of nails?I believe if you used screws you would have more of a chance of sucking the furring strips into the foam, I think using a nail gun would be the "only" way to do it, holding the nails back a few inches from the end to keep from sucking theends in. And the wider the strips the better to spread the surfacearea out.
Edited 11/22/2005 8:10 am ET by butch
A couple more things, who supplies these very high r windows? I am currently planning on andersen 400 series with permanent grills on both sides of the glass. u=.33 or so.
What specific makes and models of heat recovery ventilators and controls do you recommend?