insulated sheathing or plywood in zone 5
Hi all, I am new to your discussion group, I posted this in the construction area but then thought my best advice would come from the experts in energy, heating and insulation.
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…<!—-><!—-> <!—->
After some responses from members I continued….
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 foremost 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 sheathing) 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 sheathing 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… <!—-><!—->
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Replies
Greetings h,
This post, in response to your question, will bump the thread through the 'recent discussion' listing again.
Perhaps it will catch someones attention that can help you with advice.
Cheers
40,000 Americans are injured by toilets each year.
Hi! I'm in the same boat you are building a house and have gone through all the confusion trying to sort all this out. Don't give up! Ther is hope!
First, I suggest you go the Building Science Corp web site, http://www.buildingscience.com , and download their technical article "Vapor Barriers & Wall Design". In all my searching and reading about moisture control and condensation within wall assemblies, this finally allowed me to take real-world numbers (climate data, wall design, insulation levels, etc.) and put them all together with equations and say, "Yes, this will work!"
The above article will show you that leaving out the foam sheathing and opting for plywood or OSB sheathing works because the permeability of the ply/OSB allows any moisture to pass through the wall assembly. On the other hand, if you want foam sheathing, you can calculate the minimum foam thickness required to pevent water condensation on the back face of the sheathing under worst-case design conditions.
If you have concerns about nailing hardiboard on over thick styrofoam, you could always nail up vertical 1x lumber strips over the styrofoam to nail the hardiboard into.
Secondly, I would use at least the minimum R-value sheathing recommended by the DOE, http://www.ornl.gov/cgi-bin/cgiwrap?user=roofs&script=ZipTable/ins_fact.pl . Why? Because energy isn't getting any cheaper, and energy is a long-term expense that you have a chance to impact while you're building the house that you won't have ever again.
The DOE recommended sheathing for my area (central Indiana) is R-5. For a 2x4 wall insulated with cellulose, the minimum sheathing R-value required to insure that moisture won't condense within the wall assembly calculates to be about R 4.4, so I will use an R-5 1" styrofoam as sheathing. I found it interesting that what the DOE recommends is what is good for my wall assembly!
What do other builder's use in my area? An R-3 (1/2") foam sheathing. Why? Because that's what sells. People want up-front cheap, they don't look long-term, so builder's can't sell it. Doesn't make sense (or cents) to me. I think its just another example of the stupidity of the masses.
I don't think that "If the air seal is not perfect, then cold air would still get behind the sheathing and cause moisture to condense" describes the actual problem. Typically outside winter air is drier than indoor air, so outside air leaking in would be warmed well above its dew point and not condense. Its the moister inside air leaking out and cooling that causes the condensation problems within wall assemblies. And you are right to be concerned.
My approach is to specify a 100% wall coverage (corners included) of 1" R-5 Styrofoam, all joints taped (horizontally and vertically) and joints offset from the underlying structural sheathing by at least 12". The DOW website says this approach will produce an air-tight barrier an obviate the need for a housewrap product. Under the foam sheathing, I am spec'ing a 100% wall coverage of Thermoply structural sheathing in 0.113" thickness http://www.ludlowcp.com/pages/thermoply.html . Since I am GC'ing my own home, I also plan to caulk the stud/Thermoply interface inside the wall cavity as a further reinforcement to the air barrier. It might be overkill, but I want a tight wall assembly. FWIW, the Styrofoam/Thermoply combination above lets me use an off-the-shelf Andersen 5 1/4" jamb extension, so I don't have to jump through hoops trimming out the windows.
Also, if you decide to use a foam sheathing R-value based on the calcs above, the potential for condensation only exists near the worsts-case design conditions. For the majority of the winter, you won't approach those conditions. Another way to say it is, if you do have some air leakage, which will probably happen in the most meticulously built wall assemblies, your exposure time for condensation will probably be very minimal. Thus, the potential for any long-term damage should be very minimal. When I view the situation in that light, I think that my home will probably stand the test of time a lot better than those homes that the masses are buying!
Sorry to be so long, but that's what I've been through and come up with. Best wishes,
Rick
Was on the APA, the plywood people, website and they had a couple of photo files produced by their survey teams after Katrina and one of the common threads in them was photos of sections built with insulated structural panels failing while plywood sheathing, on the same building, held up the storm much longer.The survey teams were out of universities and even though the APA, of course, is pushing a point of view but the difference between the panel types was pretty clear. I do know that none of the proposed codes for hurricane resistance include substituting structural insulated panels as a substitute for plywood. Even OSB took back seat to ply.Edited for clarity.
Edited 12/12/2005 7:21 pm ET by 4Lorn1
While waiting for knowledgeable people to answer, try reading the 60 or so posts in this thread: http://forums.taunton.com/tp-breaktime/messages?msg=65624.1
(Hope I did that correctly - should be a long discussion of a "Mooney wall"
HI,
"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 foremost 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..."
One thing to think about on the window situation is that a major part of your window heat loss will occurs at night (long winter nights and cold night outside temperatures). If you can design in effective inside window coverings that insulate well, you can reduce your window heat loss significantly. Since you are doing a new house, you have a lot of design latitude on how to do this (its much harder after you have built the place). A window covering that incorporated an inch of foam would add more thermal resistance the then best triple glazed argon filled wonder windows you can buy, and might cost much, much less per sqft?
On the foam board sheating, I also saw the pictures that were mentioned above on Katrina wind damage and decided I'd never use foam board sheathing. But, I suppose it might be OK for some areas.
Some of the more energy concious builders around here (Montana) are using 6 inch studs on 24 inch centers. They then spray in a couple inches of the Polyurethane foam, and then fill the remaining cavity with batts. The foam provides excellent air sealing and some of the R value, the batts provide the rest of the R value. This ends up being cheaper than using spray in foam for the full cavity, and still gives a high R, airtight wall. Maybe something to look into.
Gary
http://www.BuildItSolar.com
I'd heard reports that interior foamboard window treatings can nullify some thermopane warranties because of causing premature window failure.
So, of course, I'd been using form board with a tight tension fit on some new eglass argon windows for the second winter and decided to take a look.
Sure enough, a southern window no less, had small beading water droplet condensation between the glass panes.
yep, that was a pain.
'Nemo me impune lacesset'No one will provoke me with impunity
Hi,"I'd heard reports that interior foamboard window treatings can nullify some thermopane warranties because of causing premature window failure."I guess thats a valid concern. I was thinking of panels that would open during the day, but I suppose if you forgot to open them it could get hot behind them.
Seems like its unlikely to be a problem on the north side. On the sunny sides, maybe a reflective finish on the glass side of the inside shutter would reflect enough sun back out to prevent overheating. It seems like glass has to be good for things like snug fitting dark drapes, which would also trap heat behind the drape?We have trifold shade (the accordian fold type) that also have an "energy track" along the edges so that they seal the gaps that would normally be on the sides. I think they were advertised as R3. We have had them up for 5 years now, and no problems with window damage.We are in the process of putting triple wall, clear polycarbonate panels inside some of our very odd shaped windows that you can't get a regular shade to fit. This stuff is about R2.8. I did talk to the the engineer at our window manufacturer in Billings, and he said it would be no problem for their windows.Gary
http://www.BuildItSolar.com
harvester... do a search on " mooney wall"