Foam wrap & vapor barrier question
CSmith
| Posted in Energy, Heating & Insulation on
I’m building a new house here in Billings Montana that I’ve wrapped half inch polyiso (tuff-R) on the exterior. The insulation is fiberglass batting that is being installed starting yesterday. I had planned to eliminate the interior vapor barrier to avoid trapping moisture in the wall cavity, but the insulator said he had to install it as per code and that since I didn’t tape the seams on the tuff-R that the moisture could still get out.
I assumed he must want to install the 6 ml poly for his own reasons – hold his batts in place, looks cleaner etc.. What should I do?
Clay
Replies
The short answer is to go to the building science website and read up. I know in my climate (northern MN), if you don't have a vapor barrier on the warm side, you need at least an inch of foam on the outside or you run the risk of condensation on your sheathing. This varies by how cold your winters are.
Also, look up the perm rating of the 1/2" Tuff-R. I'm thinking that it is vapor semipermeable, in which case, your insulator is right.
Based on what you decide, you could always just slit the poly VB after the insulator is done. Course don't go crying to him if your walls rot.
What does your building department say?
Thanks for the quick reply, I'll read up on it this weekend. I'm outside the city limits so I have no building permit or inspections (other that elec, sewer & plumbing) but I want to do it right. I may go talk with the building dept on monday just to see what they say. Thanks again, Clay
"but the insulator said he had to install it as per code and that since I didn't tape the seams on the tuff-R that the moisture could still get out."
Any moisture that gets that far you WANT it to get out.
I am not sure how cold it gets in Billings and I don't knwo the R -value of the FG insulation tht youare using.
But there is good chance that the inside fo the poliso is condensing surface.
If any moisture got that far then you do want it be able to move through the wall to the outside.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Clay,
I think you had the right idea, but your foam is too thin. The minimum thickness depends on the number of heating-degree days at your location. In most heating climates, you want at least an inch of foam sheathing -- colder climates, two inches. The thicker the foam, the safer the wall, from a condensation standpoint.
The "trapping moisture" thing is a red herring. Install the vapor barrier. Make sure that your windows and doors are properly flashed so they won't leak. There won't be any moisture in the wall to be "trapped".
Consider installing housewrap over the polyiso -- it's amazing how much air can leak through the seams in sheathing.
Thanks everyone for your help. I sure wish I'd asked this question a month ago, since I'm almost finished with the siding now. AARGG! Since it's too late to fix my mistake of putting half inch foam on the exterior, I believe my only option now is to use soft foam instead of fiberglass batts on the exterior walls. That should eliminate any condensation or moisture issues, and give me the super-insulated shell that I was hoping for.
However, at the risk of sounding totally foolish, would it be possible to add another layer of Tuff-R to the inside of the sheathing? That would sandwich the OSB sheathing between 2 layers on foam, but would be quite a bit cheaper than upgrading to spray foam.
Wall assembly challenged in Montana. Thanks again, Clay
What's wrong with putting a vapor barrier on the inside?
If your view never changes you're following the wrong leader
I was concerned about the potential of trapping moisture inside the wall.
What moisture? Why would it be more "trapped" with a vapor barrier in place?
If your view never changes you're following the wrong leader
No vapor barrier is perfect, either in transmission or installation, and and moisture can slowing leak past it. That is why the term vapor retarder rather than barrier to indicate that.Say you get enoug in to condence 1 drop of water per month in a stud bay. With a retarder that is not also a water barrier, such as air tight drywall then that 1 drop of water can be absorbed and dried off into the house.But with plastic vapor barrier the only place to wet the stud and FG insulation.After a couple of years you will have enough water built up to cause problems..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bullfeathers. As you say, there's no such thing as a perfect vapor barrier, so that one drop of water can make its way out as easily as it made its way in.Moisture doesn't just keep accumulating in the wall -- at some point you reach a steady state (or rather "dynamic equilibrium", since there are constant temperature fluctuations and moisture is constantly moving through). Once that state is achieved moisture will drive out as easily as it drives in, and no further moisture will accumulate.To avoid a problem you simply need enough of a vapor retarder on the wet/warm side to avoid condensation in the cooler parts of the wall. A "perfect" vapor barrier would do this perfectly, with no adverse effects in temperate climates.Having a second "retarder" on the outside actually increases the need for a good retarder on the inside, since moisture will have a harder time driving out of the wall towards the outside. So in general, for a conventionally insulated wall, you want the inside vapor barrier to be more effective than any outside barrier.The only plausible case where (in a temperate climate) the inside barrier could cause a problem is when there is some other source of moisture (ie, rain leakage into the wall), but that's really an argument for improved flashing.
If your view never changes you're following the wrong leader
"Having a second "retarder" on the outside actually increases the need for a good retarder on the inside, since moisture will have a harder time driving out of the wall towards the outside. So in general, for a conventionally insulated wall, you want the inside vapor barrier to be more effective than any outside barrier."
The rule of thumb used to be that the outside wall membrane should be 5 times as permeable as the inside since, in a predominantly heating climate, most of the temperature and vapor pressure gradient is from inside to out, and hence most drying will occur in that direction (though there may be some seasonal reversal in summer, particularly on the south side).
Assuming this wall section has R-19 fiberglass batts, then at 65° inside and 0° outside, the temperature at the inside surface of the OSB would be 13° - not only cold enough to be a condensing surface, but also a freezing surface.
Even with 2" of foam sheathing, the OSB temperature under those conditions would be 28°, cold enough to condense at about 23% relative humidity.
Tuff R, since it is double foil faced, has a perm rating of 0.03 - it is a nearly perfect vapor barrier, and for this reason I would never place it on the outside. If you do, you'd better have a perfect air/vapor barrier on the inside.
Additionally, siding nail penetrations through exterior foam sheathing can reduce the R-value as much as 39% and create condensation points through the OSB.
Edited 2/5/2008 11:12 pm ET by Riversong
Everything, Dan.
That would place a doubled VB trapping moisture between the interior VB and the foam tuff-R exterior VB, something you never want to do
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
You can't trap moisture that doesn't exist.
If your view never changes you're following the wrong leader
So, this is where I'm at. I'm as confused as ever about vapor barriers, but yesterday I cut in another layer of Tuff-R between the exterior wall studs and sealed the edges with foam. Fiberglass (R-19) has been installed along with a poly vapor barrier and the sheetrock is starting to go up today.
Should I slit the poly to let any moisture dry to the inside? Or would that just let more moisture into the wall?
Still wall assembly challenged in Montana. Thanks for all your replies. Clay
That would just let more moisture into the wall.Think of the wall as a creek with two dams on it. Water flows out of a lake (the room) and through the spillway of the upper dam, then pools behind the lower dam. To avoid a buildup of the water behind the lower dam you need to make the spillway on the upper dam smaller than the one on the lower dam.
If your view never changes you're following the wrong leader
Thanks Dan,
I'll stop messing with it then, and let the rockers go to town. I would like to know what I should do differently next time though, other than to use 1" foam on the exterior. I had planned to try dense packing cellulose on this house but just ran out of time. This is my first spec house and I want it on the market this spring. Assuming it sells I will be starting on my personal house and I'll try using ICF's.
Thanks for your help. Clay
Dan, In re-reading your post, should I try to use an air tight drywall approach to do everything possible not to let moisture into the wall? Since it would be virtually impossible for any moisture to exit the wall to the exterior now?
The more air-tight the better. Air intrusion is probably more responsible for moisture in walls than actual transpiration through permeable materials. But generally it's easier to get a good seal with plastic than with drywall, especially around outlets, at the floor, around windows, etc.
If your view never changes you're following the wrong leader
generally it's easier to get a good seal with plastic than with drywall, especially around outlets, at the floor, around windows, etc.
I would say the exact opposite is true.
It's almost impossible to get a continous seal with poly, particularly at the trouble spots you list, unless you caulk it as you would for the ADA system (in which case, the poly is unnecessary).
The Air-Tight Drywall Approach is designed specifically to deal with those problem areas and offers a far more air-tight membrane than poly or any other vapor retarder.
To be done properly, it requires acoustic sealant (e.g. Tremco) or silicone rubber ADA gaskets (http://www.conservationtechnology.com/downloads/BuildingGaskets.pdf) at each framing assembly joint (e.g band joist to subfloor, subfloor to sole plate) and on the inside edge of bottom and top plates, as well as around door and window framing and polypans at outlet boxes (http://www.energyfederation.org/consumer/default.php/cPath/21_1272_62).
http://southface.org/web/resources&services/publications/factsheets/24ada_drywal.pdf
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 2/6/2008 2:51 pm ET by Riversong
I have used both approaches, and while airtight drywall works great in theory, the problem I run into is that I just about have to personally hang the drywall. No hanger I have run across would pick up a caulk gun unless you literally held a gun to his head. Using poly (and caulk and tape) I find it quality control easier to manage.
Yup. If you don't do the job yourself, you probably get what you pay for.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
I'm as confused as ever about vapor barriers
It's really not a confusing issue, but there is a great deal of misunderstanding out there in the building trades.
moisture and water are the primary enemies of wood-framed buildings and can cause indoor health problems by mold generation and accelerated outgassing of building materials
excluding outside leakage or basement moisture, 99% of moisture problems in a building envelope are caused by leakage of moist air from the interior (exfiltration) and only 1% is caused by moisture migration (diffusion) through the interior skin
hence, an effective air barrier will prevent moisture problems in the envelope
the air barrier can be internal (air-tight drywall), intermediate (air impermeable insulation), or external (housewrap) or any combination
the air barrier can also be a vapor retarder, but less important except in high humidity areas such as bathrooms and laundry
since there is no perfect vapor barrier, avoid condensation surfaces within the envelope and thermal bridging
since there is no perfect vapor barrier, use framing and insulation materials that are hygroscopic - can absorb, store and release moisture (e.g. cellulose, not plastic foam)
since there is no perfect vapor barrier, the outside skin of a building should be able to breath (perm rating >1, preferably 5x the inside perm rating) while preventing environmental moisture from entering
in any case, indoor relative humidity should be kept at 30-40%
Should I slit the poly to let any moisture dry to the inside?
If you're going to use poly as the air/vapor barrier, you'll defeat its function by slitting it and in fact concentrate any potential moisture problem at the slits (much as with leakage around electrical outlets and windows).
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 2/6/2008 2:39 pm ET by Riversong
Thanks for the break down very helpful. I think he meant slitting the poly on the outside surface, and wouldn't that be in line with your post?
I think he meant slitting the poly on the outside surface, and wouldn't that be in line with your post?
He's talking about the poly vb under the drywall. And no, it wouldn't.
If you're going to make the outer skin of the envelope impermeable to vapor diffusion, then it is wise to make the inside skin at least as impermeable. And, if that membrane also serves as an air barrier, then slitting it is counterproductive.
I would rather eliminate it than install it full of holes, as this could concentrate air/moisture movement.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
sorry I miss read his post.
I forgot to add that I'm at 3600 heating degree days. Also, I have housewrap installed over the sheathing (under the foam) with seams taped and the insulators haven't started on the exterior walls yet - so any changes can still be made without too much trouble. Thanks, Clay
Clay,
Actually, in Billings, Montana, your average heating degree-days are 7,164, not 3,600. See http://www.nrcc.cornell.edu/ccd/nrmhdd.html .
One other alternative to polyethylene is a so-called "smart" vapor retarder like MemBrain. (You can Google MemBrain.) More expensive than polyethylene, but it works, and solves the problem of limiting vapor diffusion from the interior while still allowing drying toward the interior.
Thanks for the clarification on the heating degree days and the info on the Membrain. I've saved that info for future reference. Clay
Well, you COULD add another layer of foam against the sheathing. Obviously labor intensive. I wouldn't use polyiso--xps is easier to cut and fit with a table saw/chop saw. You'd want to use gun foam to seal it against the studs/plates. I wouldn't really worry about the OSB unless you have reason to believe it is already soaking wet. The other way to go is to spray a thin layer of spray foam on the sheathing and fill the remainder of the cavity with cellulose. If you are going for a very well insulated house, why are you even using fiberglass? You'll want to watch the insulation sub like a hawk for voids etc. But in the end, why not just use a vapor barrier? Do they use advanced air sealing techniques in you area?
Do they use advanced air sealing techniques in you area?
I don't think so, generally it seems that they just do it the way they always do it, without question. The owner of the company did say that soft foam sales are up dramatically and I don't believe they use a VB with that foam. They don't seal it with paint either which a hard foam competitor told me they're required to do. Thanks, Clay
I think that you best do some more research. Here are some sources.http://www.buildingscienceconsulting.com/resources/walls/default.htmSpecifically#Air Barriers
November 2004, Building Science Corporation. Air barriers keep outside and inside air out of the building enclosure. Air barriers can be located anywhere in the building enclosure - at the exterior surface, the interior surface, or at any location in between.
#Vapor Barriers and Wall Design
November 2004, Building Science. Corporation. Good design and practice involve controlling the wetting of building assemblies from both the exterior and interior and different climates require different approaches.
To download the Vapor Permeance Chart click here.
#Insulations, Sheathings and Vapor Retarders
November 2004, Building Science Corporation. Two seemingly innocuous requirements for building enclosure assemblies bedevil builders and designers almost endlessly: keep water vapor out, let the water vapor out if it gets in. It gets complicated because, sometimes, the best strategies to keep water vapor out also trap water vapor in.(the links are shown on that pages)http://www.buildingscienceconsulting.com/resources/moisture/default.htmhttp://www.buildingscienceconsulting.com/designsthatwork/default.htm
(Cold Climate)http://www.buildingscienceconsulting.com/resources/homeowner.htm
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
To add to the links you gave, I provided one on the subject over on JLC Online:http://forums.jlconline.com/forums/showthread.php?s=1807d9bdffaf94724010dda977d184ed&t=38697&highlight=straubeIn it I provide a link to an article by Dr. John Straube on the subject of diffusion of water vapor through wall assemblies and the ability of materials to buffer the movement by absorbing and later releasing the water. In that post, I also proposed a summary of things to follow in building a wall for a cold climate.Edit: here is the link to the Straube article:http://www.balancedsolutions.com/website/downloads/ASHRAE_Thermal8_Vapor_Barriers.pdfFrom my post over on JLC (#37), I referred to the article:"In it, he discusses the water storage aspects of diffusion of water vapor through a wall assembly. In one example (page 3 "Avoiding Condensation" and "Controlling the Amount of Condensation"), he shows that for a cold location (Omaha), a wall with batt insulation, a 1 US perm VR inside and dry plywood sheathing outside could indeed result in condensation on the inside surface of the plywood when the outside temp is around zero F. However, the rate of water diffusion, in absolute terms, would be so small that even if those conditions persisted for an entire month, the plywood would increase its moisture content by only 2%. He noted that maximum safe moisture content of plywood is over 20%, and "dry" ranges from 6 to 12%.Further on, he discusses inward vapor drive in spring and summer conditions, even in places like Omaha. The desirability of limiting vapor retarding on the inside to enable "drying to the interior" becomes easy to understand.It would seem that the ability of wall materials to absorb moisture safely and release it later is largely overlooked (by building codes and AHJ?) when determining the need for and proper perm level of an inside VR.Is it fair to conclude that a good wall strategy for a typically "cold" location and its range of climatic conditions should:
1. Keep the rain out of the wall sheathing and interior cavity.
2. Install good air barriers, to eliminate convective transport of water vapor from either side.
3. Limit the permeability of water vapor into the cavity from the living side, but not too much, so that water can get back out when driven that way from outside.
4. Let the sheathing's ability to buffer moisture content swings protect the assembly from damage over time."---------
I should have added that for a cold climate, there should not be a vapor barrier on the outside, but this goes without saying. Of course, both OSB and plywood, when really dry, act like vapor retarders, but their permeability increases with moisture content.
Edited 2/6/2008 2:20 pm ET by DickRussell