Vapor-resistant layer – VB film or ADA?
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We are building a super-insulated house in the northeast <3″ poly-iso on the exterior, 2x6s @ 24″ OC with cellulose fill, spray foam at the box sills>. We specified an air-tight-drywall-approach, with the latex paint acting as the vapor-resistant layer. But the insulator installed a 6mil vapor barrier over the netting. My understanding is that an additional VB layer prevents the wall cavity from drying to the inside. Should we tear out the VB and go with the ADA or leave it in and skip the ADA?
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Replies
Diffusion is important when it comes to microbial infestation. You want the wall to be able to shed moisture, although generally your location would call for an open diffusion to the OUTSIDE of the building. In Louisiana, I would recomend a diffusion to the Inside. Generally, I would eliminate anything not already in the plan, but keeping the plastic might not be that big a deal.
Want to know for sure? Model the whole assembly in the WUFI software. A free edition is available at: http://www.ornl.gov/sci/btc/apps/moisture/ibpe_sof161.htm The program will simulate the moisture flows in the wall over a two year period.
Tony Olaivar- EE Division Chief - Lanz Inc. http://lanzinc.com
Chris,
How difficult will it be to remove the poly without damaging the netting underneath? You definitely don't want to make a mess of the whole thing and have the cellulose begin to spill out of the stud bays.
Your wall will be more robust if it can dry to the interior. Remove the poly if you can.
Martin,
I think we can pull off the VB without much problem, thanks.
Tony,
I'll give that software a try - looks like I have to wait until they send me a serial # to activate it.
Chris,
You absolutely must remove that poly vapor barrier or you're risking significant moisture damage to the wall structure. I would never use a vapor barrier exterior on a north-country house, but since you have chosen that route, the walls must be able to dry to the interior. Several cellulose manufacturers will void their warranty if a vapor barrier is installed.
If the exterior polyiso has taped seams, then you already have an air barrier and there's no need for ADA on the interior. Simple drywall with latex paint is appropriate as long as mechanical penetrations in the top and bottom plates are sealed.
At the risk of starting a big rubarb I will say that I have built a number of homes with high R exterior sheathing and a continuous poly vapor barrier on the inside with very good results. This is in MN however and may not be suitable for other regions.
As I am fond of saying, a wall must be able to dry to the inside, the outside, both sides or neither. You must build airtight and have your flashing details down pat. My own house built by others in 1978 has 1" extruded polystryrene exterior sheathing, R-11 fiberglass batts and a 4 mil poly warm side vapor barrier. I have been in the walls a couple of times for window replacement and the walls are pristine. No moisture going in, out, or collecting in the stud space.
Doug,
1" of XPS (perm = 1.1) is a completely different animal than foil-faced polyiso (perm = 0), and a wall with only R-11 fiberglass is as good as empty and will dry easily.
Unless you have temperature and moisture monitors in your wall, you don't know how much moisture is getting in and leaving. But I can assure you that you have a very low negative ion count in your house - and negative ions are essential for emotional and biological health.
Robert,
Oddly, I feel fine emotionally and have had some breakthroughs living in this house. I use this as one working example of a double vapor barrier. I have also used polyiso as an exterior sheathing with R-19 insulation and a warm side vapor barrier with good results.
My preferred MN cold climate wall is a double 2x4 with a warm side poly barrier, a vapor permeable fiberboard exterior sheathing, tyvek and a rainscreen. Blown-in blanket insulation would be optimum for a complete cavity fill. You can seal the edges of the 4x9 sheets of fiberboard to the framing for excellent windwash control.
I would be concerned about a wall system that relies on a vapor permeable paint that essentially has to dry against the typical vapor drive which is outward in a cold climate. Does a house built in this fashion carry a disclaimer that keeps future owners from using wallpaper, certain types of paint and other vapor impermeable finishes?
"Breakthroughs" such as double vapor barriers are OK? I think you definitely have a negative ion deficiency.
You claim you've had "good results" with double vapor barriers? Talk to me in 100 years. Unless you're building a house to last only the term of your warranty, an envelope that cannot dry if it every gets wet (and all houses do over the course of their useful lives) is a disaster waiting to happen.
All human artifacts that are intended to be durable for generations of use, have to be designed to be fail-safe - because failure is assured.
Addendum:
When we were building and living in simple, practical houses there was a high degree of resilience and tolerance to moisture.
Now that we've foolishly boxed ourselves into an entropy corner, in which we are forced to build hyper-insulated structures in order to be "responsible", we must build in an exponentially higher degree of resilience.
No hermetically-sealed house has resilience.
Doug,
Here are 5 reasons why you should not draw conclusions from your 1978 home.
https://www.youtube.com/user/BuildingGeek#p/u/6/D_IrtDR3p0c
Nearly every new home built in the Twin Cities in the last decade, numbering in the tens of thousands has been built with a double vapor barrier. Poly of 4 mil or greater is used on the warm (room) side sealed at all plates, windows, doors and wall intersections. The insulation is R-19 fiberglass and the exterior sheathing is OSB. Housewrap has been a requirement for the last several years.
This is not a great wall system, the OSB is low perm and conductive as well. It has been working although test walls show that a smart , variable perm vapor barrier would be a better choice than straight polyethylene. If this wall is code approved and is working in a cold climate, a layer of extruded polystyrene or polyiso as exterior sheathing instead of OSB should have similar vapor permeance properties but better thermal performance.
Airtight drywall approach (ADA) is approved by code as well as far as I know, but this method is little used. The continuous warm side vapor barrier makes it much easier to get reasonably low ach50 levels, ADA is tough in this regard.
Doug,
Are you "feeling lucky"
Do Twin City folks ever air condition their homes?
Best practice would be to let the moisture out in both directions.
To say "neither" is VERY risky.
A common logical fallacy: if the status quo is marginally OK, then a "slight" increase would not be much worse.
It's true that 1" of XPS sheathing will have similar hygric performance as ½" of OSB. But it's not likely that any builder in MN trying to improve thermal performance is going to limit themselves to 1" of XPS, particularly since building science requires a minimum of 2" exterior XPS for the "warm sheathing" approach in a cold climate. And adding 1" of XPS on top of ½" of highly moisture intolerant OSB is going to dramatically change the hygric performance of the wall.
Adding foil-faced polyiso on the exterior changes the hygric performance by several orders of magnitude - in the direction of disaster.
Robert,
Your conclusion that adding polyiso sheathing on top of OSB in Minnesota "changes the hygric performance ... in the direction of disaster" is an oversimplification.
While it is true that adding polyiso reduces the ability of the wall to dry to the exterior, it also dramatically reduces the chance of moisture accumulation in the OSB layer during the winter (by raising the temperature of the OSB). As Bill Rose always teaches, "warm means dry."
One method of comparing the advantages and disadvantages to the addition of the polyiso is to model the wall with WUFI. The result will vary depending on the thickness of the polyiso.
John,
My preferred wall system dries to the cold side using fiberboard sheathing and housewrap, I do not like OSB as a sheathing material. We do use AC in MN but if your house is well insulated the usage is light, we use 250 kWh per year for AC.
Martin, the wall detail in question includes a warm side poly vapour barrier and I believe Robert's statement was in reference to foil-faced polyiso (which unless I'm mistaken also qualifies as a vapour barrier).
With vapour BARRIERS on both sides of this wall, how is it an over-simplification to describe this approach as a road to disaster?
Lucas,
I'm not advocating the described wall assembly; I agree that the interior poly should be omitted when using polyiso sheathing.
I'm just saying that the polyiso sheathing does two things:
1. It reduces the ability of the wall assembly to dry to the exterior, and
2. It dramatically reduces the chance of condensation or moisture accumulation on the interior face of the OSB.
A program like WUFI can help a builder decide whether (when it comes to moisture accumulation) the polyiso is a net benefit (or not) to the wall assembly.
My own house (built in 1980) has exterior foam sheathing and interior polyethylene. I would design it differently if I were to build a new house today. But the fact remains that when I have opened up my walls, they are pristine, without any signs of moisture accumulation.
1. Foil doesn't "reduce" but eliminates the wall's ability to dry to the outside. Zero, Zilch, Nada.
2. The "warm sheathing" approach prevents only condensation from air movement or vapor diffusion from inside to out
3. The unintended consequences of the "warm sheathing" approach occurs when the house (that's when, not if) eventually develops a leak from the outside. Now the sheathing is wet, it can't dry, and it stays warm enough to mold and rot, which won't be detected until it's much too late to prevent.
WUFI, as far as I know, doesn't model an exterior leak into the building envelope, but the MEWS study did and any envelope with even semi-permeable XPS on the exterior did not dry out for the two year duration of the study and the wall cavity remained at the moisture/temperature danger zone for most of that time.
A hermetically-sealed house is a house with zero resilience. It works perfectly only as long as it remains perfect. Nothing remains perfect (and few things start out perfect). Ipso, facto: a perfectly tight house is a disaster waiting to happen.
Robert,
Not only does WUFI model water leaks from the exterior, its modeling is climate-specific and orientation-specific.
"The WUFI (R) series software models can handle contributions from rain, solar radiation and other crucial weather events on an hourly basis. Both vapor and liquid transport are included, along with the sorptive capacity of building construction materials. WUFI ORNL/IBP, one of the WUFI family software, offers an easy and user-friendly interface for data input and output and is free of charge. WUFI ORNL/IBP comes complete with weather data for scores of North-American cities which can easily be selected from a map."
More information here:
http://www.ornl.gov/sci/btc/apps/moisture/index.html