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Vapor barrier paint over drywall with poly under it? overkill or will it trap moisture?

trekka's picture

1) Four mil polythene was installed as a vapor barrier on the inside face of the exterior  walls of a new construction house on the water near Seattle WA,  per inspectors advice rather than using a vapor barrier paint.  (drywall, poly, 2x6 studs, unfaced fiberglass batts, plywood, tar paper , and hardiboard lap siding).  The drywall is now on and the installer is suggesting spraying it out with a vapor barrier paint (for 400 more dollars) rather than use the standard primer.  Does this create a double moisture barrier with attendant problems or is it a good idea or just a waste of money? 

2) Ten years ago I was required to use polythene in the houses I built in Portland, Oregon, but have been unable to find out if vapor barrier paint  is a superior solution for this climatic area.  Using paint in place of the polythene would allow greater permeability - is this a good thing with the previously mentioned wall construction?  

 Thanks for your input .

In NW OH. (post #187206, reply #1 of 9)

it was and probably is still fairly commonplace to use unfaced fibreglass, cover with visqueen, drywall and prime/paint.  I would say that no one uses a specific vapor barrier paint (except perhaps in a bathroom).  I have not seen any attendant problems caused by this construction.  If there was a moisture problem IN the wall, it came from somewhere else-the exterior.  Your climate is different than ours.

David Meiland who posts here is a builder on an island outside seattle.  Might hunt him down and ask.

Jim Blodgett is a builder just south of Tacoma-he can be found here also.

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I do not and would not (post #187206, reply #2 of 9)

use poly over the studs, but I have not seen problems that I thought were caused by this. It used to be fairly common practice in a lot of places, and I have remodeled houses here with poly under the drywall. Like Calvin says, a moisture problem IN the wall is usually coming from outside (i.e. a window that's leaking, a roof-to-wall flashing that's missing or poorly done, etc.). My opinion is that we don't get enough weather here that's cold enough to lead to problems with condensation from the inside--the occupant load of the house would have to be high, the house would have to be fairly tight and have poor ventilation, and it would have to be very cold outside for longer than we usually get. Most houses are not that tight and there is enough natural air change to dissipate excess vapor, if any.

Now, the one thing you mention that I don't like is the Hardi siding. Fiber cement can hold a lot of water, and if water is getting on the back of it then it might get and stay quite damp, and the moisture it wants to give off will migrate at least partly into the wall. Same goes for cultured stone and brick. If you have a consistently wet area behind siding, and poly on the inside of the studs, the drying potential of the wall will be reduced and you may have problems, or at least greater problems than you would without the poly.

It is fairly common here to use unfaced batts and a vapor barrier primer. I have not heard anyone that I consider highly authoritative give a ringing endorsement of vapor barrier primer, but there are many products sold as such, and they are usually first on the wall up here. Benjamin Moore Super Spec 260 is one.

Anyway, I think walls do better when there are fewer barriers to drying. It's probably too late to get a rainscreen under your Hardi but that would have been ideal. 

Rainscreen (post #187206, reply #6 of 9)

Thanks for your comments re the vapor barrier.  Regarding the rainscreen, I assume you mean the use of coravent at the bottom and top of the siding and lath strips to create an air gap between the siding and the house's building paper/sheathing.  I have only seen this done once and did look at a Canadian web site posted by the province of British Columbia.  For future reference are there any other web sites you recommend.  Thanks. 

Lots of rainscreen info online. (post #187206, reply #8 of 9)

You can use furring strips, you can use a product like HomeSlicker to space the siding off the wall. It lets the back side of the siding dry, and it virtually eliminates the possibility of water moving from the siding to the wall structure by capillary action. I am starting to insist on it for all jobs, and I think it would be especially important with FC siding.

It might be (probably is) a (post #187206, reply #3 of 9)

It might be (probably is) a waste of money in the Seattle area, but it won't cause any problems.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

I have never had any (post #187206, reply #4 of 9)

I have never had any confidence in those primers they claim are vapor barriers.  Using them instead of faced fiberglass or a poly vapor barrier has been the standard here in Western Washington for many years, though. 

You know how we prove it's a pva type primer instead of something cheaper and more permeable?  We leave the can onsite for the inspector to see. 

Now you tell me, what's to say  people don't  use the cheaper primers and move the (almost) empty can from job to job?  Fact is they do.

All that being said, if there is a poly vapor barrier under the drywall, what would be the point of a vapor barrier on top of it as well?

Waste of money in my opinion.

I have seen cases where poly (post #187206, reply #5 of 9)

I have seen cases where poly was used on the walls but not the ceiling, in which case using the VP primer on the ceiling might make sense.

And it makes sense (in colder climates) in older homes where just the craft VB on the FG insulation was used -- the integrity of a craft VB is highly suspect.

One reason you don't see more moisture problems in pre-1960 homes is the multiple coats of oil paint on the walls.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

From everything I've read (post #187206, reply #7 of 9)

air leaks are a much bigger factor in moisture movement than vapor diffusion. So you put poly on your walls, you cut it around all the electrical boxes, and then do the same with sheetrock. Close all the doors and windows, turn on the bath fans and range hood fan, and see if you feel air coming out of those same electrical boxes. Probably the best thing you can do from the interior is airtight drywall. The coming period in construction will be interesting, as everyone tightens up their houses and a lot of drying potential goes away. Air leaks waste energy but in a lot of cases they help buildings last. In our climate I suspect the dew point is outside the wall most of the time, and the whole VB thing is mainly a non-issue. A couple thousand feet up and the picture changes.

No issues ... but it is a (post #187206, reply #9 of 9)

No issues ... but it is a waste of money me thinks. Provides no benefit.

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