Where to put a vapor barrier and what kind to use?
Hi,
I have a 1910 house in Seattle. We’re doing a gut job remodel on a tight budget and are looking for some advise on vapor barriers. With other rooms we’ve completed, we’ve followed the logic of putting the vapor barrier (6 mil poly) on the inside face (warm side) of the stud wall and taping the seams. I’ve heard that the best method is to put the vapor barrier on the outside so water has no chance to get in the wall assembly in the first place. We just restored the original cedar beveled siding last year so doing anything on the outside is not an option. Is our interior VB the wrong approach? We have 3/4″ beveled siding over 3/4″ over building paper over 3/4″ sheathing over 2×4 construction with plans to insulate with cotton batts. With no wall insulation, the house has always been able to dry out “naturally” but as we tighten things up we don’t want to create a breeding ground for mold or other issues.
Thanks!
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Replies
Lauren,
Interior poly is a bad idea in your climate. You'd be better off with vapor-retarding paint.
Do your best to install the drywall as airtight as possible (with gaskets or caulk between the drywall and the top and bottom plates, and with airtight details at the electrical boxes).
If you are beyond the point of gasketing or caulking behind drywall, how about a bead of sealant on the base/crown mouldings?
Kevin,
Yes, that approach can work for air sealing in an existing house. A blower door can help find the leaks if you have already hung and taped the drywall. For more information, see "Blower-Door-Directed Air Sealing."
Lauren,
An exterior vapor barrier is appropriate only in a house that is air conditioned most of the year, and absolutely wrong in a house in a heating-dominated climate.
Cellulosic insulations, like cotton batts, are highly tolerant of moisture fluctuations and can help buffer indoor humidity. But to avoid mold and other moisture-related problems, it is imperative not only to install a good interior air barrier but also to control point-sources of moisture such as baths and kitchen range with exhaust fans. As you tighten the house, it also becomes important to guaranty a minimum hourly air exchange both to control humidity and to maintain a healthy indoor environment. This can be accomplished either with a heat-recovery or energy-recovery ventilator or with a programmable timer on a bath fan set for 0.25 air changes per hour.
Lauren,
In answer to your the second part of your question: the code no longer requires a "vapor barrier", such as the ubiquitous 6 mil poly. It requires only a "vapor retarder" of 1 perm or less. This will stop most vapor migration into the thermal envelope while still allowing some drying to the interior when the moisture drive reverses in the summer, and allowing hygroscopic insulation materials such as cellulose or cotton batts to act as a "catch and release" humidity buffer.
So, in almost all but the most extreme climates, the only vapor retarder required is a latex vapor retarder primer under your latex finish paint.
lauren,
both of these gentlemen are correct in seatle it would be a very bad i dea for a vapor barrier you need an allowable amount of difusion and air exchange to alow drying of interior cavities. this barrier would only make a breeding ground for mold and a poor IAQ,-indoor air quality,
Thanks for all the advice! So - my understanding is that the PVA or latex vapor retarder primer (also, what is the difference between the two?) deals with keeping moisture from getting in the wall cavity while the airtight drywall approach deals with keeping air from coming in our out? Is there any type of membrane that could go behind the drywall that would effectively deal with both moisture and air?
We installed a high quality Panasonic fan in the bathroom and will be installing a 600 CFM hood in the kitchen so hopefully these will take care of source specific moisture issues.
Here's one more question, we're converting about half of our attic to a vaulted ceiling and/or loft space, so we'll be installing about 4.5" of polyiso rigid foam between the rafters (maintaining a 1" airspace) to meet the r-30 code requirement. Would the same advice be true for vapor retarder paint and airtight drywall at these locations?
-Lauren
Lauren,
"Latex" is the generic name for water-based paints - they generally use acrylic binders. PVA, or polyvinyl acrylic is one type of acrylic binder (often the cheapest).
The reason that codes no longer require a vapor BARRIER, such as polyethylene film, is because we now know that 99% of interior moisture that gets into the thermal envelope does so through air movement and not vapor diffusion. Codes generally require a vapor retarder of 1 perm of less and a vapor retarder primer meets this standard.
A foil or poly film is the only membrane (with the possible exception of the vapor-variable MemBrain) that stops both air movement and vapor diffusion. But the problem with any vapor-impermeable membrane is that anything that keeps moisture out will also keep it in, and can cause - rather than prevent - moisture related problems.
The Panasonic bath fan is a wonderful improvement, but a kitchen range does not require more than 200 CFM to adequately eliminate source moisture and smells. 600 CFM is far too much for an energy efficient house.
And, yes, a cathedral ceiling is one of the most important areas to air and vapor seal, because of the high stack effect pressures and air temperature stratification.
Lauren,
The performance of your cathedral ceiling would be much better it you installed the polyiso under the rafters instead of cutting the polyiso into narrow strips and putting it between the rafters.
Each rafter is a thermal bridge.
Thanks for the all the advice. We've decided to forgo the 6 mil poly and use a PVA primer instead. My only concern is that this seems to be a high-voc product. Are there any less toxic vapor retarder primers out there? I can't seem to find much online. (We have to live in the house during the priming.) Thanks!
Lauren,
Benjamin Moore makes a good latex vapor retarder primer, marketed as Moorecraft SuperSpec 260.
Your cathedral area should be sprayed with open cell icynene and not be vented. Cut board foam will leak air and be worthless.