We're painting the seams before applying Zip tape on the OSB. Will a Latex Primer work? How long does the paint have to dry before we can tape?

Thanks!

Good question.  Nothing adheres well to OSB.  For a good while we only used traditional plywood for wall sheathing because of this.  We still will not use OSB for roof decking.  After having failures with window flashing we started priming with oil based primer.  This is based on nothing more than our assumption that the oil is going to penetrate better than the latex.
Hugh
StearnsDesignBuild.com

My husband bought water based Latex Primer. He didn't want to wait to hear what he should have bought, so we went ahead with it. My plan is to have an inch of foam board over the top, and I was debating taping that as well. I guess the problem with latex is that it could peel right off if it gets wet? So, should we definitely tape the foam board, and will that work as an air barrier if the OSB/Zip tape fails? I'll probably fill the wall with cellulose, should I put a foil faced foam on the outside to cut down on the moisture that might get to the OSB?

You are really going to be covering you bases with tape at both locations. Perhaps as important is the seal at top and bottom plates and around all penetrations such as windows and doors. We use a low VOC adhesive and use double continuous beads in these locations.

I would tape the foam and avoid foil faced foam.

Posted By jonr on 05 Jun 2011 10:22 PM
I would ... avoid foil faced foam.

Why?

In most of the US, a non-permeable (technically Class I) film on the exterior adds to moisture risk. Edit: Even in the perhaps more arguable remainder, there are, IMO, better alternatives.

So it depends on where the original poster lives. I haven't seen where they stated they were building this house. What is the recommendation if they live in Houston, TX or Miami, FL? And what factors does the exterior cladding play? Say they have a brick exterior with a 1" air gap in a hot humid climate?

No, in all cases, I would avoid foil. Best to read the latest IRC (or ask Dana). But you won't find any case or climate where a exterior wall Class I foil is required. So I would always avoid non-permeable exterior foil (in favor of some other things) to let the wall dry better with some permeability - even if some people argue that it doesn't actually add to moisture risk in some situations.

Jonr, As a builder in the south, we have always put the vapor barrier to the outside, which is consistent with Building Science recommendation for hot humid climates. Moisture and energy move to the cold surface. Do you know where this is mentioned in the latest IRC? Actually, the question I have is for you Yankees who put the moisture barrier to the inside. It seems that this would leave your wall assembly vulnerable.

Hugh
stearnsdesignbuild.com

You need to distinguish between "vapor barrier"/retarder and what we are discussing, "foil". Follow local code and the IRC for when and where to put a wall vapor retarder. But personally, I would never make it Class I exterior foil (ie, use one of the acceptable alternatives). This is at least acceptable to every code, test and recommendation I've seen - in all climates. More info at: http://www.naima.org/insulation-knowledge-base/residential-home-insulation/print/insulation-and-vapor-retarders.html

This has become off topic but I hope to learn something here so I will start a new thread dealing with material permeability.

Posted By HughS on 06 Jun 2011 10:01 AM
Jonr, As a builder in the south, we have always put the vapor barrier to the outside, which is consistent with Building Science recommendation for hot humid climates. Moisture and energy move to the cold surface. Do you know where this is mentioned in the latest IRC? Actually, the question I have is for you Yankees who put the moisture barrier to the inside. It seems that this would leave your wall assembly vulnerable.

Hugh
stearnsdesignbuild.com

jonr's issue isn't with the placement, but with the extremely low vapor permeability (0.01 perms) of foil as compared to something more vapor permeable, but still vapor retardent.  In cooling dominated climates yes, keeping the interior more vapor permeable than the exterior is important, and exterior vapor retardency is a must for air conditioned buildings, but even 1.0 perms (100x more moisture transfer than foil) of exterior vapor retardency is "good enough" anywhere in even the hottest most humid climates of the gulf coast or southern FL, the air conditioning can easily handle this very modest increase in latent load.

Drying capacity is a surface area  x perms kind of deal, so with 2 perm latex on the interior and 1 perm on the exterior, the assembly can shed incidental moisture more readily. But even under rain-soaked stucco in direct 100F Florida sun a one perm vapor retarder isn't going to allow moisture in fast enough to condense on the 75F wallboard of the air conditioned room and cause a mold problem.  It would be a problem if there were interior poly (or vinyl/foil wallpaper) though.  But with the slightly-permeable exterior the seasonal drying of the assembly is usually improved (depends a bit on the siding type & construction) and the average moisture content of in the wood & wallboard usually lower.

In Yankee climates the moisture drive & risk is primarily from the interior. Winter time outdoor dew points are quite low, but the dew point of 70F 35% RH air is about 40F, well above the average outdoor temps from December-February for most of New England, so air leaks or vapor diffusion from the interior can and will condense and wick-into studs & sheathing all winter with very little drying until spring. Making the interior air tight and less-permeable is one method of treating that, but in masonry or stucco buildings the summertime moisture drives can still be high enough to cause problems if the interior perm rating is too low, and the sheathing perm rating too high.   Polyethylene sheeting ( 0.06 perms @ 6mil thickness) was used a lot during the 1980s & 90s, but it causes about as many problems as it solves.  The vast majority of true moisure problems are caused by air leaks bringing bulk moisture in and condensing, not vapor diffusion, and an air leak into a poly or foil clad stud bay still dries primarily via vapor diffusion, which is reliably blocked by poly or foil. 

Putting insulating foam outside the structural sheathing and eliminating low-perm vapor retarders/barriers on the interior is becoming a common, better solution. It works by raising the average temp of the wood sheathing high enough in winter that moisture accumulation is well bounded, and with 2+ perms of interior drying (as well as some exterior drying, if an un-faced foam is used) the assembly can always dry in reasonable time frames.  The colder the climate, the higher the fractional-R of exterior foam relative to the cavity fill R needs to be to avoid the wintertime moisture buildup.  In Zones 4 or lower just about anything works, but in Zones 5 & up one can either take the IRC prescriptive method, or do a dew-point calc for the specific assembly and climate. 

The reliable rule of thumb is that if the average temp for the coldest month at the sheathing stays above 40F  (the dew point of 70F, 35% RH air) when the interior is 70F you're golden.  In places with an average January temp of 30F, only 25% of the total center-cavity R needs to exterior foam.  But where it averages 20F, 40% is needed.  This is a very conservative method- the IRC prescriptive minimums by US climate zone are somewhat more relaxed than that, but some state codes are even more restrictive (particularly on roof assemblies as opposed to walls.) 

Some folks in the north humidify the air in winter to as much as 50% RH, which would put an even higher requirement on the exterior foam R, (or a lower perm interior vapor retarder), since the dew point of 70F 50%RH air is ~55F, bringing the susceptible region of the stud bay well inside the wall cavity.   In high interior humidity cases using higher-perm foam (such as unfaced Type-I EPS) and vented/rainscreened siding to enhance outward drying also helps.  In climate Zones 1 & 2 some sort of lower-perm facer is preferable (particularly under masonry siding) when using EPS as insulating sheathing, since it can otherwise be well under that of latex paint, raising the moisture content of the wood.

Posted By rosalynn on 04 Jun 2011 11:50 AM
We're painting the seams before applying Zip tape on the OSB. Will a Latex Primer work? How long does the paint have to dry before we can tape?

Thanks!

Would be taking about a day or so, in order for the paint to become completely dry......

What's the consensus on latex vs. oil priming for the OSB seams? Jonr , Dana?

I don't know the answer, but I wouldn't count on either one and would tape the foam or add tyvek. These are "safe bets" that won't be effected by moisture.

acrylic-latex is what the pros have recommended. Oil paints may be too vapor retardent & blister/peel over a few decades, but it's probably less of an issue if there's enough exterior foam that the moisture content of the OSB never rises much. (Rapid and deep moisture cycling is what causes oil paint failures on exterior paints, but that's probably orders of magnitude higher than OSB under an inch or two of foam would experience)

As a backup to something else and by not covering the entire sheet, I expect that either paint would be fine.

I did an airtightness test on two Irish Passive House's lately using silicone clamped between the OSB and the stud. The joints weren't taped, the OSB wasn't painted and we got air-tightness tests of 0.38ac@50kPa and 0.45ac@50kPa. So I don't think there's any need to paint OSB to air-seal it, its airtight enough already. You can read about the houses on the UK Green Builders Forum Passive Houses with no tapes/membranes, no ducts, no back up heating. The houses were built for similar prices to regular construction.

I find the claims that the HRV will work without ducts and that a CO2/humidity sensor is all that is needed to control it interesting. I'd like to see more proof - specifically with individual closed door rooms and where toxic substances are involved (like almost all houses have).

Rosalynn was referring only to the seams - so if all joints are supported, then silicon would be an alternative to tape.