Paint as an Air Barrier
CZ-4. Net Zero Energy Slab-On-Grade Ranch. Planning for 0.1ACH50. Huber Zip-R sheathing.
Above the ceiling, and within the envelope, will be 16″ ceiling trusses containing the HVAC ducts. On top of the trusses will be a flat, uninterrupted OSB deck without perforations, that will be applied, taped, and sealed before construction of the vented roof begins. I will seal the seams of that deck with Zip-Tape but wish to further seal the OSB itself, which I feel will be necessary if I wish to reach that 0.1 ACH50. I propose to obtain good quality Acrylic Latex House Paint “mismatches” my local paint store for a couple bucks a gallon and paint the top of the OSB with a couple coats of that. ( a heck-of-a-lot less expensive than spraying on a 3″ thick open cell foam lid on top of the OSB). I have been unable to determine the air permeability of the paint but I’m presuming that if it’s of low vapor permeability then it must be of very low air permeability.
The seamed and painted deck will be covered with 18″ of cellulose.
I have read of the natural air leakage of some varieties of OSB during blower door testing. Do you think that that the Acrylic Latex Paint will prevent this leakage?
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Here is a link to an article that you might want to read (if you haven't read it already): Is OSB Airtight?
In that article, Richard Pedranti was advised that a specific category of paint (an elastomeric paint) might solve the leaky OSB problem. Pedranti decided not to pursue that approach because of concerns that the paint wouldn't be compatible with Siga Wigluv tape. I have no idea whether (a) Zip System tape has similar compatibility issues, or (b) the cheap paint that you are thinking of using would solve the "leaky OSB" problem.
If you care enough about air leakage to anticipate problems with air leakage through OSB, then your best bet is probably to use plywood or Zip System sheathing instead.
I continue to try to answer my own question.
From BSI dealing with air barriers I found the following: "Painted masonry block walls have an estimated air permeance of less than 0.20 l/(s-m2)@ 75 Pa"
My presumption is that masonry blocks are, by themselves, quite air permeable. But with paint they are respectably air tight. But again I may be interpreting this incorrectly, and I don't know if this would be the case with painted OSB.
I think that the paint could well do the trick. I'm just not sure (considering your ambitious airtightness goal) that saving a few bucks to buy inexpensive OSB is worth it. Why not just upgrade to plywood or Zip sheathing? (I know the answer... your budget is tight. Right?)
Sure, the budget is always tight. We all try to get as much value as we can for the money that we have to spend. It's also an extension of the "Pretty Good House" principle - Why have more of something than you really need. Why buy an expensive item when a lesser cost item will be perfectly serviceable.
In this case...why spend a thousand when 50 bucks might do the trick. Elastomeric Paint at $25-30/gal or Acrylic Latex Paint ("mismatches") at $4/gal. I will be checking with the paint manufacturer and will post their rely here.
Tech Services at PPG Paints indicates that they don't test for air permeability.
My approach will probably be to go ahead with the attempt to air seal the OSB Attic deck with a couple coats of Acrylic Latex Paint. A pressure/fog test of the envelope will occur subsequent to that. If further sealing is necessary, then I'll go with an elastomeric paint on top of the Acrylic Latex.
I'm betting that the Acrylic Latex/OSB combination will do the job. I will report back.
With any luck, you'll end up with a brand of OSB that is already airtight, making the paint unnecessary. But it's good to have a Plan B in case your OSB is leaky.