# Painted plywood = vapor barrier?



## Nimrod (Jun 8, 2010)

I am building a barn. Got halfway done last fall and buttoned it down for the winter. The walls are covered with CDX plywood. It will be a while before I can afford siding so I was considering painting the plywood. I may insulate and heat the barn in the future. Would painting the plywood create a vapor barrier? Would this trap moisture if I insulated and heated?

The paint is oops (misstints) paint. No two gallons are the same color. I may have to buy some of the puke orange and snot green. It will be colorful.


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## AmericanStand (Jul 29, 2014)

Yes It will but by putting a vapor barrier on the inside after insulating you shouldnt have a problem.


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## Bearfootfarm (Jul 13, 2006)

Nimrod said:


> The paint is oops (misstints) paint. No two gallons are the same color.


I needed to paint a room once and had a couple of leftover half gallons of mismatched paint.
One was an off white looking tint, and the other was a really dark green.
I couldn't figure any logical way to divide the colors so I just mixed the paints together and it somehow came out a lovely shade of blue.


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## melli (May 7, 2016)

Nimrod said:


> I am building a barn. Got halfway done last fall and buttoned it down for the winter. The walls are covered with CDX plywood. It will be a while before I can afford siding so I was considering painting the plywood. I may insulate and heat the barn in the future. Would painting the plywood create a vapor barrier? Would this trap moisture if I insulated and heated?
> 
> The paint is oops (misstints) paint. No two gallons are the same color. I may have to buy some of the puke orange and snot green. It will be colorful.


I have often wondered about this question that you pose, yet I cannot find anything definitive on it...kind of maddening, as throwing on a whack of acrylic exterior grade paint is often easier and cheaper than expensive housewraps.

Here are a few links to housewrap...
http://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-water-resistive-barriers
http://www.greenbuildingadvisor.com/blogs/dept/musings/housewrap-can-liquid-applied-wrbs
https://bct.eco.umass.edu/publicati...-felt-paper-and-weather-penetration-barriers/

Only in one do they mention painting (in passing)...and now they have 'Paint on WRB'. The main idea of housewrap is to shed bulk water when the siding leaks. The better stuff has an ability to dry out quickly if it gets wet and allow moisture behind it to migrate outwards (dry to exterior). The problem I see with paint, is once water gets behind it, how quickly can it dry out? Seams, cracks are always present in plywood walls. Water will get in, and soak ply. Depending on thickness of paint, how quickly can it dry? Now, if you put on an interior vapor barrier, you need outward drying, as poly is near impermeable.

It has always perplexed me why manufacturers are not keen to introduce paint on 'housewraps' (other than a couple). I often wonder perhaps the housewrap is better being off the ply, so drying can take place. And if a good housewrap, will allow moisture to escape, but retard moisture from getting driven inward (like gortex).

I decided to go with a dimpled woven poly, feels like a canvas. My hope is, it will restricts bulk water from hitting ply, and allow outward drying.
Yet, I wonder if painting is just as good?

I think in your case, I'd paint the sucker. And when the day comes to put on siding, you can see if paint has done it's job...certainly better than nothing. If you put on a housewrap, it needs to be covered in a reasonable length of time, as the housewrap will degrade with UV rays, probably more quickly than exterior paint, which has UV inhibitors.
I wouldn't put poly on interior, especially for a barn (vapor drive in both directions), but also wouldn't put poly on interior, until you've put on siding (at very least).
I used acrylic paint on ICF, more to seal pores than anything, but I have found it also restricts condensation. I have a side wall I couldn't paint (weather turned), and I've noticed huge amounts of water condensation on wall compared to wall that I painted...huge difference! (unpainted wall...hand is dripping wet...painted wall...nothing). So, as a stopgap measure, I'd paint it.

Addition: As I think about it, the one big benefit of housewraps is they are generally impervious to expansion and contraction issues...with paint on 'housewraps', there is considerable attention to seams (fiber tape) and cracks. With the former solution, expansion and contraction of building materials don't materially affect the housewrap.


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## oldtruckbbq (Aug 8, 2016)

Housewrap would probably be best as it would be there ready for putting siding on in the future. But paint would be better protection for the wood than leaving it bare. Painted wood, unless water gets under the paint, will last years. Unpainted wood won't hold up for very long, and plywood is way too expensive to have to replace down the road.


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## Birchwood (Nov 12, 2017)

oldtruckbbq said:


> Housewrap would probably be best as it would be there ready for putting siding on in the future. But paint would be better protection for the wood than leaving it bare. Painted wood, unless water gets under the paint, will last years. Unpainted wood won't hold up for very long, and plywood is way too expensive to have to replace down the road.


As a guide - when we bought our place the previous owner had built a large work shop and just painted the plywood - with the plan of adding siding later, which happened about 3 months after we bought the place - but was ten years after he built the shop! Have to say, that plywood was in perfect condition. When I cut an opening for a new door I could see the plywood had been painted two different colours during the 10 years, so they must have painted at least twice and the ply was solid. The over hangs are pretty large so that may have also helped. I believe exterior formulated paints that are designed for wood 'breathe' so I don't think I would worry, if our place is any indication paint will do the trick. Good Luck!


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## M5farm (Jan 14, 2014)

Why not just spray it with deck sealer . It will repel water till siding is applied


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## Nimrod (Jun 8, 2010)

Because I can get oops paint for $5 a gallon or $20 for a 5 gallon pail. This has to be cheaper than Tyvak or deck sealer.

Mixing paint is a good idea. Don't know if mixing in a 5 gallon pail will be enough for all the walls. May need to mix 2 batches. I do have a gallon of lovely plum purple for the trim. I used up all the battleship gray painting the second level floor prior to covering it with a tarp.

Thanks for your help.


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## Steve_S (Feb 25, 2015)

Much also depends on the TYPE of Paint you are using as to what it will do. An Exterior Latex (likely best) will allow the wood to breath a bit and will act as protection. An oil base will seal the wood alright but that can often come back to haunt you if water get's into the wood behind it. Heated & Conditioned space will do one thing while an unheated space will behave differently. Alternately you could use Stains on the ply but forget painting over that at a later date and I'm not sure you'd get any mis-tints. I have seen some "creative colours" result from mixing mis-tints... likely not a bad way to go BUT have a can of pure white at hand so you can lighten it... (BTW If you paint it with something --- ummm - creative ? , it would certainly motivate you to put the siding on quicker... ie: anti-doddling anti-procrastination aid) feature... You could always say that was your intent / motivation IF you end up with something a touch colourful... hehehehe You still need to prime before painting, so remember to add that to your costing.

Wrap like Tyvek/Typar are mostly UV Stable (they are intended to be covered with something) and allows air through but not water, they allow moisture to escape the wood sheathing as well. It isn't that much, if mem serves I paid $120 for a 10' x 100' roll, so it is more than off-tint paint.


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## Sole soil (Jan 29, 2018)

House wrap , remove it to paint , put back up. If the wind damages the wrap, plywood is protected by paint until you can put the wrap back up.


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## doc- (Jun 26, 2015)

oldtruckbbq said:


> .... But paint would be better protection for the wood than leaving it bare. Painted wood, unless water gets under the paint, will last years. Unpainted wood won't hold up for very long, and plywood is way too expensive to have to replace down the road.


A study out of the U of IA actually showed that untreated wood weathers better & longer than painted wood ....paint apparently traps more moisture *in* than it keeps *out*.

I don't know if this applies to plywood/CDX.


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## Sole soil (Jan 29, 2018)

The science of paint has come a long way.


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## Danaus29 (Sep 12, 2005)

doc- said:


> A study out of the U of IA actually showed that untreated wood weathers better & longer than painted wood ....paint apparently traps more moisture *in* than it keeps *out*.
> 
> I don't know if this applies to plywood/CDX.


Do you have a link to this info? I did a google search and couldn't find it.


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