Sealing 3/4 Plywood Window Boxes

[Dan Aims]

Hi everyone, I have some plywood boxes that I built to extend my windows out. I am adding a foam and new siding so the windows had to be extended a few inches out. No the plywood boxes I built (5 ply plywood) sticks a few inches out. I will be flashing the top of the boxes. But as an additional precaution I wanted to seal the plywood in case water (rain) got past the flashing. Also wanted to seal so the wood would not change dimensions if water got past the flashing too.

I was considering redgard since its for water proofing or shellac. But figured I would ask the wood experts.

Attached is a sample of the box, which sticks out of the wall sheathing.

Figure_challenge_03_web.jpg

[Yonak Hawkins]

Well, since you asked for an opinion, Dan . . . I think you should make the boxes out of marine grade plywood or some other good paintable wood that will stand up to the weather. Water is insidious.

[ian maybury]

+1 that the critical issue is to use a waterproof ply Dan. Be careful of grades though, there's a lot masquerading as water something or other but not actually compliant with any standard. (over here anyway) Marine ply is quite a bit more expensive, but often the safer bet.

I'm not familar with US construction practice, but you basically need an arrangement that will shed/drain off water without pooling or leaving any wood sitting in water. Also that avoids creating bridges between the outer layer and the inside wall that might make a path for water to flow in and wet your inner wall and cause damp patches and ultimately rot. It might be advisable to dig up a how to book, or to tap a carpenter familar with what's good practice for some detailed advice.....

[Dan Aims]

Thanks for the reply guys. Marine grade plywood is over kill in this situation. Aside from cost, construction practices all use standard cdx plywood. The boxes are flashed and no plywood is actually visible from the outside once the siding and rain screen are flush with the rest of the wall.

I was honestly looking for an additional layer of protection.

Here is a sample of the plywood box in a retro siding install.
http://www.greenbuildingadvisor.com/...nergy-retrofit

Aside from what has been suggested (marine grade plywood) what other chemical ways could the plywood be sealed as a backup to water getting behind the flashing?

[Rick Potter]

How about that spray can of rubber "as seen on TV". There is an article on building planter boxes in 'Family Handyman', which came out recently.

[ian maybury]

Back again Dan - i took a look at your link out of curiosity.

It seems like your CDX ply is not waterproof as such - but that it does have a waterproof adhesive. Sounds similar to what we call WBP over here. It's rated for exposure 1 - that's short term wetting during construction and the like.

The pictures in the link describe a totally different window installation to what we use with our concrete block cavity walls and inset windows here. It looks very much as though your ply if done that way should (presuming the outer sealing works properly) be fully sealed from exposure to water by the window frame, the various sealing tapes/flashings and then whatever waterproofing membrane (if any - it may be incorporated in the foam board insulation) and cladding is applied over the insulating foam. CDX ply will tolerate occasional exposure to water provided it can dry fairly quickly - as in e.g. a small leak. Might it be OK flashed with tape just as shown? One concern might be that if water gets in that there's no obvious venting/little opportunity for it to evaporate away?

It's not easy to reliably seal plywood with a coating. Polyurethane varnish would have to help and be convenient. Higher level approaches commonly seen are polyurethane or acrylic based tanking/membrane based systems used on decks or to seal shower enclosures (which would be a PIA to apply here) - or thin epoxy sealing resins purposely made to penetrate and seal. (google should bring up some options e.g. http://www.smithandcompany.org/CPES/ ) One concern with any coating is that it'd need to be compatible with/bond very well long term to whatever caulk/sealer was used between your ply boxes and the insulating foam, and also with the flashing tapes used on the outside. This isn't by any means a certainty and would need careful checking - lots of e.g. reactive coatings longer term ooze uncured material that could mess with the bond of a tape or whatever….

On marine ply. Marine ply stamped to an insurer's spec is very expensive ( x4 about), but there's other stuff sold which is called marine ply but not certified which isn't a lot more expensive than WBP/CDX here. ( it's maybe not much different either)

[Rich Engelhardt]

B*I*N pigmented shellac on all six sides.
Since the wood is going to be covered, its not really an exterior surface.

If I'm reading things right, the boxes aren't going to be exposed to anything & a coating on the plywood is just a "plan C" kind of thing - where plans A and B have failed.

3M also makes this flashing tape - http://sprayfoamsys.com/store/3m-4in...Ku8aAled8P8HAQ

You can apply over the plywood.
I'm redoing some windows in a brick house and they recommend installing new 2x4 frames and covering the 2x4's with the 3M tape. It's sort of a similar situation...

[Jamie Buxton]

Peel-and-stick butyl flashing tape. Way more waterproof than paint.

[Erik Christensen]

what jamie said - you can get it in different widths - best thing is it self deals around nails

[Matt Day]

Marine grade ply for residential construction?! Seriously?

Use PT ply and any of the many waterproof membranes available.

[ian maybury]

As above Matt. It's not standard, but would have been very normal for a higher spec build to use an uncertified (i.e. very little more expensive) marine ply here for stuff that may occasionally get wet - sheeting under roof eaves and the like. Definitely WBP. These days they are typically covered in PVC cladding so it's less likely to be of any great benefit.

You'll note that i suggested that the flashing/tape might do the job on its own. I was however being cautious as i'm not familiar with US specs and construction practice, and wasn't 100% sure if the ply would be exposed to the weather or not. Even now knowing that it's not i'd be quite cautious about relying totally on a self adhesive tape for waterproofing without e.g. a cladding overlay or similar. I'd also prefer to verify that it was certified for such an application and layout.

This isn't to unnecessarily raise hares, and ignore all this if the flashing is a well proven solution. I can't for one moment argue with established practice (if that's what it it is - but we've had a rash of engineered/chemically based building/supposedly modern products hit the market, achieve a high level of sales only to turn out to be troublesome over here. The trade guys that threw the stuff up and the companies that didn't properly test before putting stuff on sale are long gone by the time problems become obvious...), but for sure stock ply or even the CDX variety will run into problems if they end up soaked in water for extended periods of time.

This shoves the ante pretty high, in that the flashing can't leak so that the ply becomes wet for more than very short periods in the life of the house. I'm not familiar enough with the method of construction to know, but an important question is whether or not the very small amounts of water that might get through can evaporate/be dissipated in the event of slight leakage.

Ventilation rather than reliance on paint coatings or achieving a total seal is the strategy that tends to be used to protect wooden elements that are not directly exposed to the weather against moisture in more traditional construction designs.....

.

[Peter Quinn]

Been thinking about this, I don't like the idea of putting any type of topical coating like paint on the plywood, it's more likely long term to trap moisture under itself than protect the plywood. I figure you run the house wrap long, cut it around the box, treAt the top of the box like a rough sill, ie tack a piece of beveled clapboard to it to create a drainage plane outward, tape the house wrap, seam the corners with scraps as necessary, move on. Treat it more like a regular RO. You don't need marine grade anything on your windows, might make you sleep better if you live in a rain forest, but CDX is pretty tough, you are not at grade or below the water line, a properly flashed and vented rain screen installation should not be a problem. I think they have coatings that are rollable which act as a house wrap like tyvek if that's easier to apply than all that folding and taping, just make sure the coating can breath out and doesn't act as a vapor barior to trap moisture between itself and the wood, this will cause rot quickly.

read his for more info on appropriate coatings for your application.

http://www.greenbuildingadvisor.com/...d-applied-wrbs

[Dan Aims]

Hey guys great read. Rich hit it right on the head, I ended up putting some Redgard on the edges of the frame and will be using vycor to flash the outside.

Its a backup just in case water did ever manage to get by the flashing.