Drywall over plywood (and insulation)?
In my porch-to-room conversion, the house side, of course, has plywood under the original siding, and fiberglass in the cavity. All the siding has been removed, and part of the plywood has or will be — to work on electrical and HVAC for the new room. The contractor (whose judgment I had reason to question on other matters) is planning to reinstall the plywood he had removed, and apply the drywall onto that (he can still get the screws into the studs — so, that’s not a concern. I brought up the issue — a bit late, unfortunately (but I am not the professional here) of why he didn’t remove the whole plywood, exposing all the studs, and fastening the sheetrock panels directly to the studs. He will do that for me, if I want him to. Yet, I’m not sure that’s something that is worth insisting on — hence my question for folks on this forum, as I trust the forum more than my contractor (sadly).
Is there anything I gain in leaving the plywood on? Maybe sound insulation between the new room and the rest of the house (which means the family room on the same floor but also (for maybe three feet) a bathroom upstairs for example?
Indeed, even if it is not recommendable to apply the Sheetrock over the existing plywood, should I leave that fiberglass in the cavities, maybe even replace it with 3” safe and sound, which might work as a bit of a sound insulation.
To be clear, I don’t have any special concerns regarding sound. I just don’t want to have my contractor remove stuff (the plywood or the plywood and the insulation), if it doesn’t constitute a problem, or might even turn up being beneficial. This is new construction around an existing structure, and I’d like to, so to speak, use what was already there when it is to my advantage, but go with the “new construction” rules wherever it is needed. (By the way, I do know that the new room pushes the thermal envelope outward, where it used to be a porch.)
Thanks for any feedback!
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Without knowing the exact details of your home construction, I would lean to leaving the plywood.
a) It will make it really easy to hang things on the wall - no need to hunt for studs or use anchors.
b) It may play a structural role in preventing racking of the house.
c) There isn't really any harm in leaving it.
Thanks, Iwatson! Actually, all three of your considerations are relevant and enlightening. I had not even thought of a) and b). I was hoping c). This really helps.
The only reason to remove it would be to see what's in the walls, and do any necessary remediation. To add to the benefits iwatson posted, a layer of plywood on the interior also acts as a variable perm vapour-barrier.
Thank you, Malcolm! I’m definitely going to leave the plywood and the existing fiberglass insulation. I’ll likely replace some of the fiberglass with some of the Safe ‘n Sound stuff that I happen to have, in those areas that have been made accessible.
For my own learning: what is a variable perm vapor barrier (though I sort of know), and what are their benefits? Is it by any chance more beneficial because variable? (If you have time to answer those questions. They’re really just for my learning. Thanks!)
A variable perm vapor-retarder is a material that becomes more vapour-open when it gets wet. So it responds to the conditions it finds around it to enable the wall to dry when necessary. You can buy membranes that perform like that too: https://www.certainteed.ca/building-insulation/products/membrain/?gdpr_accept=true
The plywood may also be acting as a shear panel or structural sheathing to provide racking resistance (studs falling over like dominoes) to the wall or side of the house. If the plywood is acting in this way, it's best to fasten it directly to the studs.
I don't think there is any downside to leaving the plywood in place, and even if it's not needed structurally, it's still beefing up the structure of the wall.
I've been doing structural repairs all this week, so I'm in structural-think mode today :-)
In some European countries it's standard to install plywood on the interior before hanging drywall.
Thanks, Andyfrog! That's quite interesting to know.
Bill already mentioned it. But I'll repeat. It is VERY possible that the plywood is providing essential shear resistance in your walls, necessary for surviving high winds or earthquakes. Removing it could be disastrous.
Your builder may or may not know this. Clearly he didn't mention it to you if he does know it. But in my book, nobody should be allowed to become a contractor if they don't have such basic knowledge.