moisture resistant drywall in a tight house?
BrunoF
| Posted in General Questions on
I am almost ready to insulate and drywall my house in the mixed humid climate 4a of central NC. One of the drywallers has recommended that I use moisture resistant drywall in the bathrooms but I am concerned that it could trap moisture since I am relying on my walls to dry in and dry out depending on the season. I am also concerned that if I tile over the moisture resistant drywall that it will have even more trouble drying in.
for surfaces that will be either painted or tiles on a wall assembly meant to dry in and out, is moisture resistant drywall necessary?
Thanks!
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Replies
Moisture resistant drywall absorbs less water, the paper facing is replaced with non-absorbent plastic.
Drywall is only an appropriate substrate for tile if the tile won't get wet. If it's wet the substrate needs to be cement backer board or the like and there needs to be a separate waterproofing layer.
Tile is not vapor permeable, if a wall is tiled it needs to either be able to dry to the exterior or composed of impermeable materials.
So if I’m using backerboard and tile on a particular wall, the drywall is moot as far as moisture transfer is concerned anyway?
You use moisture resistant drywall everywhere in the bathroom except where you're putting tile that will get wet, like the shower walls. There you put cement board and then the tile.
I agree with your subcontractor and put in a good exhaust fan.
nynick,
What is it about a bathroom that necessitates special drywall on walls that won't see wetting?
It often sees very high humidity. Like 99% RH, on a daily basis.
But DC, that's for very brief periods. That 99% should be cleared in minutes by the vent fan, and a layer of latex paint as a moisture retarder ensures the drywall is not significantly wetted and is able to dry fully. MR drywall cannot compensate for bad ventilation.
AndyCD,
That's my feeling. You need to deal with exhausting the moist air, not trying to create a room that is designed to deal with excess humidity. I don't have strong feelings either way though. Maybe it's a good belt and suspenders approach.
In my thirty years of remodeling experience, I find MR drywall to be a waste of money. First of all, what is sold as moisture-resistant is NOT plastic-faced in place of paper, that's a different animal altogether. It's merely normal paper-faced drywall with some treatment for mold resistance; perhaps the gypsum recipe is also modified slightly. My sense is that if you have a problem with moisture that degrades regular drywall, then your problem is far more grave than MR can handle.
When someone asks me about mold in the bathroom, it's usually on the walls and ceiling above the shower, and it's invariably on the surface. What's molding is the dirt and scum sitting on the paint, NOT the drywall facer. Drywall typically molds and falls apart at the edges of tubs and showers, where bulk water has cascaded. MR deteriorates here, too.
Also, the most common substrate for professional bathroom wall tile is the Schluter system and copycats. The manufacturer allows installation over regular drywall; the membrane is fully protective. To enhance stiffness I sometimes use Fiberock gypsum board, but never cement board. Ain't nobody got time for cement board, but YMMV!
Is it really that much more expensive to put MR drywall instead of regular drywall in the bathroom? A few hundred dollars for belt and suspenders approach?
I've had mold in a bathroom once. No fun. It was on the ceiling and high on the walls and not above the shower. I put in a second exhaust fan near the shower and painted the entire room with a mold resistant paint like KILLZ. Haven't had a problem since.
Was it the extra fan? The paint?
Who cares? It's gone.
Sure, why not, belt and suspenders. It's just that I'm wary of the upcharge for a product that appears to be different only in color, and the OP's query was "is it really necessary?" My field observations tell me that the other factors have more sway, to the point that MR or not simply doesn't matter.
I think it is a fair question as to whether the drywall sold as "mold resistant" really does anything. As for the upcharge, it's about $6 a sheet more than regular at my local Home Depot, and what does a bathroom take, 5-6 sheets?
I live in a place where humidity is often very, very high, and my experience is that mold will grow on any surface if it's wet enough -- plastic, metal, stone, concrete, even glass. That said, it favors surfaces that can hold moisture. And the facing on unpainted regular drywall is pretty attractive to mold.
Does painting it seal it enough to block absorption of moisture? Probably. Does using a sealing primer help? Probably too.
DC,
Hope you are on the mend!
Take care, Malcolm
Thanks.
nynick,
"Was it the extra fan? The paint? Who cares? It's gone."
The problem with addressing high humidity by using surfaces that are resistant to moisture is that excess humidity is bad for the whole house. Consistently running with high interior RH makes building assemblies less safe, and more prone to damage.
If you are going to spend extra money there’s probably a better product than moisture resistant drywall. I have seen bathrooms with tile from floor to ceiling and bathrooms with drywall everywhere but shower, and that use to bubble and mold. Tile seems to work best everywhere but they may be overkill or outdated
Moisture resistant drywall is such a small up charge (17%--just checked my supplier) for the relatively small areas it goes in that it doesn't warrant value engineering out of the equation, in my view.
Moisture resistant drywall may contain mold inhibitors such as sodium pyrthione (https://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/sheetrock-mold-tough-faq-en-WB2419.pdf).
I don't think of it as avoiding mold from a persistent environmental issue. I think of it as a little extra insurance in case of an unexpected event such as a leak that goes undetected for a bit.
Others may choose differently, but $20-40 per bathroom is worth it to me. Heck, I could easily see someone spending $20-40 worth of time going back and forth with a drywall sub to convince them you didn't want moisture resistant drywall in a bathroom.
If you're really concerned about water, paperless drywall is a fantastic product. Just know that it's rather expensive and requires a skim coat if used in a space where you actually want a smooth wall.
Great info folks! This is new construction so I will have plenty of ventilation. Two moisture sensing Panasonic fans will be in this space (one near the shower and one extra code required one in the water closet)