Bathroom remodel - Looking for advice on drywall and vapor barriers.

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ourzoo

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After 50 years, it's time for my tired old bathroom to be renewed. The room is only 5' by 7', with a sink, toilet and tub/shower. Five of us live in the house with five showers per day - lots of moisture.

I plan go back to the studs and start fresh, but I'm not sure about how to approach wallboard and vapor barrier selection.

A bit of background: One wall of the bathroom is on an outside wall (southwest) and the ceiling is exposed to the attic. I will have a 70 CFM ventilator fan in the ceiling. There is a double hung window in the tub/shower area. The outside walls are standard 16" OC 2x4 framing. Ceiling lumber is 16" OC.

After reading this and many other forums, I have some ideas about wall material selection but I don't have the whole picture. I'd greatly appreciate any thoughts you have on this. Here's what I'm thinking:

- For the tub area, prior to tub installation, insulate the outside wall with R13 insulation, slice the insulation's vapor barrier, and install a poly (moisture resistant rated) vapor barrier from floor to ceiling. Run the poly vapor barrier continuously on all three walls in the tub shower area. Install the tub and use HardieBacker anywhere tile will be installed in the Tub/Shower area.

- In the tub area, above the HardiBacker (about 2 feet), to the ceiling, install 1/2" mold resistant drywall?? Question - will water vapor pass through the drywall and condense on the poly vapor barrier, destroying the wallboard?

- For the ceiling, install a poly vapor barrier, and install 1/2" mold resistant drywall. Install insulation batts from the attic side.

- For all other areas in the bathroom, install 1/2" mold resistant drywall over studs with no moisture barrier.

Because of the heavy bathroom use, I will likely install tile on the walls outside the tub area up to about 48" from the floor.

Am I on track? Also is there any need for special paint for the walls to block moisture?

Thanks - r/Tom
 

Jimbo

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While 70 cfm meets building coded for that small room, you will be much happier getting a whisper-quite panasonic in the 100 to 120 cfm range.
 

Jadnashua

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If you ventillate well, you don't need the mold resistant drywall outside of the tub/shower area...a good primer and paint should be sufficient. I'm an advocate of surface waterproofing in shower areas. There are paint-on and applied membranes. An example of each would be RedGard by Custom Building Products, and Kerdi by Schluter systems. A properly applied surface waterproofing would protect the wall board and insulation from any water vapor from the shower. I'd consider tiling to the ceiling in the tub/shower area and maybe even tiling the ceiling. The waterproofing needs to go up at least the the level of the showerhead. Depending on the tub you choose and the shape and size of its tiling flange, you may find it works well to either notch the studs slightly or shim the wall out so that when you apply the cbu, it clears the tiling flange. This should get down to about 1/4" or so from the horizontal tub surface. You'll get lots of tiling help at www.johnbridge.com. And you can check out Redgard at www.custombuildingproducts.com and Kerdi at www.schluter.com. Noble and Mapai and Latticrete make their own versions of many of those products.

Also note that if the walls aren't plumb and straight, sistering a stud, when required, will make tiling much nicer. Also, industry guidelines call for caulk at any change of plane or materials. If you'd like to build a shower without caulk, various companies make expansion joints for the corners and transitions. Schluter has a wide variety in various colors and material finishes. You'll find that corners install much better if the corners are square and plumb, though. Something to consider, since it may be another 50-years before it gets done-over again...nice to not worry about caulk in that timeframe!
 

Dana

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What climate zone is this, and what sort of exterior siding do you have? What are the interior wall finishes on the rooms adjascent to the bathroom?

People confuse waterproofing with vapor retardency all the time, and the differences matter. A surface can be water proof to liquid water, yet pass water vapor. Putting a strong vapor retarder like poly in the wrong place in a wall stackup can make matters worse, not better. Even using vapor retardent paints in bathrooms can result in premature failure of the paint (or even plaster) due to moisture trapping.

Ventilating the bathroom with humidistat control to keep it under 50% (in fall/spring), or 35% (winter), and using vapor-permeable materials with low capillary draw would be a better bet.

Use waterproof grouts (or sealers) your tub-surround tiles, but let the interior walls release vapor into the wall cavities as-needed. If your climate is 3000-7000 heating degree-days (base-65F) use blown cellulose rather than fiberglass in the wall cavity, and semi-permeable 3/8" XPS rigid foam sheathing (pink or blue) between the tile backer and the studs on exterior walls. The XPS will allow the wall assembly to dry in both directions (assuming you don't have an exterior vapor retarder), and the cellulose would keep any temporary condensation issues within the wall from ending up in the structural wood. Water vapor can and will get throught the drywall, but it can't condense on anything that's warmer than the dew point of that air. If it's condensing on the bathroom walls, it's condensing anywhere it can get into the wall but wallboard & tile backer (and cellulose insulation) can absorb & release large amounts of water without damage- it's the wood you have to take care of. XPS foam won't wick any short-term condensation into the wall, but will allow the cavity to still dry inward via vapor diffusion during the 90% of the time the shower & tub ISN'T in use. (In 8000HDD+ climates you may need a poly vapor retarder, but that would be true of all rooms, not just the bathroom.)

On the ceiling below an attic a poly vapor retarder might be called for, but air-sealing around any fans, or electrical/plumbing penetrations are far more important than vapor retarders for avoiding attic condensation issues. (Spray foam is your friend here!)

Use only vapor permeable or semi-permeable latex paints (most latex is fine at 2-3 perms unless otherwise specified) and the whole thing will be happier- avoid vinyl or foil wall coverings in the bathroom or on in the rooms on the other sides of the bath to allow any water that finds it's way into the wall via capillary draw or vapor diffusion to escape in both directions. (If you're feeling ambitious or rich, filling those cavities with cellulose will give also it considerable hygric buffering, while reducing noise transmission. Use only borate fire retardent cellulose though- the sulfated fire retardents can corrode metals should sufficient water get into the walls. Using wet-sprayed cellulose is fine, even if dry-blown, since it's always borate-only. Wet-spray goods contain a water-activated adhesive, but that's relatively unimportant if installed at 3lbs/ft^3 density or higher, and for a paritition wall it's no disaster if it sags 3" in 30 years at low density.)

Most moisture issues in bathroom wall & ceiling surfaces are from capillary draw into walls, not vapor diffusion. Going with higher-gloss paints & finishes, waterproof grouts, and hard-fired or glossy glazed tile helps. Keeping it all vapor permeable allows the water to get back out without damaging anything rather than getting trapped in mold-food (wood/paper) since the surface condensation and splash aspects are temporary conditions. (Cellulose insulation is paper, true, but it's fire retardents are also mold inhibtiors and there is a dilution factorin minor wetting incidence since it distributes liquid moisture quickly.)
 

ourzoo

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Great Advice!

All - Thanks for the advice. I've learned quite a bit.

Jimbo - I was thinking about a larger and quieter fan but there's a definite privacy issue with a small house: bathroom sounds that you might want to mask. Seems for this house the more noisy the fan the better. But thanks for the recommendation.

jadnashua - Good idea about tiling up to the ceiling on all tub surround walls. I'll do this. And thanks for the reminder about sistering studs to straighten out the walls. Dana suggested a 3/8" XPS board for the outside walls as a backer for the hardibacker. I'll do this which will allow the backer board to pass over the tub tiling flange. Good info and thanks for the time.

Dana - I'm in a 4500 HDD area. This cape-cod style house was built in 1955, has a brick exterior on the lower level, but siding on the shed/raised roof on the second floor where the bathroom is. The sheathing is a 1" black celotex board with Tyvek installed over top and vinyl siding on the outside. The rooms adjacent to the bathroom are all drywall with latex paint finishes (no wallpaper).

I need to re-read your excellent post, but what I'm taking away at first read is to install extruded polystyrene foam board behind the tile backer on the exterior wall and backfill with borate fire retardant cellulose. For the ceiling, I think I'd like to use the same 3/8" XPS board as a backer for the drywall with no poly vapor barrier. I really want to make sure I don't have any cold spots on the ceiling which might prompt mold growth inside. An install of the XPS over the truss bases, with fiberglass insulation above seems like it will do the trick.

Also thanks for the finish recommendations. We already planned to go with glossy tile in the tub surround and will lean toward the glossy finish paints as well. Thanks for adding some science to the explanations. Now it's time to re-read to make sure it all sinks in.

VR/Tom
 

Dana

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In a 4500HDD climate you don't need (and shouldn't use) anything NEARLY as vapor retardent as poly (~0.05 perms). Asphalt loaded kraft backers on batts are ~0.4perms. By contrast 3/8" XPS runs about 1 perm (still a minimal class-II vapor retarder, but far from a vapor barrier), which should be about right. Odds are you don't have enough hours in a year below 37F (the dew point of 70F 30%RH air) to accumulate much wintertime moisture in the wall or ceiling even without specific vapor retarders (3-perm latex paint is enough), as long as the interior surfaces are air-tight and there are no exterior vapor retarders. (Watch out for those leaky recessed light can lights though, the cause of much attic mold and heat loss.) If you have the space and want the extra R-value you can go up to about an inch of XPS without it inhibiting inward-drying too too much. (3/8" goods are about R2, 1" s about R5.)

Celotex fiberboard (not to be confused with similarly named iso insulation) as well as Tyvek are fairly high-perm- it'll dry toward the exterior just fine (as weather conditions allow). A ~1-perm no-wicking layer on the interior and some blown cellulose limiting intra-insulation air-movement & infiltration as well as moisture buffering will keep peak humiditiy events in the bathroom from ending up in the wood to create a mold problem. If you want to go all Mr. Science on this you could model the stackup with WUFI using your local weather data to assess mold & rot-risk on different surfaces, but I have confidence you'll be fine.

If your attic insulation is standard-density fiberglass batts at levels less than R40, a 3" overblow of cellulose (about an addtional R10) will usually improve both summer & winter peak-load performance of the fiberglass layer, which would otherwise lose R-value when it's coldest out, and absorb radiated heat into the upper layers when the roof deck is over ~115F (unless you have radiant barrier at the roof deck). The effect is beyond just the R-10 you're adding- it fills in any voids or gaps, and reduces wintertime convection losses that occur in standard density fiberglass by a huge amount. Under hot roof decks the hottest part of exposed fiberglass batting is about an inch or so into the insulation- it's actually hotter than the attic AIR, which means you're effectively insulating against a higher temp with a couple inches less insulation(!). With a cellulose overblow radiated heat doesn't get even 1/8" into the cellulose, and while it's surface will get somewhat hotter than the attic air under a hot roof deck, convection at the surface limits how hot it'll get. It's usually cost-effecitive if the attic batt insulation is in the R30-40 range, less so if you're already at R50+ on the fiberglass. With trusses you often can't go hog-wild with deep cellulose due to chord loading issues, and fiberglass has much better insulation value per POUND, but just a little bit of cellulose can go a long way, if you're in the mode of re-installing the batts.
 

ourzoo

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Dana - Thanks for the additional info. and the analysis. You've provided me with quite an education on the subject!

I'll likely stay with the 3/8" poly on the ceiling, though the 1" is tempting. The bathroom fan I've purchased has a side port for the exhaust which doesn't tolerate much of a build-up of wall material. The 3/8" XPS and 1/2" drywall will max out the mounting depth allowed and still allow me to connect to the duct, although I could build-up a collar to extend the face of the fan unit. I'll have to look into this.

The vapor ratings are new to me. I understand how these act independently, but not so sure how they act in layers. For example, if a poly layer is 0.05 perm, I assume that two layers are still 0.05 perm since whatever passes through the first layer can then pass through the second layer. When talking about a layer of paint (3 perm) on drywall, then a 3/8" layer of XPS (1 perm), then include the vapor capacity (if there is such a term) of the drywall and the XPS.. wow-it gets complicated fast. I'll have to try out the WUFI software just to see how the numbers play out for different construction.

Also, OK on the overblow of cellulose in the attic. The actual thermal profile/performance in the bats is new to me. Seems like a little known fact. We do have about R30 in the attic so it looks like there's room for improvement. I'm willing to experiment and give it a try.

Thanks for the advice, I've learned a great deal. I'll have to find a good book on the subject describing the science, math, and materials behind all of this. Cookbook solutions are good, but it sure helps to understand the theory so that it's done right.

VR/Tom
 

Dana

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It's not exactly linear, but perm ratings are more or less additive. The ASTM tests determine how much water gets through the layer per unit area per hour under the specified vapor-pressure:

1 perm=1 grain of water vapor / ft^2 of area / inch of mercury

If you have 2 layers of 1- perm matierial, the total permeance of the stackup is ~0.5 perms, but not quite, since the vapor pressure at the secondary layer has been reduced by the first layer. Under the various ASTM tests a 2-layer stackup of 1 perm each will test out somewhat higher than 0.5 perms, but still way less than 1 perm.

Anything less than 10perms is considered semi-permeable, a minimal (aka "Class-3") vapor retarder. Anything under 1 perm is considered semi-impermeable (aka "Class-2") vapor retarder. Under 0.10 perms is a vapor barrier (or "Class-1 vapor retarder"). It can take YEARS for macroscopic amounts of liquid water to dry through a 0.05 perm vapor barrier at normal temperatures & pressures.

In a 4500HDD climate you're typically looking at only a few days/weeks of wintertime humidity accumumlation risk in an assembly, and the amount of water that can get through even a 5-perm surface isn't going to be enough to cause a problem- as outdoor temps rise, the vapor pressure in the water that accumulated in the assembly also rises, and it drys through the same walls that it came in on. But in 7000HDD+ climates temps inside the assembly can stay below the dew point of the interior air for months, and a higher level of vapor retardency is called for. (Canadian national building code spells out 6mil poly vapor retarders, but that's not always the best approach in milder Canadian climate zones where weaker vapor retarders would do.)

Building Science Corp has a bunch of primers on these subjects available online, some more technical than others. The most basic easy-reading version lives here.

The key to using vapor retarders correctly is to grasp that

A: water gets into building assemblies by many routes

B: water only gets OUT via vapor diffusion

Blocking the primary routes in means making it air & liquid-water tight and inserting capillary breaks to stop wicking. Poly can do this, but it also blocks the escape route. Spray polyurethane and un-faced rigid board foam insulation can be good air & liquid barriers that also allow drying, as long as they aren't too thick. With rigid-board insulation it's important to seal the seams with spray foam/FSK-tape/caulk whatever to ensure air-tightness. Staggering it's seams with the gypsum board helps too, as does glueing the gypsum to the foam at the edges with foam-board constuction adhesive (standard construction adhesive solvents can attack the foam- get the right stuff.)

Using the thinnest possible XPS as a capillary break & minimal vapor retarder on the ceiling is fine- there's no real advantage for going higher in your climate. 3/8" XPS sheathing can usually be found at the big box stores in both 4x8 & 4x9 sheets, or sometimes even bigger fan-fold form. To meet code all foam insulations need a thermal barrier against ignition (1/2" gypsum works, as does most tile-backer, etc).

The air movement performance losses of low-density batt insulation at high delta-T has been studied in depth at least since the early 1980s- it's well known in the industry (and lots of foam & cellulose vendors try to capitalize on it.). High density batts (sometimes called "cathedral ceiling" batts) have far less of an issue, and some of the newer best-in-class sprayed fiberglass (eg JM Spider) are even better. Sprayed at over 2lbs/ft^3 density Spider has comparable resistance to air movement as cellulose and slightly higher R-value per inch but still no hygric buffering capacity. (The specified standard sprayed densites for Spider are only 1lb & 1.8lb/ft^3 though- 2 or 3lb densities have to be quoted as special "dense packed" installations in much the same way as dense-packing cellulose.) In terms of R-value fiberglass is pretty good, but half-pound density (open cell) spray foam will beat that, and forms a perfect air-barrier as well (at a slight price premium).
 

ourzoo

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The link you provided gave some great examples of vapor movement and the end results of bad decisions. The shuttle example is especially significant.. you'd think the engineers would have thought that one through when they changed the outer coating of the fuel tank.

Thanks for mentioning the FSK tape and suggesting the foam friendly adhesive. I wasn't sure of the best means for closing the seams. Looks like I have a few options to play with.

OK on the (almost) halving nature of vapor barriers. This makes sense given the units/definition of a perm.

Well, I have to say that this discussion has completely changed my thinking about the use of vapor barriers. There's quite a bit to consider and definitely not one solution even within the same house. Varying exterior surfaces can make a big difference. Then adding geographical location throws in another twist.

After the bathroom is finished, I'll need to rethink my basement insulation using the principles you've presented. Good stuff!

Thanks again! VR/Tom
 

Dana

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There are several "gotchas" on basment insulation too. Below grade foundation walls have significant vapor pressure from groundwater, but if you don't allow it to dry toward the interior the capillary action of the concrete will draw water vertically upward, potentially rotting the sill or causing efflorescence on the above-grade exterior side of the foundation (or both.) If you use a studwall & batt solution, the batts must be unfaced, but you still run the risk of wintertime moisture accumulation in the studs creating mold conditons.

A better solution is to use some amount of semi-permeable foam-board between the studwall & foundation along with unfaced-batts. Unfaced EPS (beadboard) is cheapest, but easy to damage. Using an inch (or up to two inches) of XPS sheathing (sealed at the seams, of course) behind a 2x3 R8 or 2x4 R11-R13 studwall can get you a fairly high R with reasonable permeability. With the foam between the studwall & cold foundation the studs will stay warm enough to not condense moisture from the interior air during the winter, and the somewhat-permeable foam keeps the foundation from saturating and driving moisture into the foundation sill.

Insulating the band-joist is also something where it's much better to do with foam in an air-tight manner. Cut & cobble with XPS works if you're religious about sealing the edges, but it's often easier to just calculate how many board-feet of closed cell spray polyurethane foam it would take to get there, and buy one of those 2-part kits (FrothPak, TigerFoam, Fomo-Foam), or higher a pro (figure on ~$1.25/board-foot, installed from a pro, if it's over 1000 board-feet, which is about what the larger kits cost at 600 board-feet.) An inch of ccSPF is ~ R6/2 perms and at 2" it's ~R13/1 perm. If you used fiberglass it would need to have a vapor retarder and perfect air-sealing on the edges (good luck with THAT!). But with an inch of foam against the band joist you can use kraft-faced batts to raise the R and be a little less fussy about how perfect the air-sealing is.

Building Science Corp. has done a whole bunch of real-world testing of basement insulation stackups, and have a good set of recommendations here.
 

frmrnyker

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Have you considered using roxul insulation ? Might be overkill for your area but more moisture resistance than fiberglass and recycled from lava stone, they are unfaced, and the sound deadening is phenomenal. You could proceed with vapor barrier and backer board or look into the Kerdi shower which would let you use drywall. Im no expert but currently using these products and have found them user friendly...
 
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