Hey guys, I've got a 12x'x20' Shed, ICF foundation, R-20 XPS reinforced slab, double 2x4 walls, 2" polyiso over OSB (I got most of it for free, so it is actually 1.5" regular with 0.5" foil faced staggered seams), 1/2" rain screen via furring strips and cor-a-vent, and will soon have fiber cement siding. So, I was always planning on a cellulose (either loose or dense) packed wall cavity, but i'm not sure if my high moisture situation makes it a good idea. My shed will house an indoor above ground pool (endless pool) for year round use. When the cover is on it is "supposed" to trap 100% of the humidity, and I'll just run a standard bathroom fan exhaust when it is in use, and a dehumidifier afterwards. We are an avid swimming family, so between all of us it is likely to get 1-3 hours of us each day. If I use DensShield or some other paperless drywall rated for moisture, coupled with the exterior grade paint on my walls, would I be ok? I think spray foam is outside of my budget, and fiberglass seems like it would be even worse. Thoughts anyone?

So, you put 2" of foil-faced iso (== true vapor barrier) on the exterior of the OSB.  Indepentently of the presence of the swimming pool this could create dew-point issues at the OSB in a heating dominated climate if the R-value of the fiber layer is too high.

Where are you located, and how much space is there for the fiber insulation layer, between the OSB and the wallboard?

Located in Utah County, Utah - lots of snow, but we do barely break 100 degrees periodically in the summer. Wall cavity is 9" deep. If needed, I could just go with the single layer of 1.5" foam (and maybe add a tyvek or similar to aide the rain screen) and not use the foil faced insulation. Either way, I know I won't be using traditional paper-backed drywall.

9" @ 3.6/inch (typical for dense-packed cellulose) is R32, 2" of iso is about R12, so you have ~R44 at center cavity, with 27% of the R on the exterior of the OSB.

It actually matters where in Utah County, since elevation can make a relevant difference in wintertime temperature averages, which is the relevant point here. But taking the Weatherspark.com dataset for Payson UT, it looks like the January mean temperature will be about 26-27F:

http://weatherspark.com/#!dashboard;a=UT/Payson

At an interior temp of 70F and an exterior temp of 26F, and a total R value of R44, that's about 1F/R. So the mean January temp at the OSB will be 27F + (R13 x 1F/R)= 40F. If the humidity of the interior air averages much above a 40F dew point, the OSB will have moisture accumulation issues, so that's the standard you have to live with. A 40F dew point @ 70F corresponds to an interior relative humidity of about 33%, which is a healthy range for humans. But keeping a pool shed at a humidity that low in winter would require a heat recovery ventilation system (HRV) running a fairly hefty ventilation rate, but running it under a dehumidistat control set to 30% would probably be the right thing to do.

If you want to buy more margin with the insulation you can either add another inch or two of foam (recommended) or cut back on your fiber layer to leave the OSB at a higher winter mean temp.

You can also buy a bit of margin by putting an air tight "smart" vapor retarder such Intello Plus or Certainteed MemBrain on the interior side under the gypsum, which will limit the rate at which moisture gets into the cavity via diffusion, but becomes fairly vapor open if the air in the cavity goes to 50% RH or higher, as it will during the warmer weather, allowing the moisture to leave much quicker than it got in, assuming you kept the interior RH under 35% most of the winter.

In all cases, using cellulose is preferable to other fibers, since it will share the moisture load with the structural wood without loss of function or damage, offering signficant protection to the cold OSB.

Without knowing the permeance of the facers on your "regular" polyiso it's hard to say for sure, but most of the time fiber-faced iso is ~0.5-8 perms, a class-II vapor retarder which would only offer modest outward drying for the OSB at best. The extra R of the foil faced layer would be more protective of the OSB than drying through thinner but still very vapor-retardent foam into the rainscreen cavity.

The overall 12 coldest weeks of winter mean temp for Payson is about 32F, which makes the mean temp at the OSB about 44F, so with an air tight interior and cellulose humidity buffer you'd still be in good shape with an interior dew point average of 44F. At 70F that correlates to a relative humidity of 39%, which may be a fairly easy target to hit with HRV ventilation only.

A standard bathroom exhaust fan isn't going to cut it- they don't really cut it for showering in bathrooms, taking many minutes to fully purge the shower moisture from the air. You need something that can really deliver 250-300cfm to keep up during swiming-time only operation, which would put a real chill on the place. But a 100cfm HRV under dehumidistat control could be left on and the spikes of high humidity would be dealt with more quickly than the room dehumidifier, without the chill.

Awesome, your detailed response is much appreciated! I had to re-read it twice, but it all makes sense to me now (i think). And yes, my "regular" polyiso is fiber faced (gray in color). I think going with the MemBrain would be a good idea - I read up on it from their site

http://www.certainteed.com/resources/30-28-080.pdf

The only thing is that it specifically warns against pools...but with mine being coupled with a cover (about 90% of the day) and an HRV it doesn't really apply to the "regular" pool conditions I gather. I'm not familiar with the dehumidistat operation, but I'll do some reading.

Also, what about when I paint the drywall? If I used an exterior grade paint would that help or hurt?

Evaporating water absorbs lots of heat and you get it back if you condense the water. So I'd like to see an analysis of the economics of winter dehumidification vs dumping the moist air outside via extra HRV flow. But I suppose that condensation also occurs in the HRV.

When it comes to mold, a few hours/day of extra dry conditions helps. Ie, it will allow a somewhat higher average humidity level.

A secondary concern of ventilation is chemical fume. There is no place for casual design with enclosed pools. I've seen buildings rotting from the inside out where this is done.

If you heat the enclosure above the pool temp you hardly need to cover it at all. I would dig very deep including into your insurance policy before proceeding. This will likely call for permits if you have an inspection authority, so you will want to research local codes. I doubt a bath fan is going to get it done. Where are you getting and how are you conditioning outside air when you replace exhaust...........etc.

It's true that the MemBrain et al become vapor open when the room humidity is high, but if you're only using it 3 hours out of 24 and actively dehumidifying to 30% it'll still be doing a world of good.

The speed at which you can dehumidify matters, which is why HRV is preferable to mechanical dehumidification alone. Joe's point is valid corrosive atmospheres can take a toll on heat exchangers, whether a standalone dehumidifier or HRV. There are commercial mini-split type dehumidifers like the DryMax designed specifically for pool sheds, but that has the potential to over-cool the space in winter. (and it's not a cheap piece o' hardware either.)

http://www.appliedpowergroup.com/dehumidifier.html

Spending the money on MORE exterior foam (taking it up to say 4-5" total so that it averages above 50F) may be a cheaper way to protect the OSB than heavy duty dehumidifiers.

So if I went with 4" total then (that's the highest I can go using 7" HeadLok screws and 1x4 furring) would I be safe using a "exhaust fan style" HRV like this:

http://www.amazon.com/Panasonic-FV-04VE1-WhisperComfortTM-Ventilation-Patent-Pending/dp/B000XJNZ1Y/ref=sr_1_cc_1?s=aps&ie=UTF8&qid=1379352000&sr=1-1-catcorr&keywords=heat+recovery+ventilator

AND use in combination with a dehumidifier. For an interior dimension of less than 200 square feet, I'm thinking this should be sufficient with dealing with the humidity and maintaining fresh air without significant heat loss. BTW, I'm not using chlorine, but instead bromine in combination with a UV/ozone system so chemical "off-gas" is almost non-existent.

The above link is actually an "ERV" @40CFM , will that been enough/make sense though?

Otherwise this seems to be the cheapest HRV I could find that would match my small space.
http://ecomfort.com/vh-704-heat-recovery-ventilator-hrv-4-top-ports-67-cfm-single-speed-12392.html
likewise only 56 CFM

An ERV isn't what you want here, since it returns a fraction of the humidity to the incoming air stream, and that's even before investigating whether it would tolerate a chlorine or bromine-rich environment. Ventilate with HRV or exhaust-only, but under dehumidistat control.

I've given you the tools in this thread to do your own dew-point math on 4" of iso + 9" of cellulose. (I could recalculate it for you but it's not hard math- I'm trying to convince people to actually learn the basics.) You need to use your real zip code in the weatherspark.com to estimate YOUR average wintertime outside temp, which may be different from Payson. To translate between dew-point and relative humidity, consult a psychrometric chart, or use an online psychrometric calculator.

http://www.dpcalc.org/

http://www.vaisala.com/humiditycalculator/vaisala_humidity_calculator.html

The average sheathing temp will be the proportional with interior & exterior R value roughly linearly with the average interior & outdoor wintertime air temps. The sheathing MUST stay well above the average dew point of the interior air. Solving for the sheathing temp, then translating that temperature as a max dew point for the interior air give you the hard upper bound on the interior relative humidity than you would need to maintain.

i'm following you, that makes sense. Let me see if I got this...

I'm in Provo (the elevation is the same and temps are almost identical day to day, and averages are the same). By using Certainteed Membrain barrier, and instead loose packing my 9 inch cavity, I'll be blocking a good deal of the moisture with the Membrain (along with a dedicated room dehumidifier to keep it from going past the manufacturers specified effectiveness) and from what I understand about dense-pack cellulose it has virtually the same R-value as loose-fill just better protection against air penetration (http://www.greenbuildingadvisor.com/blogs/dept/musings/how-install-cellulose-insulation) which saves me money on install/materials and is less important since I am use the Membrain product anyway (or am I a bit off on my logic?). Like wise, on the cold side going to a full 4" of polyiso allows about R24 of protection to the OSB bringing the total R value to a daunting R56 (if my math is right) which would bring the OSB temp in winter to about 50F? which means the psychrometic tool is tell me I'd be "safe" up to 48% humidity - a very easy target to maintain with my dehumidifier. I think at this point I'm still debating the HRV vs Exhaust - even with humidstat the exhaust is $350 cheaper (with such a small heating footprint it would be a while to see a ROI)...but the HRV is more of an "complete" solution i suppose.

Celluose R/inch peaks at about 2.8lbs density, and has slightly falling R/inch with increasing density. But with humidity cycling you'll definitely want to dense pack, since moisture cycling is the primary cause of settling in lower density cellulose.

Yes to the MemBrain, yes to the R24 polyiso.

Exhaust only is probably the way to go on the ventilation if you can keep it well under 48% in winter with the dehumidifier. Expect standalone room dehumidifiers to fail every few years from chlorine corrosion on the coil fins though.

Dana1 - sounds great! And do you think a Panasonic FV-30VQ3 WhisperCeiling @ 290 CFM controlled via wall humidistat is a good fit?

At 290cfm you'll be getting a complete air exchange every 8 minutes or so.  To achieve that ventilation rate you'd probably have to provide an an air vent on the opposite side of the shed, but even if it's half that volume it's probably enough.

Plotting permeance against relative humidity, MemBrain is fairly vapor open at 48% RH, ~3-4 perms, so dehumidifying to 35% during the colder months would be important, but you can bump that up to 40% during the shoulder seasons:

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