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chsfink
01-12-2009, 09:55 AM
I have a 28x32 attached garage with in-floor heat. The humidity level in the garage has been upwards of 60% this winter. I have run a furnace fan periodically to circulate the air and that seems to help a little. Today I put in a dehumidifier before I left for work, but I think it will be ineffective and freeze up as the temperature is only set at 55 degrees.

What would be a good humidity control? I am thinking of an air exchanger, but would installing a vent through the wall connected to a humistat work the same? Or would I be better off installing a fan type heater to move the air without losing heat?

Thanks for your help

Pete Engle
01-12-2009, 10:35 AM
First you need to figure out where the moisture is coming from. If you're in MN and keeping the garage at 55 degrees, it should be bone dry. If it's damp, there's a moisture source somewhere.

Some possibilities jump immediately to mind:
1. water leaks from the walls/roof (of course)
2. subslab - is there insulation and/or a VB under the slab? Are there any big holes in the slab/exposed earth?
3. What are you doing/storing in the garage? wet lumber? finishing? wet vehicles coming & going?

Once you've identified the moisture source(s) and determined that they're not causing immediate damage, you can start looking at solutions. Identifying the sources might suggest the solution itself (fixing leaks).

If the garage is well insulated, use an HRV to exchange the air with dryer air from outside. If the garage is not well insulated, the HRV will be a waste of money. Spend money on insulation first, then the HRV. Even without an HRV, bringing in some outside air will certainly dry the place in winter.

chsfink
01-12-2009, 10:44 AM
Hi Peter, Thanks for your quick reply. The moisture is most definitely coming from the vehicles. The garage is 2x4 construction with r-11 fiberglas and 10 inches of cellulose in the attic.

I try to "squeegee" most of the water and snow that falls from the vehicles, but the floor is still always wet from them.

With this info what would you suggest? Air exchanger / open a window?

Thanks again,

Chris

Riversong
01-12-2009, 04:19 PM
I have a 28x32 attached garage with in-floor heat. The humidity level in the garage has been upwards of 60% this winter...the temperature is only set at 55 degrees.

What would be a good humidity control?

If you're not experiencing severe condensation or other obvious moisture problems, then you don't have a problem. That RH is perfect.

Air at 55° and 60% RH has the same dewpoint temperature (41.4°) as air at 68° and 38% RH, which is nearly perfect for a conditioned space in winter. In other words, if you pushed up the garage thermostat to 68°, you would find the RH to be 38%. The only reason that it's reading 60% is because you're keeping the air temperature lower. The amount of moisture in the air remains the same (0.000421 lbs water/cf air, 0.27" HG partial pressure).

David Meiland
01-12-2009, 04:33 PM
Agree with Robert. Try heating the space to 68 degrees and see what the RH is then, it should be perfect. It is very common to see folks posting RH readings on these forums without mentioning the air temp.

My shop is currently 53 degrees and 58% RH. If I go in there to work and turn on the electric heater, the temp will be at 68 in about half an hour, and the RH will be around 40% I would guess. No moisture will have come or gone.

Pete Engle
01-12-2009, 05:08 PM
That's still a dewpoint of about 42 degrees. In MN, that's too high, especially for a garage where none of the equipment would suffer from lower humidity.

The good news is that if the humidity is that high, it means he's got the place sealed up pretty tight. That begs another question - if the place is that tight, do you have problems with the vehicle exhaust? Or is that pipes outside well enough to keep the air clean? Have you got CO monitors inside? If there's chance of vehicle exhaust leakage, a little bit more outside air wouldn't be such a bad idea.

I think it would be a good idea to reduce the humidity. There are some high-performance dehumidifiers on the market that operate as low as 35 degrees. Try here: http://www.thermastor.com/Commercial-Dehumidification/

If air quality is not a problem, dehumidification would be a better solution than ventilation, because you're not wasting heat. All of the energy you put into the dehumidifier will remain in the conditioned space as additional heat, and in MN that's not a bad thing. Dropping the indoor humidity to 40% or so will give you 10-20 degrees of additional dew point protection. That can make a big difference in the amount of water building up in your walls and roof.

Then again, what's your construction like? If you've got materials that are pretty insensitive to moisture, then don't bother with any of this. There's nothing necessarily wrong with 60% RH, it just increases the risk of condensation within the envelope. If your envelope doesn't care about moisture, then neither do you.

David Meiland
01-12-2009, 05:16 PM
One thing that's not mentioned is how old the garage is. At his latitude his slab will be giving off moisture for quite a few years. If his slab is sitting on poorly draining fill such as sand, and has no barrier underneath, it may be giving off moisture permanently.

Pete Engle
01-12-2009, 05:31 PM
He's wetting the slab every day. It will certainly be giving off moisture forever.

David Meiland
01-12-2009, 06:29 PM
Ah, I missed the part about water and snow coming from the vehicles. Still don't see how that in itself is a problem. Your concern is that the moisture brought in is migrating to the back of the sheathing? I agree with your point about the dehumidifier not wasting heat but is it needed in the first place? The ones I have run are not cheap to operate.

chsfink
01-12-2009, 07:55 PM
Thanks for all the info! Just a few more details: the garage is attached, built in 2001 on 5 feet of taconite tailings with drain tile. The walls are 2x4 R-11 with a 6 mil vapor barrier and 1/2 drywall. The ceiling is 5/8 type X with 10-12 inches of blown in cellulose. I have it sealed pretty tight, but have no problems with exhaust.

My main concern was condensation behind the drywall because the windows have been so wet, but after reading the replies I think I will dial the heat up to 60-70 degrees and see what happens......except not for a few days; We have high temps of -2 forecast until the weekend.

Thanks again for all the help!

Chris

David Meiland
01-12-2009, 08:00 PM
Just curious, what are taconite tailings like? Is that crushed rock?

Mark Parlee
01-12-2009, 08:25 PM
He is using in floor heat so this has got to exacerbate the problem in that it will speed the drying process of the slab through temperature assisted evaporation, much the same as a pan of water warming on the stove.
What if one were to seal the slab to eliminate this absorption and allow the moisture to be mopped up or squeegeed out of the heated environment.
Then this added moisture would be able to be controlled

chsfink
01-12-2009, 10:09 PM
Taconite tailings are the byproducts of mining the ore in our area. It is common to use this in place of sand for backfill when building. It compacts so well after watering it and running over it with a wacker a few times it does not move...at all.

Attached is a link to a major project that used the tailings this past year. These are coarse tailings. I used fine in my garage and coarse in my driveway

http://www.nrri.umn.edu/egg/tacagg/




Chris

Pete Engle
01-12-2009, 10:20 PM
Mark,

Sealing the top of the slab is a good idea. It would certainly make squeegieing (how can you possibly spell that?) and mopping more effective.

Chris,

Don't bother turning up the heat. It will just drive the moisture into the walls harder. Relative Humidity isn't really the issue, it's just a convenient way to measure moisture. The real issue is the amount of water in the air, by weight. once you know RH and temp, you can use a psychrometric chart to calculate everything else you need to know. But you really don't need to know anything else, except the dew point.

For your air, 55 degrees and 60% RH, the dew point is around 45 degrees. If you warm the air up to 68 degrees, the RH will go down to about 38%, but the dewpoint will remain the same. However, the warmer the air is, the higher the vapor drive will be, so you could end up moving more vapor into the walls.

If your poly film was well detailed, it will prevent vapor drive into the walls and stop the problem of condensation behind the sheathing. If your electric outlets and other penetrations of the exterior walls are not foamed, panned or otherwise sealed, go ahead and do that. That and sealing the floor might be all that you need.

Riversong
01-12-2009, 10:46 PM
Taconite tailings are the byproducts of mining the ore in our area. It is common to use this in place of sand for backfill when building.

And there's concern that MN taconite tailings contain respirable asbestos.

Mark Parlee
01-12-2009, 10:50 PM
Pete
Spell check is my friend (squeegeeing)

When we have the problems with concrete that does not have the poly underneath it I have started thinking why not seal the top side.
As I see it the amount of moisture in the building is the biggest problem coupled with the good evaporation device installed,in floor heat.
It does seem that it is a rather simple solution to reduce the reservoir capacity of the slab and most of the problem may dissapear.

Riversong
01-12-2009, 11:00 PM
It does seem that it is a rather simple solution to reduce the reservoir capacity of the slab and most of the problem may dissapear.

But the problem is not moisture coming up through the slab. It's moisture from the vehicles melting and pooling on the slab. Sealing the slab will do nothing to reduce this moisture load.

Mark Parlee
01-12-2009, 11:09 PM
Robert
You are correct in that but it does make moisture management easier by taking out the reservoir potential of the slab and turning it into surface moisture. He still has to be diligent with the puddles if the problem is to go away.

Riversong
01-12-2009, 11:25 PM
but it does make moisture management easier by taking out the reservoir potential of the slab and turning it into surface moisture. He still has to be diligent with the puddles if the problem is to go away.

I'll agree that eliminating the reservoir may be helpful if the surface moisture is dilligently squeegeed away.

But since that kind of maintenance is unlikely, the reservoir capacity of the slab might actually act as a moisture buffer, preventing spikes of humidity every time a car melts off.

One of the least comprehended and least used moisture-management strategies is to employ moisture-buffering with hygroscopic, high moisture storage materials. In the same way that thermal mass evens out indoor temperature swings, moisture buffers will even out humidity fluctuation.

Mark Parlee
01-12-2009, 11:50 PM
The moisture buffering capacity of the slab is what is being used now but apparently with unsatisfactory results so he does need a change, this change will require more work on his part.

Pete Engle
01-13-2009, 05:19 PM
He's also already told us that he's pretty diligent about sweeping/squeegeeing out the moisture. If he wants to lower the humidity, he'll just have to do a bit better.

Also, if the slab is sealed and properly sloped, the water from melting snow/ice will run out the door. Currently, much of that water is probably being soaked up in the slab for later release into the air. Getting it out first and fast might make a big difference.

Besides, it gives him an excuse to do one of those really pretty epoxy floors with the granite-looking chips thrown in.

chsfink
01-13-2009, 05:33 PM
Yes, I plan on sealing the floor this spring! I did leave the wet vehicle out longer today and allowed the floor to dry completely. The temp is 54 and the RH is at 43%. I guess I will need to work a little harder to move the water out. A sealed floor will definitely help.

Thanks again for all the info.

Chris

dgbldr
01-14-2009, 03:51 AM
Gents, I agree with all the theoretical advice, but I'm missing the practical part.

If this is a garage used for shelter for daily driven vehicles, why are you heating it? And particularly why in-slab heat? Seems to me all you have is an expensive and unnecessary humidifier.

If you leave the heat off, you 1. save energy, 2. avoid the excessive melting, evaporation and attendant humidity and 3. keep your car(s) from rusting.

And since it's not a living area, I don't see the point of heating the slab. If you work in there occasionally (hopefully not barefoot), a regular electric or gas radiant or convective heater will heat you faster and more economically.

I'm in a similar climate. It's 5F outside right now. My insulated but unheated attached garage is at 43F and quite dry. I'd actually like it colder, so I don't melt as much snow off the cars. If I need to work in there, I just leave the door to the house open, fire up a small portable heater for an hour, get the work done, then turn it off.

Riversong
01-14-2009, 04:05 AM
If this is a garage used for shelter for daily driven vehicles, why are you heating it?

Hey, radiant-heated driveways and walkways are the next big thing. And let's not forget the outdoor heated pool.

dgbldr
01-14-2009, 04:10 AM
And let's not forget the outdoor heated pool.

Yeah, but if they're heated by wood stoves and used for bathing they're OK :)

Actually, seems to me a couple of floor drains in that garage would have been infinitely more useful than the slab heater.

DickRussell
01-14-2009, 08:07 AM
I'm in a similar climate. It's 5F outside right now. My insulated but unheated attached garage is at 43F and quite dry.

For the house I'm designing, there will be an attached unheated garage. While that part of one garage wall won't be exposed to outside winter temperature, I don't expect it to provide much heat to the garage (R-40 wall). However, I figure on insulating the garage walls and ceiling. Also, for the garage I plan on NOT putting in sub-slab insulation, although I would insulate the perimeter of the garage foundation walls. The idea is to let ground heat moderate the temperature within the garage.

So, the question is: does anyone know of a good source of info for approximating the amount of heat potentially provided by the garage slab? From a total U*A for the garage I could make a good guess on how much warmer than outside the inside would remain.

Pete Engle
01-14-2009, 10:44 AM
So, the question is: does anyone know of a good source of info for approximating the amount of heat potentially provided by the garage slab? From a total U*A for the garage I could make a good guess on how much warmer than outside the inside would remain.

The ground will provide heat to the garage, but it varies significantly with your climate and garage foundation treatment. If you use the insulation levels provided in the IRC for a frost-protected slab, you will pretty much guarantee that the soil around the foundations & slab will not freeze. Increasing the level of soil perimeter insulation will increase the ground temperature a bit.

The ground temperature a few feet down tends to stabilize at the average annual temperature for your region. That starts to give you an idea of the potential slab temperature. Of course this is going to be affected quite a bit by heating from the basement, surface cooling outside, and any number of other factors. You'll also be concerned about edge cooling at the aprons.

If you are in a relatively moderate climate, the ground and slab will probably provide enough heat to keep the garage above freezing during relatively cold spells. My own stone garage stays above freezing in NJ unless we get a week or so of consistently subfreezing weather, and that doesn't happen all that often. And this is with uninsulated (but very thick) stone walls, weatherstripped but uninsulated overhead doors, and R40 roof.

If you have weeks of single-digit temperatures, I doubt that you're going to be able to keep the garage above freezing with only earth-source heat without super-insulating (and/or solar heating) the garage.

Riversong
01-14-2009, 11:10 AM
So, the question is: does anyone know of a good source of info for approximating the amount of heat potentially provided by the garage slab? From a total U*A for the garage I could make a good guess on how much warmer than outside the inside would remain.

Start with the deep ground temperature for your area which will be within a degree of the mean annual air temperature. Climate Data (http://www.worldclimate.com/) Then estimate the ground temperature at perimeter insulation depth by graphing the gradient between ground temp and average winter air temp, including average snow cover (R-1/in).

Then use the attached chart to approximate the apparent R-value of your slab. If you have vertical insulation around the perimeter to at least 1 foot below grade, you can probably increase the apparent R-value by 50%.

thadjohann
01-14-2009, 11:28 AM
Hello, interesting replies, Chris the origianl poster calle me with this question, and I encouraged him to visit this site. I have to add that just the other day I had another contractor in town contact me withthe same question and problem.

I have the same system as Chris, and live just miles apart. I have a sealed slab, floor heat, etc. The only difference i have is i have 2X6 walls filled with Icyenene. I did not use a vapor barrior. I have not had any moisture issues.

I have windows on my garage door, and no signs of moisture even on the windows. last night in was over 30 below.

I guess what confuses me is that the only real difference between Chris and I are the amount of insulation, and type. There is also the fact that I don't have a vapor barrior just 5.5 inchs of foam. ??????? Any mor thoughts as to why i don't have the similar problem as these two.

I haven't checked the humidity in there simply because I havent noticed a problem.

Riversong
01-14-2009, 02:55 PM
I guess what confuses me is that the only real difference between Chris and I are the amount of insulation, and type. There is also the fact that I don't have a vapor barrior just 5.5 inchs of foam. ??????? Any mor thoughts as to why i don't have the similar problem as these two.

I haven't checked the humidity in there simply because I havent noticed a problem.

You might check the relative humidity. It may be that it's staying low enough not to cause a condensation problem. The fact that you see no condensation on the windows is a good sign.

But, if the humidity level were to rise, you might have a problem at the wall sheathing. Icynene has been known to cause saturation of sheathing in high humidity conditions. The manufacturer recommends painting the inside surface of the foam with vapor retarder paint.

David Meiland
01-14-2009, 03:01 PM
I tend to think the OP's poly behind the sheetrock sounds better than the recent poster's Icynene.

thadjohann
01-14-2009, 03:54 PM
But I haven't had a problem! remember we are in a area without high outdoor humidity levels, and in the summer with the door open often there is little chance of a problem. I would think the best chance for a problem would be in the winter with the melting snow on the heated floor. Again with the cheap windows on the garage door I would think that I would have some frost. I do have some on the windows in my house when it hits -20+.

David Meiland
01-14-2009, 04:12 PM
People in construction always say "I've never had a problem". You could have vapor moving from the garage interior and into the Icynene, condensing either in the insulation or on the back of the sheathing. You might not know it for quite a few years.

You might NOT have a problem, too. I do not live or build in the kind of cold you have, so I am not used to opening up walls that live in your climate, and never have.

thadjohann
01-14-2009, 04:32 PM
I understand what you are saying, just trying to figure this out. In my case I have tongue and groove pine on one of my walls, I am pretty confident that there is no condensation forming in the walls, I would also believe that it would also show up relitively quickly on the1/2 inch sheet rock. I would also think that with no vb that a musty smell would be evident. I have worked on a lot of places with water intrusion problems and I really don't believe I have one. If some one has any ideas as to what I may be missing I would appreciate the info, like I said its been a few years now and no evidence at all.
I do also understand that the foam does have a higher perm rating, but I have worked on several places with no vb, and have never noticed a hint of a problem. That is why I felt comfortable just covering the foam.

Remember the amount of water these guys are having is pretty substantial and our circumstances are not that different. Again I am just trying to figure out why they have a problem and I don't. (Yet???)

Boy these moisture/vapor conversations can get pretty lengthy!

dgbldr
01-15-2009, 03:55 AM
The difference could be as simple as you having a leakier overhead door. Garage doors are very seldomly well air-sealed. Some seal better than others. Some are installed better than others. With winter air bone dry, a little extra leakage could make a big difference.

Or it could be you wash your car more often and therefore put it in the garage mostly without snow on it.

Or he may be heating the slab more than you do, which makes for more melting and evaporation.

Or any number of other factors. If we were there we could tell you why in 5 minutes. That's why forum discussions are an aid to, and not a replacement for, good analytic skills.