Insulating / finishing an unheated garage?

[TimberMan]

I am not planning to heat or cool my barn / garage build but would like to condensation. Would simply using plastic under the concrete slab and insulating the wall cavity / ceiling be enough to keep any condensation that would occur at night on the outside of the building?

Thanks

 

[AEAdam]

I am not planning to heat or cool my barn / garage build but would like to condensation. Would simply using plastic under the concrete slab and insulating the wall cavity / ceiling be enough to keep any condensation that would occur at night on the outside of the building?

Thanks

No. The moisture is in the air. You would need the barn to be air tight to stop condensation. The easiest way to protect metal from rusting is to heat it, to stop water vapor in the air from condensing on it.

You should put a moisture barrier under your slab to prevent ground water from permeating it, regardless of your condensation concerns.

When I did my house slab I did 4” of compacted gravel, 6 mil heavy poly film, taped off, then 4” of rigid foam insulation, then the slab. I also taped off the top layer of foam. That floor has been nice and dry. But in the spring, when the weather suddenly gets warm, and the floor remains cold, water will condense on that cold floor. I think no amount of weather proofing will stop that. You need your barn to be warmer than the dew point.

My advice is to run 1/2” HE PEX in any and every new slab. Estimates in my area were astronomical. I ran all the pipes in my 1800SQFt building in about 1-1/2 days. It would have been one day if the weather was warmer. I installed a light duty 6” steel mesh, and just used clips to clip the PEX to the steel. It’s was very easy to do. It was a couple hundred dollars worth of PEX.

You can heat the water in a number of easy and economical ways anytime in the future.

 

[TimberMan]

No. The moisture is in the air. You would need the barn to be air tight to stop condensation. The easiest way to protect metal from rusting is to heat it, to stop water vapor in the air from condensing on it.

You should put a moisture barrier under your slab to prevent ground water from permeating it, regardless of your condensation concerns.

When I did my house slab I did 4” of compacted gravel, 6 mil heavy poly film, taped off, then 4” of rigid foam insulation, then the slab. I also taped off the top layer of foam. That floor has been nice and dry. But in the spring, when the weather suddenly gets warm, and the floor remains cold, water will condense on that cold floor. I think no amount of weather proofing will stop that. You need your barn to be warmer than the dew point.

My advice is to run 1/2” HE PEX in any and every new slab. Estimates in my area were astronomical. I ran all the pipes in my 1800SQFt building in about 1-1/2 days. It would have been one day if the weather was warmer. I installed a light duty 6” steel mesh, and just used clips to clip the PEX to the steel. It’s was very easy to do. It was a couple hundred dollars worth of PEX.

You can heat the water in a number of easy and economical ways anytime in the future.

What part of the country are you in; it sounds like it may be much colder there?

 

[AEAdam]

What part of the country are you in; it sounds like it may be much colder there?

1 hour southwest of Philadelphia Pennsylvania (South East PA. It's shown under my user name. You should add yours so people know where you are and can provide better answers.

 

[66Caprice]

Air circulation is very helpful as well.

 

[Lou's Garage]

My tubular steel building has a vapor barrier under the slab with R-5 insulation in the walls and roof. The insulation slows the temperature change enough so that there is never condensation on the inside of the building. A dehumidifier keeps it dry and comfortable except on the very hottest and coldest days here in SC. I store vehicles, parts, tools to work on the vehicles, and use it as my "basement." After 5 years there are no ill effects from excessive humidity. I'm not sure where you live but my experience is what you might expect in the "Upstate" near Greenville, SC.

Lou Manglass

 

[AEAdam]

Keep in mind, condensation occurs when warm moist air finds its way to cold objects. The cold surfaces chill the warm moist air causing the water molecules in the air to condense into liquid.

Thermal insulation doesn’t really help that and can kinda cause the problem. If the contents of your barn were always the same temperature as the air, you wouldn’t get condensation.

A moisture barrier can help. On those early spring days when it’s been 30-40F and suddenly we get a 60degree humid day, I won’t open the door to my stone machine shop. What I do now is install a small space heater to get the machines up over the dew point.

Removing humidity with a dehumidifier is another option. You have to figure out whether it’s more economical to remove the humidity or live with the humidity and heat to avoid condensation.

 

[racecougar]

What climate/location are we working with here?

 

[TimberMan]

I am in central NC, Raleigh area

 

[racecougar]

Insulation is going to help, but with your typical humidity and dew point vs temp swings, you'll very likely need a dehumidifier at least to manage humidity in your building.

 

[dcg9381]

Would simply using plastic under the concrete slab and insulating the wall cavity / ceiling be enough to keep any condensation that would occur at night on the outside of the building?

In TX our slabs are typically done with a plastic moisture barrier.

It's fine to have condensation on the outside of the building. That's no problem. That's what exteriors are for. Any time the building is surface is cooler than the moist warmer surrounding air, condensation is possible.

Usually the concern is moisture or condensation on the inside of the building. I've seen that happen a few times in uninsulated steel sided buildings when the temp inside the building is lower than the outside and it's humid. If you want to avoid this, you need to insulate and/or dehumidify. Visible moisture on the inside of the uninsulated steel building didn't happen very much in my climate. Maybe once a year or less. YMMV.

 

[racecougar]

I think the OP meant to ask if the underfloor barrier and wall/ceiling insulation would prevent interior condensation. That's the way I took it, at least.

 

[WisJim]

Remember that the only time to insulate and vapor seal under the floor is before the concrete is poured. You can always insulate the walls and ceiling later.

 

[TimberMan]

I think the OP meant to ask if the underfloor barrier and wall/ceiling insulation would prevent interior condensation. That's the way I took it, at least.

sort of...What I meant to ask was if in a mild climate like central NC where our frost line is only 12" deep, will there be enough thermal mass in my slab to keep the inside of a sealed and insulated barn above the dew point. I have never owned a detached garage / barn before but the attached garages I own / owned were all insulated and never had condensation but they were climate controlled above and next to them so not apples to apples with my new structure.

 

[AEAdam]

sort of...What I meant to ask was if in a mild climate like central NC where our frost line is only 12" deep, will there be enough thermal mass in my slab to keep the inside of a sealed and insulated barn above the dew point. I have never owned a detached garage / barn before but the attached garages I own / owned were all insulated and never had condensation but they were climate controlled above and next to them so not apples to apples with my new structure.

Yeah I think no. I suspect the opposite is more likely. Even an insulated slab will probably stay pretty cool all winter in NC.

When spring comes, your slab will probably sweat with condensation. When that day comes the solution is to heat your slab to dry it, by opening your doors, rendering your insulation worthless. A humidifier will work by removing water vapor, but will only work as well as your vapor barrier and door seals allow.

Remember, insulated or not, because it’s unheated, your slab will roughly be the average of the Winter air temp. And depending on your windows you probably wont get the benefit of passive solar heating. So your slab in winter will likely be closer to the daily low temp than the high.

You can do what I do. No need to heat your space in the dead of winter, but when spring comes, it would be smart to plug in a space heater to bring the interior temp up in line with a warm humid day.

No amount of insulation or vapor barrier will stop condensation. The root problems are the temperature swings and that your space is uncontrolled.

I think lowest possible cost and effort would be insulate and heat the slab and only insulate the roof.

 

[mm08822]

Simply put, the surface temperatures have to remain above the dew point of the enclosed air. Heating your floor is a good way to raise its temp and that of anything else in the enclosed space.
Adding a fan will help circulate the air in the space and any that infiltrates. This will disperse the more humid air infiltrating with that of the dryer hotter air.

Heating may be better than running a dehumidifier.

 

[TimberMan]

Thx folks. Concrete went in last week so it’s too late for in-floor heat but I am considering insulating and getting a cheap mini split for heat and cooling / dehumidifying in the summer. I am only hesitating due to budget creep since climate control means Insulation & drywall which is more $$

 

[finn]

The garage that I owned with the worst propensity to slab sweating is an unheated with a thicker than typical slab, ie at least 6” and maybe more, compared to the typical 4”.. it didn’t help that the door faced north, so there was no passive solar to help heat the slab.

That slab would sweat from May until early July.

The radiant floor heat keeps the shop floor dry, and my regular house detached garage has windows that get sun to the floor, plus there’s foam under the slab, so a little heat from the overhead propane heater in the spring seems to keep that floor dry.

 

[TimberMan]

The garage that I owned with the worst propensity to slab sweating is an unheated with a thicker than typical slab, ie at least 6” and maybe more, compared to the typical 4”.. it didn’t help that the door faced north, so there was no passive solar to help heat the slab.

That slab would sweat from May until early July.

The radiant floor heat keeps the shop floor dry, and my regular house detached garage has windows that get sun to the floor, plus there’s foam under the slab, so a little heat from the overhead propane heater in the spring seems to keep that floor dry.

Was it only the slab condensing water or also the walls / windows as well?

 

[HogDude]

sort of...What I meant to ask was if in a mild climate like central NC where our frost line is only 12" deep, will there be enough thermal mass in my slab to keep the inside of a sealed and insulated barn above the dew point. I have never owned a detached garage / barn before but the attached garages I own / owned were all insulated and never had condensation but they were climate controlled above and next to them so not apples to apples with my new structure.

My buddy has a place on Lake Tillery. Ran into the condensation issue a few years ago. Put a low budget dehumidifier from Lowe's in there and voila! Problem solved. His shop/garage is about 1/3 berm or underground into the slope. So its temp swings are mild.

 

[finn]

Was it only the slab condensing water or also the walls / windows as well?

Slab only. No puddles, just wet ans slippery.

 

[Jazz1]

It’s currently 80F outside. 65F Inside insulated garage

 

[TimberMan]

It’s currently 80F outside. 65F Inside insulated garage

It is a heat index of 109f out side and inside my barn.