Garage Foundation: Slab, No Stem Wall

[cls89]

- I have a 22x32 detached garage located in southeast Michigan. Probably built in the 1950s.

- The foundation is on a concrete slab with a footer.

- The framing / wall plates sit directly on the slab. In other words the wall plate and the rest of the garage floor are on the same plane.

- The problem with the wall plate sitting on the garage floor is that the floor sweats / gets a little wet when it’s humid / hot outside. Assuming this happening because there’s no vapor barrier when they poured the concrete slab. This obviously exposes the wood framing to moisture which over time will be a problem.

- I want to insulate, finish and heat the inside of the shop, but I don’t want to invest in all this without addressing the moisture issue and wall plate / framing getting wet from the situation described above.

- If the wall plate / framing sat on a stem wall this wouldn’t be an issue, but instead it sits right on the slab.

- Ultimate question, is there any way I can add a stem wall / raise the framing off of the slab? If not is the option to demolish and pour a new foundation with a stem wall and build a new garage? Without raising the wall plate off the ground with a stem wall how else can I protect the framing from moisture?

 

[ericm]

It's likely that the floor isn't sweating- it's condensation from the hot humid air onto the cool floor. You're going to want to keep that air out of the building anyhow if you condition it. That and A/C ought to be able to deal with it. If you're opening the door a lot you might want to add a dehumidifier. Those work well in hot weather.

 

[cls89]

It's likely that the floor isn't sweating- it's condensation from the hot humid air onto the cool floor. You're going to want to keep that air out of the building anyhow if you condition it. That and A/C ought to be able to deal with it. If you're opening the door a lot you might want to add a dehumidifier. Those work well in hot weather.

That makes sense. I have no plans to add AC just an electric heater (240v) for the winter months. You think a dehumidifier is appropriate for this situation? Is there any other type of ventilation or fan I can incorporate to help?

 

[The Cobbler]

is there any way I can add a stem wall / raise the framing off of the slab?

easiest in my opinion would be jack it up & put a strip of high density styrofoam between the plate & floor

 

[cls89]

easiest in my opinion would be jack it up & put a strip of high density styrofoam between the plate & floor

Is this something I would be able to do myself using some jacks, or should it be contracted out to a professional service? Also how effective is this approach with respect to protecting the framing from moisture from a long term perspective?

 

[The Cobbler]

depends how able you are depends on if you can do it yourself . if you have electrical in there, the service will have to be extended. doors will have a gap under them now. what some one calls easy can be very difficult for someone else . I guess you have to ask yourself if you can do it yourself or not . as far as longevity, it will be a lifetime fix for keeping moisture from the floor and framing

 

[cls89]

depends how able you are depends on if you can do it yourself . if you have electrical in there, the service will have to be extended. doors will have a gap under them now. what some one calls easy can be very difficult for someone else . I guess you have to ask yourself if you can do it yourself or not . as far as longevity, it will be a lifetime fix for keeping moisture from the floor and framing

1. Can you clarify what you mean by electrical service needs to be extended? Are you referring to the wiring?

2. The high density styrofoam that you are recommending, what thickness would it be?

3. Can you link the high density styrofoam product you are recommending?

 

[ericm]

That makes sense. I have no plans to add AC just an electric heater (240v) for the winter months. You think a dehumidifier is appropriate for this situation? Is there any other type of ventilation or fan I can incorporate to help?

Hotter air holds more water. When you cool it, the relative humidity goes up until it condenses. If you can blow cool dry air into the space then you won't get condensation. But the outside isn't a source for that.

Doesn't it get hot in Chicago? I'd put in a mini split and run it as A/C in the summer. You don't have to crank it down super low like a lot of people do, just run it enough to cool and dry the air some. It'll make the space more comfortable. And it will cost a lot less to run for heat than the resistance heater.

 

[The Cobbler]

1. Can you clarify what you mean by electrical service needs to be extended? Are you referring to the wiring?

2. The high density styrofoam that you are recommending, what thickness would it be?

3. Can you link the high density styrofoam product you are recommending?

if your electrical is underground you will have to accommodate for that since it will become too short
1" would be ample
any xps type extruded foam will work

 

[finn]

It's likely that the floor isn't sweating- it's condensation from the hot humid air onto the cool floor. You're going to want to keep that air out of the building anyhow if you condition it. That and A/C ought to be able to deal with it. If you're opening the door a lot you might want to add a dehumidifier. Those work well in hot weather.

This. The cold floor is sweating when the hot, or warm air hits it, especially in the spring.

The thicker the slab, the longer it takes in the spring to get above the dew point, since thick slabs have more thermal inertia.

If you heat the garage in the winter, the problem will go away, or at least be minimized.

Might be easier to replace the bottom plate with pressure treated than to bust up the slab. Jack up the wall a little, cut out the rotten plate, replace with pt and move on.

This came up before, so do a search here.

 

[mike93lx]

if your electrical is underground you will have to accommodate for that since it will become too short
1" would be ample
any xps type extruded foam will work

Putting an inch of foam board under the framing hits me as quite odd. I wouldn't trust that at all from a strength perspective.

A piece of PT 2x would be my choice if the desire is to keep the height minimal. A course of block would be better, but that's a lot taller

 

[cls89]

Hotter air holds more water. When you cool it, the relative humidity goes up until it condenses. If you can blow cool dry air into the space then you won't get condensation. But the outside isn't a source for that.

Doesn't it get hot in Chicago? I'd put in a mini split and run it as A/C in the summer. You don't have to crank it down super low like a lot of people do, just run it enough to cool and dry the air some. It'll make the space more comfortable. And it will cost a lot less to run for heat than the resistance heater.

I’m in the metro Detroit area. Yes it can get hot and humid in the summer.

What type of mini split system allows for both heat and AC? How well do they work for heating? Temperatures get extremely cold in the winter time.

My workshop is in the back of the 22x32 garage. The condensation that occurs on the concrete floor honestly only occurs in a specific section. Mainly the front left section of the garage. Knowing this how should I locate the mini split so I can effectively heat/cool the shop space in the back of the garage and also cool the front area that has the condensation issues?

 

[mike93lx]

What type of mini split system allows for both heat and AC? How well do they work for heating? Temperatures get extremely cold in the winter time.

Pretty much all of them. Efficiency does drop off at lower temps, but some can output heat at pretty extreme temps

 

[cls89]

This. The cold floor is sweating when the hot, or warm air hits it, especially in the spring.

The thicker the slab, the longer it takes in the spring to get above the dew point, since thick slabs have more thermal inertia.

If you heat the garage in the winter, the problem will go away, or at least be minimized.

Might be easier to replace the bottom plate with pressure treated than to bust up the slab. Jack up the wall a little, cut out the rotten plate, replace with pt and move on.

This came up before, so do a search here.

It actually doesn’t do it in the winter. Only during the summer months when it’s hot and humid and only in a specific section (front left area of the garage.

At the moment none of the bottom plates show any signs of rot, so no need to replace right now.

 

[HoosierBuddy]

Are the plates already made out of treated lumber?

 

[tom in nh]

Cheapest way out is to install a wall mounted AC unit.
Ger er' done !

 

[cls89]

Are the plates already made out of treated lumber?

I don’t think so. I’ll try to get a photo later.

 

[cls89]

Cheapest way out is to install a wall mounted AC unit.
Ger er' done !

You’d recommend having two separate electric units? One for AC and one for heat? Instead of 1 like the mini split?

 

[OccupantRJ]

What is on the outside preventing water from getting under that sole plate?

 

[CraigStu]

First I'd look at youtube videos of jacking up a garage to get an idea of what's involved. You can probably find both diy and pro versions. I'd also look at jacking and building a stem wall. Could be that an extra 18"-24" of ceiling height would be worthwhile long term. Minisplits are a great solution but it may be that a window AC unit and a propane heater would work for you too. That is what I have in my 28x32 attached garage. I run it off of a bbq propane tank. Works out great as the bbq and heater tanks act as backup for each other since generally they are used in opposite seasons.

https://www.sportsmansguide.com/product/index/mr-heater-vent-free-radiant-propane-heater-30000-btu?a=1843562&szc=000&clrc=000&utm_source=bing&utm_source=bing&utm_medium=pla&utm_medium=pla&adlclid=d0b31aa50f7e1d1b4bc17dc3b1f14191&utm_campaign=31664717&utm_channel=shoppingads&utm_product=wx2*0651033000000&msclkid=d0b31aa50f7e1d1b4bc17dc3b1f14191

You might be able to use the separate units in different locations to your advantage in dealing w/ the moisture as well as making you work area comfortable.

 

[theoldwizard1]

- Ultimate question, is there any way I can add a stem wall / raise the framing off of the slab?

Yes, it is possible, but not cheap/easy ! Lift the wall a small amount (2"-3") at a time around the perimeter. Use SOLID concrete blocks for your "stem wall". While it is lifted, replace the footer with pressure treated lumber.

A concrete floor will always be "damp". If you want a DRY floor (not safe for vehicle traffic) look into something like DriCore or Dampro.

 

[finn]

Pretty much all of them. Efficiency does drop off at lower temps, but some can output heat at pretty extreme temps

I think there are still lower end mini splits that are cool only, and don’t have much in the area of heating capability.

Do a search, though, as major manufacturers all have higher end units that can deliver heat, at reduced efficiency, down to the teens.

Ours is a Mitsubishi Hyper Heat.

For really low temperatures, a propane or natural gas heater, vented, is superior.

 

[tom in nh]

You’d recommend having two separate electric units? One for AC and one for heat? Instead of 1 like the mini split?

A wall mounted AC unit and a wood stove should be your least expensive option.

 

[BillK]

My 24 x 24 detached garage is built the same way. The footers and the floor were all one pour. The sole plates are pressure treated and there is a strip of thin fiberglass under them. When I built the garage they did not have the poly strips.

There is also no vapor barrier under the slab. That was not a "thing" 40 years ago.

It gets plenty hot and humid here in Maryland and I have never seen a drop of water on the floor. Mine is not insulated at all and that might actually help because it is not sealed up real good at all. If yours is only getting wet in one spot I would have to wonder if there is something else causing it ? Maybe water getting under the slab some way ?

If yours is truly from condensation then I bet a simple fan to circulate some air will solve the problem.

 

[Beauregard]

It actually doesn’t do it in the winter. Only during the summer months when it’s hot and humid and only in a specific section (front left area of the garage.

At the moment none of the bottom plates show any signs of rot, so no need to replace right now.

You stated that the garage was built in the 1950s, and the now close to 70-year-old bottom plate shows no sign of rot.
I'm not sure I would even mess with it.
Why the concern now?

 

[NUTTSGT]

I would suggest jacking the walls and laying a course of block down, fill them solid and add a PT sill plate ... Cut out old plate if rotted.

However, this creates a new issue. Your building now has walls about 10" taller, which is nice, your door will need some work. Either taller door or relocate the tracks and box in the opening at the top.

 

[ericm]

I think there are still lower end mini splits that are cool only, and don’t have much in the area of heating capability.

Do a search, though, as major manufacturers all have higher end units that can deliver heat, at reduced efficiency, down to the teens.

The Daikin units speced for our house build are rated for full output at 14 degrees. They're not special cold weather models. (14 is plenty for our location, its extremely rare to go below that and we'll have a wood stove) There's been a lot of improvements over the last few years, especially in cold weather performance.

They're also rated to cool at full output at up to 121 degrees! Which is insane. I hope it does not get that hot.

The EG4 units I am looking at for the shop have a working range from 5 degrees to 131. Also not special cold weather units.

 

[Bert_]

You stated that the garage was built in the 1950s, and the now close to 70-year-old bottom plate shows no sign of rot.
I'm not sure I would even mess with it.
Why the concern now?

My thoughts too. If it's a worry put down a new pressure treated plate. Good for another 70 years

 

[theoldwizard1]

What type of mini split system allows for both heat and AC? How well do they work for heating? Temperatures get extremely cold in the winter time.

Do a search, though, as major manufacturers all have higher end units that can deliver heat, at reduced efficiency, down to the teens.

Min-splits are a "Buyer Beware !" Finn is almost correct. Some will make heat down to 0°F !

If you are trying to heat a garage from "dead cold", you really need an auxiliary propane/kerosene heater to get it up to "tolerable" (50°F-60°F) in a reasonable amount of time.

 

[finn]

Min-splits are a "Buyer Beware !" Finn is almost correct. Some will make heat down to 0°F !

If you are trying to heat a garage from "dead cold", you really need an auxiliary propane/kerosene heater to get it up to "tolerable" (50°F-60°F) in a reasonable amount of time.

That’s partly because the slab is such a huge heat sink. It takes a lot of btu’s to budge that thermal inertia.

Even the Hyper Heat struggles a little when it gets into the low teens. Look at the efficiency curves if you plan on shirt sleeves and sandals at really low temperatures.

 

[WisJim]

The Fujitsu Mini splits, XLTH series, are rated down to minus 15 F for heating, and up to 115 F for cooling. We had a pair of them in our old farmhouse and they provided all of our heat most of the winter--we didn't use our wood furnace until our solar electric output didn't keep up with our heating needs, although one winter we didn't use the wood back up at all, and this is in NW Wisconsin where we get actual cold weather.
But to get back to the OP's problem, I'd like to see pics of the building, inside and out. I'm thinking that some outside foam board insulation buried around the edge of the slab might help if it is actually a cold floor condensation problem.

 

[Jon h]

Personally I wouldn’t worry about it. But what about lifting it just enough to slide shingle material under it? About 1/4”…..I seem to remember seeing rolls of shingle about 6”-8” wide x number of feet long? I could be wrong on that. Or just get a couple bundles of 3 tabs.

Jon h.

 

[mike93lx]

I seem to remember seeing rolls of shingle about 6”-8” wide x number of feet long?

Yes, it's starter strip, used under the first course of shingles