Foundation and Slab Insulation

[barnee]

Building a 28 by 38 detached garage which will be well insulated (6" Studs)and parts of it heated/cooled with mini splits.

Building will have a deep foundation and our frost line is at 18" here in VA. Plan on pouring the footings and then either using block for the stem wall (to one foot above grade) or a poured stem wall depending on cost.

2/3 of the ground floor will be my hobby garage, and 1/3 will be a studio/garden room. I know I will heat and cool the garden room and might condition the garage portion as well. Winter use will be limited.

I Plan on putting insulation board around the foundation down to the footer, but placing insulation board under the slab doesn't seem to be worthwhile based on my use and limited winter. Id rather spend that money on other things.

Am I thinking about this correctly? Is even the foundation board overkill for my conditions?

 

[ProCharger]

Pour your footings, pour your walls, before filling with your grading material line the entire inside with 1 1/2" -2" foamboard set on its side (4'deep) and then fill with your gravel. I would recommend using a 6 mil poly for a vapor barrier under the slab. That is how I would do it. Almost all of the floors I pour that use foam under the slab use some sort of heat, whether it be radiant heat or electric wiring actually placed in the floor.

 

[barnee]

So you put the insulation on the inside of the wall? Since the footer is only 2 feet down I assume that you would put it to the top of the footer and then trim it at the bottom of the slab.

Wouldn't this allow the stem wall to cool the slab or is there a thermal break between the stem wall and the slab?

 

[ProCharger]

Correct. We do this even on slab homes/tri-level homes. On these they form the wall and on the inside at the top of the foundation they add a 2x4 with the top of the 2x4 at foundation height around the entire wall. When they strip the wall this gives the slab a 1 1/2" x 3.5" lip for the interior floor to sit on. When the builder adds the foam he brings it to the bottom of that lip. As far as I know they have no problems with it cooling. The siding starter strip starts at the bottom of the sheeting outside so there is roughly 3.5" that is not insulated. The foam board is prescored as well to break down the center at 2 feet as well, if it is going to be shorter you will need to trim.

 

[Denwood]

There's still a good benefit to insulating your slab, regardless of how you're heating. We did a 9500 square foot warehouse renovation, and did the unthinkable. We insulated over the existing slab (2" closed cell foam, plus thermal reflective layer), added radiant heat in a 2" over pour. We also used a furred air space assembly on the inner walls. So 6" walls with fiberglass, 2" foam on the inside, thermal reflective vapour barrier, horizontal 2x2 furring then drywall. All wiring runs on the inside furring channels, so no envelope penetrations for network, power etc. This building costs less than my house to heat..and we see -30C regularly.

There was no question we were insulating the slab, as a lot of heat escapes this way via thermal conductance between slab and ground. The radiant tubes were added just in case..and we ended up going Triangle Tube on demand hot water heat for the entire building.

Radiant heat is nice, but to be honest, in a very well insulated building it makes a bit less sense. Our floor never feels warm, ever. If it did feel warm, we'd be overheating the main floor. We have 5 air handlers anyway for summer AC and HRV fresh air to the building. Doing it again, I'd insulate the slab, skip the radiant and just use the air handlers for everything. We do keep our loading zone (14" overhead door=not very efficient) at 55F and for this space, radiant does make sense as at -30, an open door in a 20ft ceiling room means 100% of the heated air goes away in seconds.

So, short answer. Insulate your slab.

If you look at the edge of the slab, you can see the start of the 2" inner SM envelope going up the walls. This was part of the furred airspace wall assembly. Having just had a commercial appraisal done, we pretty much confirmed my estimate in that this space operates with a $6000/yr savings for hydro and heat. I should mention it's no longer a warehouse, but has our production, offices, film studio etc.

 

[wssix99]

So, short answer. Insulate your slab.

This. ^^^

No matter what you do, your floor will naturally go to 55%. If you want it to be comfortable, you'll need to put radiant. If not, then you still need the insulation if you are going to do any heating.

Skipping the slab insulation in a heated space is similar to heating the room and then leaving all the windows open on a 55 degree day. It will get very expensive, quickly. That insulation will slow down the loss of heat dramatically.

 

[tlmartin84]

I tend to disagree with several of the above comments.

http://www.toolbase.org/PDF/DesignGuides/revisedFPSFguide.pdf

Go read that, In your case it is better to go with insulation on the outside of the foundation. The way described above leaves a cold bridge that transfers heat loss through the edge of the slab.

Also, you need to take into account your climate. You are heating 3-4 months a year and cooling for 6 with a couple that require little heat or cooling.

Plus if you insulate the floor, you lose the natural cooling affect through the summer. It's 6 one way, half a dozen another. Pick your poison.

 

[barnee]

I think a lot of this decision is based on where the project is, how the space will be used, and limits on budget. For my situation I'm much more interested in cooling during hot months than heating in cold months since I likely wont be in the space the majority of the winter. I don't plan on leaving the heat (or cooling) on and will pre-heat/cool the space if I know I have a project to do. If I had a shop that was in continuous use I would not hesitate to insulate the slab.

The foundation wall insulation ($300) make sense, but the slab insulation cost ($1,200) is a good portion of getting my racedeck floor or bendpak lift.

 

[tlmartin84]

Couldn't have summed it up any better!

 

[wssix99]

I didn't think anyone in Fairfax County lived off a budget any more! (It's not like it used to be...)

I don't plan on leaving the heat (or cooling) on and will pre-heat/cool the space if I know I have a project to do.

This changes the equation some. I'm not sure how long a mini-split would take to heat things up. You may find the need for an auxiliary heater later on, but I suppose you can always add that later. The floor will just always be cold. You can probably get around that with flooring in the room and maybe that isn't an issue for the garage?

The foundation wall insulation ($300) make sense, but the slab insulation cost ($1,200) is a good portion of getting my racedeck floor or bendpak lift.

After going through the building process, (and now the rip-it-out-and-do-it-over-right process) I don't think so much about what I can buy with saved money. I think like - "What will it cost me to rip out the slab and re-do it later?"

 

[bcradio]

After going through the building process, (and now the rip-it-out-and-do-it-over-right process) I don't think so much about what I can buy with saved money. I think like - "What will it cost me to rip out the slab and re-do it later?"

Best answer so far hands down.

 

[wssix99]

My lift fell out of the near term "budget" when we encountered a late 1800's cesspool while digging the foundation. The risk of continuing as-is and having to jack up the foundation later in the future outweighed the lift. ... and the hard wood stairs. ... and some tile in the bathrooms. ... and the stripper pole.

 

[GYPSY400]

You are on the right track with insulating the stem walls below the frost line.. the idea is to keep frost out from under the pad.. most of the time i say insulate everything.. but for occasional winter use, you should be fine.. i do recommend that if you can, keep it heated above freezing.. the pad can act like a big ice cube, or a big heat sink - which makes it easier to warm up when you do want it heated.

 

[barnee]

I didn't think anyone in Fairfax County lived off a budget any more!

There are a few of us that still care about budgets in this area, but not many. I work hard for my money and I'm not a fan of wasting it.

For heating I'll just get a $200, 20,000 btu kerosene or LP heater for the 5 times Ill be in the garage over the winter. For the summer Ill have some very good fans, and if need be turn on the AC. Right now I have a smaller un-heated and un-insulated garage and haven't found it to be detrimental to what I do. Virginia is very pleasant 8 months of the year.

I'm a perfectionist and have to try hard to balance state of the art mentality with cash on hand, and I will have my lift (but the stripper pole may edge it out).

 

[StevenMorgan]

For a mild winter climate, could 1" insulation be adequate? Thats R-5 instead of the commonly recommended 2" R-10 for the under slab insulation. Quick check on prices says that should cut the estimated $1,200 for slab insulation in half.

 

[12ozd]

^ +1....

 

[dirttracker18]

Having used my shop for two years before setting up my infloor heat, I would always insult under a slab in the future regardless. In made a huge difference in all seasons even without conditioning.

If I were to do it again I would use ICFs from the footer up to slab level and then foam down under the slab sealed to the ICF.

In fact we are planning an addition to the house that includes an attached garage with infloor heat and that is exactly how it will be done.

 

[Elginz]

With our humidity in the summer there are occasionally some damp floors. My dads is one, in both his old shop and now his new one. I don't know if your climate could have that problem. My shop, and studio are insulated with 3/4" foam, no damp floors like the neighbor, or dad. I heat it with an overhead electric radiant, that keeps things comfy, even at floor level.

 

[Denwood]

I should mention that our over pour is indeed in a cold climate. That said, what happens when warm moist air contacts a cool slab? I know what it's like in my older home basement (not an insulated slab!) in summer...we're running a dehumidifier to address moisture issues. Same climate.

I'd agree though that conditioned dry air and a buried slab in certain situations may in fact favour no insulation. Case in point would be earth berm builds in very hot climates. Your climate and thermodynamics always have to be considered. I think it's also important to look at climate change, and consider the life of the building in making these decisions. In general, many predictions are turning out on the too conservative side.

 

[pantera1]

We have 2 shops. One with and without insulation.

Insulate the slab, it'll be cooler in the summer and warmer in the winter. Doesn't matter what climate you live in.

You'll never have a wet slab again.

 

[barnee]

The wet slab issue is real. My current building does get a little wet during our humid summer months, so that may be a reason to throw 1" under the slab to help with that, especially if I don't run the AC much.

 

[MagKarl]

Don't let folks pressure you into spending money you don't want to, on insulation that may in fact not even help for your usage. Insulation doesn't care what season it is or where you live. It doesn't make things warm or cold. It simply slows the transfer of heat from warmer areas to colder areas. Where do you want to slow the transfer of heat?

You've stated that winter heating is not a priority, but summer cooling is. If you insulate the slab, you will get less free ~50? or so degree heat from the ground, and you'll less 50 degree heat sinking in the winter. Think root cellar. Think geothermal.

 

[joes169]

Using a simple, cheap vapor barrier will also take care of most, if not all, sweating of the slab in humid months. Add me to the list of people who think that underslab insulation is a waste of money and resources in this situation.

If on the fence, I'd likely place 2' of 1" - 2" foam vertical from footing to bottom of slab, and 2' in from the edge under the floor. This is where most of the heat loss takes place. 1/2" foam as a thermal break at the floor/wall intersection is cheap and easy as well.

 

[yeldogt]

In a cold climate I would never put down a new slab without some insulation -- even if I did a thicker 4' band around the perimeter and less in the middle.

You don't mention it ------Radiant slabs need insulation -- don't want the ground acting as a heat sink.


I own a large older house built in the 50's with a huge basement -- the walls (block) are now insulated on the inside. It was too costly to rip out the slab -- the rooms are not uncomfortable as the heating is correct -- the floors are still cool to the touch

 

[dirttracker18]

Don't let folks pressure you into spending money you don't want to, on insulation that may in fact not even help for your usage. Insulation doesn't care what season it is or where you live. It doesn't make things warm or cold. It simply slows the transfer of heat from warmer areas to colder areas. Where do you want to slow the transfer of heat?

You've stated that winter heating is not a priority, but summer cooling is. If you insulate the slab, you will get less free ~50? or so degree heat from the ground, and you'll less 50 degree heat sinking in the winter. Think root cellar. Think geothermal.

As noted by myself and others that have gone with and without insulation under their slab, it makes a difference year round, conditioned or not.

Your root cellar comparison is moot as the slab is not underground, like a root cellar. The slab is sitting on top of the ground.

 

[tlmartin84]

As noted by myself and others that have gone with and without insulation under their slab, it makes a difference year round, conditioned or not.

Your root cellar comparison is moot as the slab is not underground, like a root cellar. The slab is sitting on top of the ground.

Actually he has a very good comparison. My floor is always cooler in the summer time than anything else. If you put insulation between the ground and the slab the slab will inherit more of the properties of the air above it. So if you are cooling the air to 70 degrees then that's what the slab will tend to stay at.

Kind of reverse thinking here on some of your parts, pay money to cool the air and cool the slab, or let the slab help cool the air above it. NOW THIS IS A DIFFERENT STORY FOR HEATING IN THE WINTER.

As far as being under ground or not, it is still in direct contact with the ground, and any radiant heat from the sun is removed due to the roof.

 

[Denwood]

Using a simple, cheap vapor barrier will also take care of most, if not all, sweating of the slab in humid months. Add me to the list of people who think that underslab insulation is a waste of money and resources in this situation.

If on the fence, I'd likely place 2' of 1" - 2" foam vertical from footing to bottom of slab, and 2' in from the edge under the floor. This is where most of the heat loss takes place. 1/2" foam as a thermal break at the floor/wall intersection is cheap and easy as well.

Moist air condenses on cooler surfaces. That's a sweating slab.

Vapor barrier will have little to do with this. This is also why there are severe limitations (meaning some cooling is possible) on cooling a building via the floor's radiant system. Condensation on the slab results in mold issues.

When regulating a building, thermal mass is your friend. Insulation just gives you more control over what that temp will be.

 

[dirttracker18]

Actually he has a very good comparison. My floor is always cooler in the summer time than anything else. If you put insulation between the ground and the slab the slab will inherit more of the properties of the air above it. So if you are cooling the air to 70 degrees then that's what the slab will tend to stay at.

Kind of reverse thinking here on some of your parts, pay money to cool the air and cool the slab, or let the slab help cool the air above it. NOW THIS IS A DIFFERENT STORY FOR HEATING IN THE WINTER.

As far as being under ground or not, it is still in direct contact with the ground, and any radiant heat from the sun is removed due to the roof.

Again

Having had both insulated and uninsulated as well as insulated but not conditioned, insulated is far better in all seasons, including summer.

As far as the root cellar, the immediate ground is not at a lower temp as it absorbs heat all day. You must go underground to get lower temps not sit on top of it.

Anyway, having been there, I would always insulate under now.

 

[tlmartin84]

Again

Having had both insulated and uninsulated as well as insulated but not conditioned, insulated is far better in all seasons, including summer.

As far as the root cellar, the immediate ground is not at a lower temp as it absorbs heat all day. You must go underground to get lower temps not sit on top of it.

Anyway, having been there, I would always insulate under now.

HAVING SLABS WHERE? Everyone here for it, lives in cold climates, or extremely hot climates.

Vapor barrier does just that, stops vapors from coming through. 6mil plastic, stops moisture and vapor coming up through the ground, it still allows heat transfer. In a conditioned space you will not see sweating. open up the garage door for an extended time and let humid air in and it will sweat. (this occurs anytime the ambient temperature is hotter outside than the concrete with humid air). Why? Because the moisture in the warm air condenses on the cooler concrete. Just a few degree change is enough to cause it regardless of insulation or non-insulation under the slab. The main factor at work here is the HUMIDITY.


Check your asphalt temperature in the shade, and in the sun. it will be cooler where something is over it. Always is, always will be. It TAKES a LOT to change the temperatures of concrete. The thing you are missing is he has a THERMAL BARRIER around the perimeter of his building........it keeps the perimeter temps, (whether hot or cold) from being transferred into/under his slab. THERE ARE ONLY TWO (2) FORCES AT WORK HERE. The natural ground temperature under the slab, and the temperature inside the building.



I GUESS HE SHOULD PAY SOMEONE EXTRA TO BURY HIS PIPES 6' DEEP (OR DEEPER) BECAUSE THATS HOW IT IS DONE WAY UP NORTH TOO.



I posted a link earlier, I am not giving information based on my few experiences or my opinion. Mine is from facts. Look up ASCE 32, look up the link I posted earlier. Everything there is based on climate and location. Pull up the building code......

I have seen hundreds of slabs, and thousands of yards of concrete poured. NO ONE here insulated underneath, not residential nor commercial. When the rest of the building is sealed correctly there are no issues with any of the slabs. From an engineering, cost, and the intended usage standpoint, I would not recommend it in your case. If one of my clients wanted to throw money at something then by all means go ahead.

PLEASE READ THE LINK FILL OUT THE WORKSHEETS AND INSULATE AS NEEDED. Form your own opinion based on those results, and do what you feel comfortable with.

http://api.viglink.com/api/click?fo...ase.org/PDF/DesignGuides/revisedFPSFguide.pdf


My frost depth is 30", yours being 18" means your climate is a little more mild than mine. Out of curiosity BARNEE, what are the other guys in your neck of the woods doing?

 

[jpcjguy]

Hey BARNEE - what did you end up doing? I am in Richmond, VA - 2 hours south of you and will be facing the same dilemma. I am leaning toward no insulation underneath because my garage will also be limited use (3 kids 5 and under).

 

[Greg Smith]

My advice..as there is alot of opinions here and all have vald points, put your money into Heat Retention, the cost of fuel is not dropping and in the long run insulation will pay for itself.
And I can tell you there is nothing worse than a sweaty Harley!!

 

[racer-john]

The "toolbase.org" won't load.