Existing unisulated slab - what to do.

[roger440]

20 years ago i built what is now my workshop. Sadly i was young and foolish at the time, and, to cut a long story short, the slab was laid without any insulation under it, or for that matter any form of moisture control.

Fast forward to today, im now looking at refurbising the workshop. (new roof etc.)

Fortunately, i have no moisture issues at all, and the slab is still good. However, i really want to make it a bit more comfortable in there in winter.

So, to the question, in order to insulate the floor somehow, i figured laying insulation (1"?) over the existing slab, then further concrete or similar over, probably with underfloor heat. But hieght is an issue.

So, just how thin a floor/concrete can i get away with and still be able to jack up a car with a trolley/floor jack, bearing in mind the solid existing slab underneath? I have no idea just how much "give" there would be in the insulating board?

The building is part timber, part sectional concrete built off the slab, so replacing the slab would involve demolition of the existing building! So i dont want to do this!

 

[bczygan]

Dig down around the outside of the building along the foundation wall to the depth of the freeze line or the bottom of the foundation wall. Take the opportunity to renew any waterproofing and or drainage. Install rigid insulation of a thickness or R value that you desire up to grade. This perimeter insulation will insulate your floor.

 

[tcianci]

Save your money and time. While an insulated slab would be nice, Your wanting "comfort" is subjective. The floor will still be colder than the rest of the conditioned space unless of course you go with the in floor heat. Be aware that in floor heat is a very poor choice for a space that you intend to heat only when you're working in it. It's not the type of heat they you can turn on and warm up the space in an hour or so.
Why do you refer to your omission of insulation under the slab as foolishness. I'm sure your space was built for the needs at the time. Just heat the space, claim victory and go home proud. Outside of "comfort" you will likely never recoup in energy savings what it will cost you to install another slab or perimeter insulation.

 

[SPDMETL]

What would happen if You sawed out part of the concrete? Leave a couple(?) of feet alone where the walls are supported, and do a new slab section with floor heat. Problem solved!

 

[roger440]

Dig down around the outside of the building along the foundation wall to the depth of the freeze line or the bottom of the foundation wall. Take the opportunity to renew any waterproofing and or drainage. Install rigid insulation of a thickness or R value that you desire up to grade. This perimeter insulation will insulate your floor.

Mmmm, interesting. So what you are really saying is that most of the thermal loss/gain is sideways rather than downwards? The only flaw is that the slab is, on one side, up against a retaining wall, because the garage is about 3ft above the ground that side.

Save your money and time. While an insulated slab would be nice, Your wanting "comfort" is subjective. The floor will still be colder than the rest of the conditioned space unless of course you go with the in floor heat. Be aware that in floor heat is a very poor choice for a space that you intend to heat only when you're working in it. It's not the type of heat they you can turn on and warm up the space in an hour or so.
Why do you refer to your omission of insulation under the slab as foolishness. I'm sure your space was built for the needs at the time. Just heat the space, claim victory and go home proud. Outside of "comfort" you will likely never recoup in energy savings what it will cost you to install another slab or perimeter insulation.

I understand floor heat is not fast to respond. My thinking was to run the floor heat at a moderate temp to keep the chill off it and then use my existing deisel fired fan assisted (ex train) heater to warm it when i want to actually work in it. 5 degrees below freezing a is regular event here, so just warming it up with a heater when i need still leaves everything metal in the garage very cold for a long time.

Just heating it when i need it is what i do now, and, to be honest, discourages me from going in there in winter.

I say it was foolish because i just didn't consider or think about it at the time. It was always my workshop, but it seemed great when i built it because before that i worked outside! Back then a bit of cold didn't both me, but now im older ive gone soft!

What would happen if You sawed out part of the concrete? Leave a couple(?) of feet alone where the walls are supported, and do a new slab section with floor heat. Problem solved!

The slab is just 7-8 inches thick all over. There is no additional depth under the walls. I guess it wouldn't do the structural integrity much good. Plus it is all reinforced concrete and seriously tough! Hence my thought of going "over" it.

 

[venom50svt]

Hmmmm Interesting topic... I myself went out to the garage this morning to have a cig,,, god the floor was like ice.... There is no way to improve on the slab after 30yrs so i usually rely on my Mr. heater to warm the 24x24 garage. It does a good job and when I work on my cars I just use a thick moving blanket to sit or lay down. I do have a 4 post lift but I don't always use it..
My son says I should use a radiant heater, it works on the princible of heating objectd and intern it heats the room. Maybe it would warm the floor??? I just don't know

 

[bczygan]

Cheapest solution for me would be insulated underwear and electrically heated socks. Solves the comfort equation. For the heat loss, first insulate above, then walls then foundation/floor. Heat rises. Heat also seeks the coldest place first.
Where the foundation wall is exposed above grade I would carry it up to top of slab level then cover it with cement board and stucco it. Metal flashing on the top will shed rain and snow.

 

[hot4rd]

Maby Race Deck floor tiles. A lot better than working on the bare concrete..

 

[nehog]

Perimeter insulation is the answer to your problem. There are several web sited describing systems that don't even have to go to the frost line, and can be done relatively easily.

 

[BrushWhackin]

Perimeter insulation first. To keep the cold from coming in not the heat from going out. Either route you take after that it will be worth it.

 

[roger440]

Excellent, sounds like perimeter insulation needs doing regardless. Not to difficult either i guess.

But, "if" i wanted to go with floor heat, just what is the minimum concrete thickess i can get away with over the insulation?

Or could i just ignore that too. How much heat would i lose "downwards"?

 

[slickgt1]

Ok, go buy electric radiant heating. Tile over it. Problem solved.

 

[roger440]

Ok, go buy electric radiant heating. Tile over it. Problem solved.

Indeed, i have looked into this. Just how much heat will i lose into the slab, compared to if it was insulated though. Sounds like an expensive way to burn money? Or am i wrong?

 

[wrenchguy]

Insulate around the outside about 3' down. But to me this is minimal help depending how your slab is set up to its foundation. The thermal mass of your floor and whats below it is warmer than the outside. It feels cold because the coldest air in the shop is at the floor and your feet seem to always feel it worst. They're in constant contact with it. I've tried a 8'x8' area with 1" insulated foam board with 7/16 osb over it then working on that area for a few days didn't feel any improvement. The coldest air of the room is still at the floor. Good luck.

 

[slickgt1]

Indeed, i have looked into this. Just how much heat will i lose into the slab, compared to if it was insulated though. Sounds like an expensive way to burn money? Or am i wrong?

No not really. Think of it like this. Heat tends to rise, not settle. Yes you will be heating up your slab through conduction, and probably loosing some heat to it, but once the floor heats up, the stone and tile itself will retain heat. Basically become like a radiator.

If that is not enough, think of it like this. The money it would cost you to rip all that up, or even to insulate over the top, pour another slab, lose of time, money, and whatever else, it is just not worth it. Just do the radiant heating, put tile over it. Call it a day. I can't see the floor pumping heat out of the bottom. I would think it would reach and equilibrium of heat transfer.

You will probably never realize the cost savings by redoing the slab.

 

[dcs Inc]

The misconception that heat will rise from this system is incorrect. It's called radiant for a reason. You will be loosing at least 50% of your heat warming the worms under the slab. The heat radiates in all directions. Being warmer in one direction wont stop the heat loss in that direction. Installations of this nature insulate on the bottom side to force the majority of heat to go in the opposite direction. It's really an infective system right on top of concrete unless you have 4" of rigid insulation under the slab. Not to say that I haven't installed them on uninsulated concrete, even to the point of saw cutting lines and epoxying the wire directly into the surface. It works, but not to the best of the systems ability. gene ec-Indy

 

[roger440]

Hmmm, bit of a conflict of opinion there then! I guess insulating round the side will be worthwhile, but it wont be the cheapest system to run!

I havn't ruled out laying insulation over the existing floor, but as per my original post, i cant find anything anywhere on the internet ref what i could get away with thickness wise with the concrete over the top.

 

[dcs Inc]

I'll try to remember to look up an insulation system that I found way back. I bid a project that was to be insulated, wired for heat and then an overlay on top of it. gene

www.elitecreteindiana.com
www.indycrete.com

 

[roger440]

I'll try to remember to look up an insulation system that I found way back. I bid a project that was to be insulated, wired for heat and then an overlay on top of it. gene

www.elitecreteindiana.com
www.indycrete.com

 

[slickgt1]

The misconception that heat will rise from this system is incorrect. It's called radiant for a reason. You will be loosing at least 50% of your heat warming the worms under the slab. The heat radiates in all directions. Being warmer in one direction wont stop the heat loss in that direction. Installations of this nature insulate on the bottom side to force the majority of heat to go in the opposite direction. It's really an infective system right on top of concrete unless you have 4" of rigid insulation under the slab. Not to say that I haven't installed them on uninsulated concrete, even to the point of saw cutting lines and epoxying the wire directly into the surface. It works, but not to the best of the systems ability. gene ec-Indy

Yes valid point, but heat will radiate more through the 1/2" of tile and mortar, than through 4" - 5" of concrete. Then there will be a point where the ground below will reach equilibrium from the heated slab above. Remember the ground below his garage will always be warmer than the ground outside of it. But lets leave that alone.

Consider the expense of laying insulation, pouring another slab. The difference will be huge. This expense is not justifiable in this situation.

How big is this slab anyway, I think i missed that somewhere?

Either way, figure out how much a new slab will cost, with insulation and all, and then think how much you will really save on your electrical bill if you didn't have it. I am going to say, that it will not recoup the expense any time soon.

 

[The Garage Dude]

The quickest and cheapest solution would be to roll out a poly-vinyl floor. You can buy sectional pieces, i.e. 7.5 x 17, 9x20, etc... or large rolls of them to do a complete wall to wall finish. BLT is one of the better manufacturers. It's a nice product and helps to protect the floor from any water or auto fluid spills. The more expensive option is to use a garage tile (from RaceDeck Flooring)

I hope this helps. I have some pictures of the various flooring products on my website at www.thegaragedude.com

The Garage Dude

 

[roger440]

Yes valid point, but heat will radiate more through the 1/2" of tile and mortar, than through 4" - 5" of concrete. Then there will be a point where the ground below will reach equilibrium from the heated slab above. Remember the ground below his garage will always be warmer than the ground outside of it. But lets leave that alone.

Consider the expense of laying insulation, pouring another slab. The difference will be huge. This expense is not justifiable in this situation.

How big is this slab anyway, I think i missed that somewhere?

Either way, figure out how much a new slab will cost, with insulation and all, and then think how much you will really save on your electrical bill if you didn't have it. I am going to say, that it will not recoup the expense any time soon.

I didn't actually say the size, but its about 20 x 28. A new slab will mean demolition of the building Not really going to happen.

 

[slickgt1]

Yea that is understood.

So at $4 per square foot, for a new slab, which I think is on the low side. You are looking at $2240.00 . That is without insulation or anything else you want to add. Figure this will save you $10-$20 a month, maybe, depending on how the rest of the building is insulated.

So you are looking at a minimum of 10 years to pay that back. Most likely more.

I would just put electric radiant heating and tile over it.

 

[roger440]

Yea that is understood.

So at $4 per square foot, for a new slab, which I think is on the low side. You are looking at $2240.00 . That is without insulation or anything else you want to add. Figure this will save you $10-$20 a month, maybe, depending on how the rest of the building is insulated.

So you are looking at a minimum of 10 years to pay that back. Most likely more.

I would just put electric radiant heating and tile over it.

Good to see some numbers on it. I really have no handle on that. The roof is very well insulated, most of the walls bar one also, though i intend to rectify that.

TBH, this is the way im leaning at the moment. Adding thickness to the floor has quite a lot of additional issues/cost.

 

[slickgt1]

Exactly. When people call me for quotes, yes, I can sell the extra slab, but what for? Some things just don't pay off.

Heated tile floor is the best feeling too. Sitting on the floor becomes a pleasure.