I'm planning on having a "semi" heated garage with the ICF house I'm building soon. Semi means it will be heated only to like 50F or 55F full time in the winter, and fully heated to about 70F + when I'm working out there. I will be using hydronic ceiling radiant heat. The perimeter is ICF to 20" below grade so there will be good perimeter insulation. I'm planning on a thermal break between the floor slab and driveway.

How worthwhile would it be to fully insulate the slab? If this was living space there would be no question about insulating, but for part time usage I'm inclined to save the upfront $$. Since this is a garage I don't need to be concerned about keeping sock feet or bare feet warm.

you might not be able to get 70+ with radiant ceiling over cold slab, and it's much more iffy if the slab is not insulated.

I can't yet explain why, but the only real ceiling problems I've seen yet are over cold slabs.

I would recommend insulating/heating the slab for the baseline heat and using ceiling or another low mass emitter to "boost" heat as needed, especially if you want to achieve such a big differential in temperatures.

Another alternative is to plan ahead if you want to work in the garage, and just use floor heat, turned up a few hours before you go out.

Just some ideas!

Skip the insulation(save $$$). I live in an area with some of the coldest ground temps, 50°. So, with R-100 insulation on the walls and ceiling(and assuming no air infiltration) the garage would be 50° year round. Depending on where you live, on cold days, the ground will actually be warmer than the temperature outside. So, the lack of insulation will be helping to heat the garage(to ground temperature)!

If you are not actually heating the garage slab(usually via radiant floor), an above slab heating solution is not likely to affect the temperature of the slab itself. And, around here, a 50° garage feels pretty nice when it's 10° outside!

That's kind of what I'm thinking, plus the reverse in the summer. I just looked at my heat load calcs and of the total load of ~5500 Btuh for the garage, less than 1000 of it is the slab. That's design condition of 9F outside (SW Idaho). The temp drops below 20F here less than 2% of the year. Since this is the garage and a screw up here isn't severely detrimental, I think I'll go the no insulation route. With the ceiling radiant the car and pickup should absorb a good portion of the radiation and stay warm. That's the primary intent! I can always add an electric heater or hydronic fan coil unit later.

Rob, I'm wondering if the problem with the uninsulated slab and ceiling radiant is the thermal mass of the concrete and its heat conductance into the soil just won't allow it to come to room temp before everything else in the room is warmed up to the thermostat set point. Then of course bodies will radiate to the cooler floor and thus feel cool.

There is more to it than that.. we had one running in a basement all winter and it never got the area up to temp, so it wasn't a short cycle issue. Might have just been big losses through the slab, but it's hard to say. We still have to do more this fall to figure it out.

PC had a point, except you're not installing R-100 and you will have air infiltration... this is a garage and garage doors are not very tight at all. So the garage *may* not freeze, but it will probably get close... depending on heat gain from adjacent heated spaces.

I think you'll find insulation and tubing in the concrete is approximately the same cost as a radiant ceiling.

Once again, you've got me rethinking things! :-) I haven't worked out the numbers yet, but I was thinking last night that the cost may actually be close. As you know I'm doing the ceiling so I have cooling also, but in the garage I either don't need much, if any, cooling or cooling the floor may work just fine.

Regarding garage doors, I found out this summer just how much of a radiant heating panel a garage door is when it's being beaten upon by the summer afternoon sun. The door in this house is painted dark green and has the standard ~2" styrofoam insulation. One afternoon a thermometer stuck to the outside showed the face getting up to near 160F. The inside surface was somewhere close to 120F. I think I'm going to line the inside of the doors in the new house with aluminum foil to cut down on the radiation. I may see if the door mfr can put a foil face on the inside. Foil radiant barrier does have its place, and value!

Thanks for the comments.

A hydronic radiant system will work great for keeping your garage at 50-55 but it will take many hours to warm it up from 55 to 70 on those days you want to work out there. It's just not a fast response system. On the other hand you will probably find that 50-55 is very comfortable for working in the garage and 70 is too hot unless you are painting something.

For slab heating that's right. Maybe I'll just put 1" XPS under the slab and use a hydronic fan coil unit. Two reasons for considering ceiling radiant, faster response and better for radiant cooling. But even 1" XPS will cost about $400 for a 790 sq ft garage.

Dow F250 is around $20 for a 2"x4'x8' at the big box stores here. So about $500 for putting 2" in your size floor.

This is what I see at HD & Lowes. The cheaper one is the white fluffy stuff!

The Barrier would have no seams, All-in-one subsurface barrier
The Barrier acts as a thermal barrier, vapor barrier, moisture barrier, and radon barrier, all in one. It is made with recycled, extruded polystyrene and heavy-duty black polyethylene film (less than an inch thick), laminated on both sides and with patented seamless self-taping edges. It can be used under slabs, basements, foundations, crawl spaces, pools, hot tubs, and mobile or modular homes. It also deters carpenter ants, termites and other insects. Barrett Enterprises Inc. 877-904-8424. http://www.barrett-inc.com/. No breakage, no mess, no leaks. Cheapest is not always the best investment.

That's true. The Barrier, being 1/2" or 3/4" polystyrene, would also be significantly lower R-value than 2" polystyrene.

Out of curiosity I checked out the web site. Is there a tactful way to say it is littered with male bovine excrement?

Probably;-) We used The Barrier™ on a home we did a couple years ago including a basement slab (on & below grade), a two-car garage and a rather large snow melt drive & walkway. I will say that I’m still a bit skeptical this product performs as advertised. Since we’ll never really be able to compare it to traditional XPS foam board on this job, I can only give my personal observations. The product is very easy to work with. It un-rolls quickly covering large areas very efficiently. The rolls are easily attached and sealed to one another with well-designed peel back tabs that are self-sticking. (It’s fast.) It doesn’t break up over rough terrain like foam-board does and we walked on it quite a bit as we were tying down our tubing to the wire mesh. It does eliminate the need to use a moisture/vapor/radon barrier, which is a very nice feature. On the other hand, it is a bit expensive given the actual R-value that is achieved and I’m still a bit confused & skeptical about the conflicting debate over R-value vs. K-value in this or any type of application. A friend of mine in Denver recently did an experiment with various insulation products under a SIM driveway and I was surprised by his findings. Turns out that true R-value seems to trump everything else. We only have one chance to put in under-slab insulation. It’ll be there for the life of the building. Live & learn. In the case for your garage, I do like your idea of using ceiling radiant for primary heating & cooling with the use a hydro fan coil unit as a backup. I’d consider skipping the insulation all together or using the Home Depot cheapo un-faced foam-board.

dmaceld, white polystyrene is much cheaper than extruded but be careful with the regrind at home-depot. The r-values and moisture absorption ratings are fairly equivalent between extruded and poly, extruded actually gives of gassing and overtime has thermal drift. (loss in r-value) The prices you see at the depot are much more expensive than buying from an independent. You should go with 11/2" 1.5# virgin for around 15.00 per sheet.

Dave

It's my understanding from what I've read, IRC 2006, mfr literature, and elsewhere, that the white polystyrene is not suitble under slabs because of compressibility. In fact the IRC says horizontal insulation must be extruded polystyrene. The prices around here are the same for both Dow and Owens-Corning XPS. In general, prices at HD & Lowes for building material are comparable to contractor pricing at the independent lumber yards, and in some cases less. I'm looking at somewhere around $0.50/sq ft/in thickness for XPS regardless where I get it. Of course, I would probably get some break when I buy enough for the entire house project.

I checked the local Menards and the 2" Dow F250 was 21.95 so a little higher than when I last bought it but much less than your area. Maybe regional price differences?

Wow! That's a big difference. We've had a HOT, and I mean HOT, building boom in SW Idaho for the past 2 + years. That's probably driven building material prices up across the board.

Last year I sold my house, fully fixed up, in post-Katrina Louisiana for $108/sq ft, including about a $20 post-Katrina boost. The cost estimate I'm using for the loan application for the ICF house I'm getting ready to build is running right at $125/sq ft. That's for a high quality non-extravagant design and what it would cost to hire most everything done. I'm going for enough money so if I bust my back or have a heart attack there'll be enough money to finish the house.

I think you can see why I'm concerned about the price of blue board!!

Gentlemen, without trying to advertise our products there's a much simpler way to do this that is a huge labor saver.

Don, I found a price of $161 for 64 square feet for your 2" panels. Is that a typical price? ($2.50/sqft)