Insulating Existing Slab Floor

Greetings from -20 Minnesota! I have a question about insualting an existing slab. I have a 12x28, inflated double poly greenhouse that is built on what was a barn foundation (barn burned down 30+ years ago). Insulating down the sides of the slab is not too realistic, but would laying the thin,rolled , foil backed insulation on the slab then covering that with landscape fabric do any good? I mainly use the greenhouse from late February through May for seedlings, but I have been heating it lately to keep my water barrels from freezing (I have 14 55 gallon drums along the north wall). Any other ideas for insulating the floor? Thanks.

Jim

Jim, I'm not sure what kind of insulation you are referring to. If it is standard housing insulation I think that will just soak up water, eventually fall apart, give you a mushy floor to walk on and be more headache than help.

I'm wondering if you'd be better off with a cork underlayment that builders use to insulate concrete slabs when they install radiant floor heating. Instead of using the floor heating just roll out the cork matting then cover that with your weed mat/landscape fabric, or maybe better yet, lay out the cork, lay down plastic to keep the cork dry, then the weedmat/landscape fabric (to protect the plastic). And heck, if you want to put down some gravel on top of it all to absorb the heat from the sun.

Sorry...just thinking out loud here!

Do your best to stay warm! It must really be cold your way!

Shoe

jlee503,

I looked for this information when you posted on GW but I couldn't find it. Now, suddenly, it pops right up...

http://ohric.ucdavis.edu/Newsltr/fn_report/FNReportSp82.pdf

Look at the bottom left, first page. The heat loss through the floor is apparently so insignificant that they didn't even bother including it. Cuestaroble gives the following figures from other sources: approximately 56-62% of heat loss is through the roof, 27-29% sidewalls, 2-3% perimeter and 6-13% from air infiltration.

Now the common advice is to insulate the perimeter with rigid foamboard insulation placed vertically. But if these figures are correct, all that work is to conserve at most the 2-3% losses through the perimeter. It is a drop in the bucket compared to the losses through the other surfaces.

Nonetheless, if I were to do it, I would use 1" rigid foamboard and cover it with some kind of stiff surface that would prevent damage to the foamboard, something I could walk on.

I hope this helps some. SB

The slab will absorb considerable heat during a sunny Winter day and radiate it back at night. I agree, leave it alone and worry about the sides and top.
Dave.

Jlee503 ~ considering the phenomenal cold you enjoy and the fact that most homes there have basements for insulation, I am sure the slab radiates cold. If I may suggest, have you considered a radiant heat coil in the floor. I have a brother that installed that in his Mn. home using heated water. The water for his system is heated by a wood burning furnace which is removed from premises. It is efficient. I would think something on a smaller scale would make the GH more usable and efficient. There is also an electrical heat strip that could be installed under floor. Maybe not as economical tho. I would assume the water barrel storage was to provide some warmth? not just water source. Are the barrels painted black? It sounds as tho they may be a hinderance by causing you to heat to prevent them from freezing?
pod

Podster, good idea, but he would have to get some insulation underneath that radiant heat floor to make it efficient. And now we're getting expensive.

Thinking outside of the box for a minute...your only problem this time of the year is freezing of the 55 gallon barrels? Why not try an aquarium heater? I see 50-100W models online for as little as $14. If you could make it work with 100W or under, you could get all of your barrels heated with 1400W or less. If you need more insulation, why not insulate the barrels themselves? You could do that with a bubble wrap type of material. It would be far better to use PVC or IR-PE than to use plain PE, because plain PE is transparent thermal IR wavelengths and these are the wavelengths at which your heat will be radiated.

Is there a problem with letting the barrels freeze? Lots of people keep water in barrels in the GH for the latent heat, assuming that they will freeze, and as they do so, protect the plants with the latent heat. Are you concerned about damage to the barrels?

There are a lot of unanswered questions here. What are the water barrels for? If they are for some kind of usage all year, then I can see why you must keep them from freezing. Or maybe they're wooden barrels that would break if frozen. That much water is a good heat buffer for any greenhouse. If the inside temps are allowed to go below freezing at night, a little skin of ice would be fine assuming your greenhouse gets good and hot during the day if the sun is out. Having a fan circulate the hot air across the barrels would help them absorb more heat during the day and keep them thawed longer at night. Wrapping fiberglass insulation around the barrels is also a good idea. If you have some other insulated building such as a garage or possibly a shed, this would be better than the greenhouse because the greenhouse doesn't have good top insulation. Insulation is the key. They used to store ice for old style ice boxes all winter long by insulating it with lots of sawdust, straw, etc.

Cold doesn't radiate, heat does. Concrete doesn't get cold just because it's winter. Concrete radiates heat to the air when it's not covered. We can have 30" of frost depth in our ground, but the ground under my unheated garage never freezes. That's because the concrete floor in the garage looses much less heat to the insulated garage versus the open winter air. The earth is ALWAYS radiating approximately 60 degrees from the ground. It's just that the winter air is far more "powerful" so the top few feet freeze. If you insulate the ground surface from the nasty cold air, the 60 degrees from underneath will come through. It's hard to believe, but you can prove it to yourself. Lay down a piece of 2" foam board or some other good insulator that's weatherproof and pick it up in the middle of winter. The ground should still be soft underneath. Same thing happens if you get a deep, early snowfall - the ground won't freeze with the nice layer of insulating snow.

Your concrete slab can lose heat to the air of the greenhouse (which then radiates heat through the skin, then to the open air), or it can radiate heat to the surrounding frozen ground. It will not lose heat from below because the ground temp is near 60 degrees. I know you said it's not feasible to insulate the sides of the concrete slab, I guess we'll have to forget about that.

I probably didn't help you one bit, so I'll try to summarize: If you're heating the greenhouse anyway to use it, storing water in there (for any reason) is a great idea because of its buffering capabilities. If you're not using the greenhouse (as a greenhouse), then it's a TERRIBLE place to store water barrels if you don't want them to freeze. Last option: If moving the barrels twice a year isn't feasible either, then I wouldn't bother with the foil backed insulation on the floor. My personal opinion is that it won't much good. As others have mentioned, your precious heat is radiating up through the top of the greenhouse, not underneath. In this case I would probably clump all the barrels together as tightly as possible and insulate the crap out of it with bales of straw, fiberglass insulation, whatever is available. If you're actively using the water out of the barrels, this probably won't work. We still don't know what the barrels are for.

Water is cheap. Why don't you just empty the barrels?

Stosh is right, I think we're saying the same thing. You won't lose heat straight down...that's why the heat loss numbers from Cuestaroble don't include the "floor" but do include "perimeter."

Thanks everyone. The water barrels are for thermal mass, and my concern is that during these wonderful cold snaps (100 straight hours below 0) that they would freeze solid. Two of them in the NW corner (less sunlight hitting them) almost froze solid last week.

If such a small amount of heat is list through the floor, I will concentrate on sealing up the end walls. Thanks again for the advice. Spring will be here soon (at least 4 out 5 ground hogs said so).