Insulating GH Floor?

15 years ago

I have a 10X16' homemade greenhouse that attaches to the south side of my house. The walls and roof are made with triple-wall pollycarbonate. This coming winter will be it's 3rd winter. I live in the northern part of lower Michigan.

I was talking with my brother, who has no construction background or anything, and we were brainstorming ideas on how to keep it warmer during the winter. He asked if I insulated the floor before installing it. I did not. He speculates that a lot of the heat in the GH might be getting pulled out the bottom of the floor, which kind of makes sense.

My questions are ... how much heat would I actually lose out of the bottom of the GH in wintertime? And if it is substantial and worth doing, what would be the best materials and process of "retrofitting" the floor.

The floor was dug down about 8 inches and then filled in with 4 inches of quarter-chip and then 4 inches of pea gravel on top of that. I grow primarily orchids in my GH and would like to keep the temp no lower than 55 degrees.

Thanks for any advice!

-ken

Featured Answer

oakhill (zone 9A, Calif.)

15 years ago

Ken,
Thanks for the photos. Looks very nice. And, your ability to post all the photos in an easy to see format is very much appreciated. As for ripping out your floor, just read the scientific info that has been presented and is readily available on-line. The "perimeter", wall-down area, can be protected with insulation and perhaps help with some minimal heat loss. However, there is not one university or other scientific study that shows any significant heat loss through a greenhouse floor. Testimonials are interesting, but have not to date proven that insulating greenhouse floors can go around the laws of physics. Until hot air "sinks", I guess there will not be much research support. Some examples commonly posted are for solar homes, some with crawl spaces and other house building information. If the greenhouse floor is insulated, the solar heat gain during the day in the winter is impeded, not saved.

Comments (6)

oakhill (zone 9A, Calif.)
15 years ago
The link is to a fairly detailed discussion on greenhouse floor insulation

Here is a link that might be useful: greenhouse floor insulation discussions
birdwidow
15 years ago
Ken:

Your brother is right.

I have a fully insulated floor both under and around the perimiter of my freestanding GH, that holds a constant 72 deg. temp in the worst deep, No. IL freezes with very little impact on our gas bill and I am convinced that all that base insulation made a difference.

However, a fully insulated floor requires drains and unless you plan them, you might have to stick with perimiter insulation alone but with a triple wall, attached to a warm house, you have a heat advantage already.

It would mean excavating around the outside perimeter, both down and a bit out to affect the reduction of heat loss you are seeking.

You would need to dig a narrow but deep trench down to your frost line immediately outside the exterior, then lay another piece from the top of that foam "wall" to form a "flange" out about 2 ft., sloping it away from the perimiter, because it's pretty dense and waterproof and you want water to run away, not back down under your foundation.

The material of choice for this type of application is 2" thick Type 150 insulating foam: the pink stuff. The Type 250 might be an even better choice. It's denser and the difference in cost for 4 X 8 sheets isn't all that great.

For a structure your size, I'm guessing that you would need about 7 sheets, five for the deep walls and two more plus leftovers from the trenchs, to be cut in half lengthwise, for the 2 ft. wide flanges.

Then, once the foam parts are all in place, use expanding foam like Great Stuff to fill any small gaps.

BTW: That deep trench is an ideal excuse for renting a Ditch Witch.
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calliope
15 years ago
I do not insulate floors, and I have a commercial operation. All the growing houses have kick boards, and are kept airtight as we can. Flooring is pea gravel over sand, and then covered with landscape barrier.

Perimeter insulation makes a lot of sense. We do have a solarium, against our house, like schwing. We have polycarb rigid double wall on it. The polycarb does not go all the way to the ground. We have an approximate two foot wall around the periphery to which the glazing is attached. We used a sand bed, and then lay brick for thermal gain and as a heat sink. The wall attached to the house is light, and reflective. How is your g'house oriented to the sun? I do not use a moisture barrier, as I want any moisture to drain through the brick and sand, and it does. This would be a 12 X 24 with a 35K btu thermal output heat source.

There isn't as much loss to ground as you'd think, because the ground under the structure is warmer than that outside. But, you will get heat loss around the lower periphery for a couple feet, and that's where I'd concentrate. I doubt you need the polycarb close to the ground, and your walls are made of What? to which you've attached the rigid polycarb. It would make sense to insulate them and fill gaps with expandable foam, perhaps. Any plant material I've kept in this lean to (mostly houseplant type stuff) are moved a few feet away from the walls, or are up on benches.
stressbaby
15 years ago
As with Calliope, perimeter, yes. Cuestaroble's studies suggest that maybe you don't even need the perimeter insulation, but I ran some other calculations (in another thread: http://forums2.gardenweb.com/forums/load/strucs/msg0312555932001.html) that suggest that maybe up to 13% of heat loss is through the perimeter, depending on the size of the GH and maybe the climate.

Floor? No.

Flange? Very questionable, I have never seen any data on that, just anecdotes from BW and maybe a dude from Australia who never posts here anymore.
schwing
Original Author
15 years ago
Thank you for your advice. It sounds like it would be good to go back and retro-fit my GH and at least put the perimeter insulation in. Now to decide if digging out the whole interior would be worth it to install an insulation floor.

My GH get full south sun so when the sun does shine through in the winter it warms up pretty good.

Since our polycarbonate goes all the way to the ground, we were thinking that maybe if we built some flower boxes (raised bed kind) that maybe that would give us some more protection from the cold. We would maybe build then up a few feet so as to not block too much of the sun to the lower shelf plants. Do you think that would help?

Also, I attached a link to the project photos we took while putting it together.

Thanks again!
-ken

Here is a link that might be useful: Ken's Greenhouse Project Photos