We have a concrete slab in our structure. Above grade over the slab I put in a 2x4 floor with outdoor preserved wood on 24" centers to fit 1.5 inch styrofoam sheets Then covered with exterior grade plywood. This has kept our floor toasty warm for the last 18 years. This can be done by yourself on any slab in a day or two. If you go the below grade route then there are drainage issues and so on.

I also put in some extra runs of outdoor grade electrical wiring in the floor (loose at the ends and not connected to anything.) I've been able to use these over the years to add a light. fan. heater and so on as I've needed them.

Re the floor. If building your GH and not using a kit (your own is cheaper and more sturdy) there are two ways of doing the above grade floors. One: on top of the slab and then add your walls. Two: add the walls to the slab and then the floor. When building small structures I've aways preferred to attach the walls to the slab (a 2x4 plate)and then build the floor separate within the walls. This allows a bit more flexibility for adding on or renovating in future. A piece of floor can be ripped out or changed with no impact on the structure. Floor panels can be screwed down and then just unscrewed in a jiffy to run a pipe, wire or anytthing else and then just screwed back in place. If the floor panels run under the wall plates then it's more of a pain.

pcdur, if you insulate below ground, you are more likely to be able to take advantage of the thermal mass that is the floor of your greenhouse. Typically 2" rigid foamboard is installed vertically around the perimeter to a depth of a couple feet or to frost line. Others have also used foamboard laid flat inside, sloped for drainage, covered with gravel or other suitable flooring material. I don't know about the optimal depth of the flooring above the foamboard...others may comment on this...it may depend upon the your particular design.

The advice from stressbaby concerning the foamboard, vertically around the perimeter is a good idea. As for the floor, however, how much heat is actually lost in that manner? In looking at various university reports, approximately 56-62% of heat loss is through the roof,
27-29% sidewalls, 2-3% perimeter and 6-13% from air infiltration. None of the reports I found mentioned the floor area in their calculations.

We have two dynamics to deal with here. I think the issue with the floor isn't loss of heat but the cold coming in from the ground. An IR shot would show the panels glowing from heat loss and the floor showing cold.

Troykd, isn't loss of heat = gain of cold?

Cuestaroble, do you have any references for those numbers? Not that I question them at all, I'm just interested in more reading.

I think the two dynamics are losing heat (gaining cold) through the ground versus taking advantage of the thermal mass that is the floor in the same way you would use 55 gal drums of water in the GH.

Pcdur is in z6 Ohio, where the ground gets really cold. Perimeter insulation will take care of a lot of heat loss in his case. The question then is, do you get any further improvement in heat loss or the thermal mass side of the equation if you place insulation horizontally under your floor?

I did install 2" of foamboard vertically all the way around my foundation wall to a depth of about 2.5' (down to the footer) and I did not place any foamboards under my gravel floor. My reasoning (faulty or otherwise) was as follows. What is the R-value of soil? 1 foot of soil approximately equals 1 inch of insulation. Therefore, 2' of the soil above the level of the footer is essentially the same as 2" of insulation. Above that is my gravel which may (or may not) end up acting as thermal mass, insulated by 2' of soil.

Thanks alot,

I really appreciate all of the great opinions and facts,
I understand about the heat loss through the roof but, I was wondering about getting cold up from the ground. would that not make a difference in the heat temp. in side or therfore the change in temp. inside.

I was thinking that the more insulation in the floor to keep cold out that the heat inside would last longer.

Foam insulation board with an R-value of 10 is not cheap, therefore I did not want to speend a great deal of money if it will not make a great deal of difference.

Thank you all for the great help.

Stressbaby,
Your reasoning for not installing insulation on the greenhouse floor seems very sound. In fact, that is exactly what I have done in my greenhouse. Many commercial greenhouses in colder climates do the same as you mention. The reference for my numbers:
Blom, T., F. Ingratta, and J. Hughes. 1982. Energy Conservation in Ontario Greenhouses. Pub. 65. Ministry of Agric. and Food, Parliament Bldgs., Toronto, Ontario, Canada. It is an old publication, but I suppose physics do not change much. Sorry, but I was not able to find this pub. posted on-line.

Thanks Cuestaroble.

Pcdur, regarding heat loss, consider where your heat will go...heat lost through the floor is lost laterally and vertically. Heat lost laterally will be lost to the cold ground around the GH. This ground could be 20*F or even 10*F so the differential between the floor of a 70*F Gh and this outside ground is 50-60*F. It is the vertical perimeter foamboard that insulates against this heat loss.

Now consider heat loss straight down, vertically to the earth below the GH. This soil is warmer than the soil surrounding the GH. Going straight down, you might have a soil temp of 50-55*F. Therefore the differential between a floor of a 70*F GH and this soil is only 15-20*F. You can see that it is less cost effective to use foamboard to insulate against this loss.

Obviously, things aren't necessarily this simple. But if you are going to insulate the foundation, by this reasoning, if you are solely concerned about heat loss, the perimeter vertical insulation would be the first priority. That's the way I see it, anyway.

Finally found an on-line pub.(from Univ. of Calif.) that gives the following numbers for heat loss:
roof- 68%, infiltration- 21%, side walls- 9% and gable ends- 2%

Here is a link that might be useful: greenhouse energy conservation

Stressbaby,

As far as insulating the perimeter vertical insulation , would you please explain to me what you mean, and how to do that.
It seems as though you ,and cuestaroble, know exactly what your are doing, so if you would help me I would greatly appreciate it.

Thank you.

pcdur - here is a link with photos.

Here is a link that might be useful: perimeter insulation

Cuestaroble,

Thank youso verry much that will be a great big help ,and I really love those pictures, thats always good.

If I have anymore foundation problems I will sure know who to ask!!

Thank you so verry much for your time and energy.

Charles

Here in Alberta we definitely insulate the floor for greenhouses, sunrooms and so on. Many do R12 but a full R20 is recommended by CMHC. The floor is a BIG heat exchange and loss. I have R12 in the floor and R 20 on the perimeter wall going down to frost line and 12" above grade.

If you want the inside warmer than the exterior then definitely insulate the whole floor. It's one of the least expensive and but best heat retaining investment you can make for the dollar.

CMHC (Canada Mortgage and Housing Corp) may recommend insulating residential home floors. However, the Alberta Dept. of Agriculture, under "greenhouse floors",
"The floor is covered with white plastic film to seal-off the soil from the greenhouse environment, reducing the problems associated with soil-borne plant diseases and weed problems... The white plastic also serves to reflect any light reaching the floor back up into the plant canopy."

Here is a link that might be useful: greenhouse floors

cuestaroble,

That link is going to give me nightmares for weeks!

13ft high pepper plants? Do they go in there wearing full body armor and carrying flame throwers just in case the pepper plants decide not to give up their offspring (seeds) without a fight?

(best I get is 3ft!)

Back on topic, looks like they needed the light more than the thermal mass. In summer reflect the light back up and keep the floor cool, in winter the sun ain't goung to heat up the floor much so reflect the light back at the plants.

It is interesting to see how commercial operations differ depending on the location.

That is why we cannot define what works best for everyone.

Cool link cuestaroble, and the links on the bottom of that page are really good reading too.

Still going to have nightmares though.....

Our G.H. has joists over the bare ground filled with 4 inches of loose gravel and covered with 2 inches of extruded pink styrofoam. All finished with pressure-treated decking boards. Around the perimeter we put wrapped hay bales. They are wrapped to minimize the mice moving into the bales over the winter. The north wall is windowless and full insulated.