tennis maynard wrote:
Rob wrote:
Of course in that case it helps to

have a bit of insulation between your rock and the ground, otherwise
most of the heat goes the wrong way.... If you are installing a new
greenhouse, consider spending the extra 100 dollars or so to put a
layer of high density foam insulation under your greenhouse floor.
Regardless of what your flooring material is. I wish I had done that.


If I can ever get the morons at the building permit to pull their
heads out of their...um...well, you know...I am planning on putting the
styro around the foundation up top glazing level, about 4" above ground.
At that point, I think insulation under the floor might be a bad
idea...at least with the insulated foundation down past the permafrost
line, the ground beneath should be at about 50 degrees, right? Or at
least substantially warmer than the outside air. This should help with
the heating, n'est-ce pas? What do you think Rob?


I feel your pain Tennis. And I won't say "I told you so", although I
seem to recall that I did... *grin* After my run-in with the building
police, I've decided that it is far easier to beg forgiveness than ask
permission. At least for a greenhouse.

As to 50 degree earth. That is great if you are heating your greenhouse
to 50 degrees. Chances are good you will want it a bit warmer than
that. I've been reading a lot of underground house books (I want one!),
and it seems counterintuitive, but you still want to insulate your
floors. If your greenhouse is 65 degrees, and your floor is 50, you are
still going to lose heat (fairly quickly) to the large mass of the
earth. Insulation slows the process of heat transfer, and allows you to
keep your higher temperature longer.

At this point you are saying "Whah? Huh? Why use earth at all and bury
part or all of your house?". Well, remember, when the earth is 50
degrees, the air might be 100 or -30. It takes a lot less fuel to
maintain a 15 degree temperature differential than a 95 degree one.
That is why thermal mass is so important... The insulation slows down
heat transfer, and the thermal mass of the earth reduces the temperature
differential.

I think that the logic behind homes and greenhouses is slightly
different, however. Obviously we want as much sunlight as possible in a
greenhouse, and that really messes with a solar house (where you collect
as much light in the winter as possible, and exclude it in the summer).
So while the concept works in the winter, insulate the foundation so
we don't lose heat, it really fails in the summer. We want the
foundation cold, ice wouldn't be too cold in August... But, vents are
much cheaper to operate than furnaces, and a wet-wall or other cooling
system probably is too. Failing that and if you want to dig deep holes,
you could have your replacement air come through buried pipes. Hot air
leaves through the vents, cool air comes in through the buried pipes.
Cheap to operate, a bit more expensive to install.

And as a final thought (hah!) on the subject... All of this is great to
think about for a hobby greenhouse. We can spend a little more to build
a neat energy efficient design. And we might rationalize our extra
expense by thinking that we will keep that greenhouse for 20 or 30 years
and it will pay for itself eventually. It is substantially more
difficult to justify the extra upfront costs in a commercial greenhouse.
It can be done, and probably should be done, but just like government
work, the contract often goes to the low bidder.

--
Rob's Rules: http://littlefrogfarm.com
1) There is always room for one more orchid
2) There is always room for two more orchids
2a) See rule 1
3) When one has insufficient credit to obtain more
orchids, obtain more credit