permaculture

[Stephen Sherman <spsherm@msn.com>]

I've been growing in a small (8' x 18') attached GH for 8 or so years 
now. It has no supplemental heat or light, but has yet to go below 
freezing even in sub 0F weather. A few thoughts/suggestions from my 
experience:

1. First a disclaimer, I am in the Colorado mountains at ~8400', we get 
plenty of sun but also cold temps and winds. People in other climates 
without our abundant sunshine may not be able to strike the same balance 
as I have.

2. It's been quite a while, but back when I was designing the GH, it 
became clear that heat storage was the main obstacle. Most GH's vent 
extra heat on sunny days, that just wastes heat that in most cases you 
just end up replacing later on...  There are several heat storage 
"schemes" for GH's, the water barrels, stone/brick. And the ones I like 
best are when you can use some other "thing" in the structure to store 
your heat for you: John Cruikshank's (sp?) soil storage system (SHCS), 
and Earthship's use of living space as the heat sink. Both of these a 
"more PC" in that they make use of an existing feature and add the heat 
storage function to it. SHCS has the advantage in that it creates warmer 
soil which aids plant growth.

Personally I did a sort of a merge of these two systems. I have raised 
beds which allow for more surface area to the soil (John actually did a 
SCHS plan for my GH, and fans and tubing in the soil would no doubt have 
been better, but when I built the GH, it worked well enough that I never 
added the SHCS system). Second the GH opens into my garage, which acts 
as a passive heat sink for the GH, taking extra heat during the day and 
returning it at night.

In combination, these two systems have kept the inside temps nicely 
above 40F all of the time. We just had a couple of days of -15F lows at 
night, with 20-40mph winds, and the GH stayed above 50F (although not 
much at times). Granted it was sunny for the week before, so plenty of 
stored heat in the system. I am still growing tomatoes and peppers 
(actually I have a 4 year old pepper plant that is approaching tree size 
in there). Much less fruit in the winter, but they are both still 
flowering and producing.

3. Air tightness, and a well insulated envelope surrounding as much heat 
mass as possible is goodness. I put in a quasi-conventional concrete 
foundation with 4" of foam insulation on the outside, down 4' below 
ground level, extending up to the level of the glazing. This gave me the 
strength I needed for our winds (design to 130mph wind load) and added 
to the thermal mass with a good thermal break between the GH's soil and 
the outside.

You mentioned that you won't be able to go below ground. That is 
unfortunate, in that it will make putting enough thermal mass into the 
GH considerably harder, and make a SHCS system difficult if not impossible.

I would spend the time to do some heat gain/loss calculations on the 
design you are planning. And cost out whatever fuel costs that design 
might need to meet expected conditions. It's alot easier to make changes 
and see what they will "buy" you energy-wise and growing-wise on paper 
rather than after you've got something built.

Steve in Colorado