Subject: Re: [permaculture] small attached greenhouses
Date: Sun, 06 Dec 2009 01:59:54 -0700
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
Re: [permaculture] small attached greenhouses,
Stephen Sherman, 12/06/2009