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  • From: Stephen Sherman <spsherm@msn.com>
  • To: permaculture <permaculture@lists.ibiblio.org>
  • 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

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