Insulating under your heat sink is not a bad idea but you will get a better return on your investment on the perimeter of your foundation. The reason is because the coldest area is above your frost line. The area below your heat sink is fairly steady and much less cold so you're are not losing as much heat and over time, you can build enough heat that is beyond the heat sink (according to Sunny John). However, often the area below your greenhouse would take far more insulation than insulating your foundation. So, the fact that you need a lot more of it and the ROI is much lower, makes people tend to want to put that money into more insulation on their foundation.

I am not surprised that most contractors are telling you that they always insulated under heated floors. They should! But that is a really different scenario. -That is fossil fuel heat, heating a floor. You would not want more than half the heat you are paying for to go into the ground. With SHCS you are getting abundant solar heat through a phase change by way of a slow-moving fan. Even if you waste some of your heat, you are wasting the small amount of electricity it took to get the phase change to release the heat into the heat sink -not fossil fuel-powered heat. You are heating a large area (your heat sink) over a long period of time. It is really not the same thing at all.

We opted to insulate under our heat sink. I hemmed and hawed. If I insulate then I don't get the benefit of using unlimited heat storage under my greenhouse, if I don't it will take me a long time to build up that heat. In the end, my architect reminded me that the insulation is only going to stop the heat to a point. Heat will still get through so if I insulate, it is not like I am not influencing that unlimited thermal mass -it will just take a lot longer. So, I figured I'd insulate under the heat sink and get better efficiency up front and then over time the little heat that did leak through the insulation would buffer us a bit more.

Best of luck with your greenhouse!

Sincerely,
Curly Girl

Thanks, Curly Girl. That all makes perfect sense. In our severe winters, I feel better insulating both perimeter and under my greenhouse. I'm still working out the details of my system. I appreciate your help. By the way, I believe I saw some posted photos of your nearly completed greenhouse and it looks amazing. Can't wait to hear about all the amazing things you grow in it.

To me it depends on how long your heating and cooling cycles are. If a month or less then insulate underneath. If you're hoping to store summer heat for use in winter you need a huge storage volume. You can only get that by having a deep storage mass.

For long cycle heat storage all my money would go into insulation around the perimeter down to the depth of constant soil temperature. You're going to drive the summer heat pulse deeper than normal but probably not by more than a few feet. I don't see much heat loss below the normal depth of constant temperature and won't insulate below for a year long cycle.

It also depends on how deep your piping runs. That determines where the summer heat ends up.

Insulation underneath defeats the purpose except for short cycle heating and cooling.

Basically what I'm saying is insulation all around is good provided you're enclosing a big enough soil mass to meet your needs.

This post was edited by fruitnut on Tue, Feb 18, 14 at 17:00