Polyisocyanurate Insulation


We go to sleep every night, hopefully in a warm cozy bed, and forget about how cold it is just beyond, in the winter night air. There are two technologies helping to create the comfort of the modern bedroom.

The first one is a heating system of some kind. Unless you are heating with the sun or a stove (or body heat in the case of igloos), the boiler or furnace is set to turn on with a thermostat. Typically the system cycles on and off keeping the room between 65 and 75 degrees, depending on your needs for warmth (we set ours to 58 at night).

Secondly, unless you are living in a building that was built before 1940, there is some kind of insulation between you and the outside. In recently built homes, say between 1980 and 2000, this insulation is typically fiberglass. When put into 2×4 frame wall cavities fiberglass insulation rates about R11 (R being the resistance to heat flow, 0 being no resistance). R11 seems good right? Well, imagine the heat that could be saved by doubling to R22. To get close to that with fiberglass it would mean at least a 2×6 wall with more expensive R21 insulation.

The bioshelter north, west and east walls are closer to an R24, which is double the insulation of most homes. How could we possibly pull that off without a really expensive thick wall?

It all started one beautiful sunny day. Eric and I rented a box truck. We were headed to a large parking lot an hour from home. Sitting there waiting for us was over a hundred panels of used insulation. This insulation board is known as polyiso board, or closed cell foam. Without it, the bioshelter would not be able to mimic Northern Florida winter temperatures.

It took us an hour or so to load up the truck with 160, 4 foot x 4 foot, one inch thick panels. Our plan was to install a 4 inch thick layer of this stuff as the bioshelter wall. See photo for how we actually layered it. Once the panels were cut to fit and glued and screwed in place, we used the same material in spray form to fill in the cracks. The spray foam comes out of the can as a liquid and quickly expands to fill in the gaps.

piles of insulation

Piles of foam board, some coming home with us.


North wall four layers of insulation, exterior layer already in place.

Now it is time to add the spray foam. Don't want any heat to escape!

Now it is time to add the spray foam. Don’t want any heat to escape!

Compared to other types of insulation (see chart), the foam won out because of its high durability. Fiberglass although cheaper, doesn’t hold up to water damage like foam does. Since we are in a wet environment in the bioshelter, it seemed that foam’s high strength and water resistance would be a better choice. Plus, fiberglass must go into a wall cavity, where foam board becomes the wall.

cost labor durability recycled R-value/inch
polyiso board high high high yes 6
fiberglass low low low no 3
rock wool medium medium medium no 3
cellulose medium medium low yes 3.5

Unfortunately most of the insulations that I researched aren’t very ecological. Almost all of them are made with non-renewable materials or high embodied energy. Foam is made of plastic, fiberglass and rock wool is made at high temperatures and is not recyclable, cellulose is the most ecological as it is made with ground up paper, but I felt like the foam board will last the longest, and it was diverted from the landfill.

I must mention the challenge we had installing the foam board (this almost made it not my number one choice after the fact). It turned out the particular foam board we got for half price, was faced with not the typical foil, but fiberglass paper. We had to shape the foam a lot to get it to fit, and all that cutting dislodged the fiberglass, which got into our skin, eyes and lungs. If I ever do a project like this again, and use foam board, I will make sure to pay double to get non-fiberglass faced insulation!

If you have had different experiences or ideas to add about using insulation in greenhouses, please post a comment.