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.


11 thoughts on “Polyisocyanurate Insulation

  1. ts not just igloos that use body heat as the only heat source! We made it 2 years with no heating or cooling (but we have been bad lately, and using it every few days)

    I’ve used that stuff before, to insulate the space under the trailer when I lived on the snowy East Coast. I only used one layer, and just enough to fill the gap (about 2 ft tall, and 65ft around), because it is freggin expensive!
    How did you find it for less?
    Also, why did you decide on 4 layers, and not 3, or 5? What insulates the roof (or, if its meant to let in sunlight, what is it made of)? Are you sure that the insulating factor goes up proportionately with the layers? How about the missing foil, does that lower the R value? Did you leave a gap between layers, or put them right up against each other?

    • Hey Jacobaziza, Good questions…

      I found the polyiso from a foam board reuse company. They take the material from deconstruction jobs and resell it for half price. There are a couple in the Boston area. In the future though, I’ve found better material at the same or lower prices on craigslist.

      We picked 4 layers for a couple reasons, first was price. Second was space available for install. The trusses are 6inches tall so we were able to fit 4 inches of insulation in there nicely. Five or six inches would have been overkill. Three might have done fine, but I didn’t want to risk not having enough insulation (there isn’t a lot of data out there about passive solar greenhouses and how they work in our climate insulated with polyiso)

      The lack of foil and gaps may lower the insulation value. But, I wasn’t able to sandwich the layers right against each other, so there is a little bit of a gap on some (the curve of the wall effected the layering). Unfortunately radiant barriers don’t really have that much of an effect, using building science, air leaks have the largest effect on heat loss, then insulation, then radient losses. The white paint we used is probably better anyway, because in greenhouses the foil can actually burn plants. With that said, next time I will use foil faced not because it is necessarily better, but because the fiberglass faced stuff is terrible to work with.

  2. I’m planning to use homemade charcoal packed between masonary or rammed earth walls. Charcoal is as insulative as polymer foam, is hydrophobic, won’t support criters, and won’t degrade trapped between the walls. I’ll be a few years until I can get to this, but I think that using whole bales of hay that have been charred may be the way to go. The inner wall then gets to act as a large thermal mass insulated from the exterior.

  3. Thanks for sharing your experiences here and the comments are quite insightful too. It looks like that foam was a pain to install but at least it was half price. Good to know you can find it cheaper on craigslist though. That site’s a great starting point for just about anything used!

    • As far as I can tell, without taking the layers apart, after 4 years, everything is doing great! The greenhouse hasn’t gone below 25 degrees F in 4 winters, with no added heat except what we gain from the sun during the day!

      • Thanks for the update! Are you planning any workshops for the coming year? I would love to see your bioshelter. We are building something similar here in SE Ohio.

      • I responded online as well:

        Hello John, Yes. We are having a Year-Round Food: Backyard Bioshelter Greenhouse Workshop November 5. CLICK HERE TO SIGN UP . Hopefully we’ll see you there!

        On Sun, May 1, 2016 at 8:32 PM, The Backyard Bioshelter Blog wrote:


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