Polyisocyanurate or extruded polystyrene for in ground use?

[strider3700]

I'm materials collecting for my solar hotwater setup and found a place selling probably used but in good shape 4x8 sheets of polyiso for half the price of what extruded polystyrenes costs around here. $10/inch for the poly iso is a great deal.

The insulation will be used to insulate my in ground solar storage tank and to insulate the pipes running in the trench. What I can't find however is if ployiso is ok for in ground use. Extruded Polystyrene is the standard for that around here and it is far more common so I see it everywhere. polyiso I've never actually seen for sale before so I'm not sure if it's just it's rarity that makes it not used or if it's an actual issue. Some manufacturer websites say it's fine, others don't mention it and thats about all I can find on it.

Any ideas?

[NeilBlanchard]

Critters like to tunnel in many kinds of foam -- some have borax in them, to try and minimize this.

[Ryland]

For below ground you want closed cell foam, at the depth that you will have your pipes critters shouldn't be a big issue, at least I've never heard of them being an issue and have worked with renewable energy system for many years, but if you are really worried you can have someone who sprays for pests spray some borax in the trench first.

[AC_Hacker]

Polyiso has a better initial R-value than EPS. The problem is that polyiso is not closed cell, and will take in moisture, which, over time reduces its R-value even in relatively low humidity situations like walls and roofs. If buried, it will be worse.

Here is a link to a Canadian site with charts...

I'd recommend the high-density 'blueboard' which is designed for foundation insulation.

And regarding cost, I'd advise going the extra bucks and getting the right stuff, because it is very hard to dig the wrong stuff up.

-AC_Hacker

[strider3700]

this page reports polyiso as closed cell
Energy Savers: Polyisocyanurate Insulation Materials

[AC_Hacker]

Originally Posted by strider3700

this page reports polyiso as closed cell
Energy Savers: Polyisocyanurate Insulation Materials

Well, paragraph # 3 of the text on that page verifies what I was trying to communicate.

As to open or closed cell, I met a boat owner who filled the cavities of his boat with this stuff, the spray-in variety, to make it unsinkable. To his great disappointment, over time it had taken up so much water that his boat had become almost 'un-floatable'. Since his boat was molded fiberglass, he would have to destroy the boat to replace the 'floatation material'.

So, since I happen to have samples of each at hand, I just performed a very crude test, where in I tried putting my mouth on samples of the foam and sucked on it to see if air could move through it. I tried a sample of the spray-in a sample of commercial Iso board and a sample of EPS board (I tried white EPS, NOT blueboard, I didn't have a sample).

Here's what I found:

1. The spray-in foam has a self-skin that is air tight. However on areas where the self-skin has been punctured or cut, air could migrate through easily.

2. The commercial iso board is denser than spray-in and has an air-proof covering which is sealed against air & moisture. However on areas where the skin has been pinctured or cut, air did migrate through, but at a much lower rate that the spray-in.

3. The commercial EPS board has an air-proof covering which is sealed against air & moisture. However on areas where the skin has been punctured or cut, air did migrate through, but at a rate lower rate that of the spray-in, but higher that that of commercial iso board.

NOTE: None of these crude tests actually address what goes on at the cell-level of the materials.

I did find this link from Green Builders.

DO leave a gap between the iso and the floor. Bare iso is somewhat hygroscopic and it CAN wick up water.

...also in a study titled, "Water Vapor Transmission and Moisture Accumulation in Polyurethane and Polyisocyanate Foams", I found this:

Regards,

-AC_Hacker

[Ryland]

A chunk of each foam in a glass of water left for a day or two can be a good self test as well.
I found that a foam they put in my house soaked up water and sank to the bottom of the glass after sitting two days.

[strider3700]

That study suggests that both Extruded Polystyrene and polyiso are going to have water issues and degraded performance in this sort of situation. Perhaps I should spend more time worrying about keeping the tank/insulation dry on the outside and less worry about the type of foam. I do need polyiso on the inside layer as temperatures can be quite high compared to the breakdown point of Extruded Ploystyrene.

[AC_Hacker]

Originally Posted by strider3700

That study suggests that both EPS and polyiso are going to have water issues and degraded performance in this sort of situation.

Except for very high heat (how hot is this thing going to get?) I think blueboard would be the way to go.

-AC_Hacker

[strider3700]

inside the tank, depends on what I limit the collectors to. Probably closer to 60 C would be my max. My test collector hit's 115 C when stagnant so steam is all that comes out initially.

extruded polystyrene breaks down near 100C poly iso closer to 200 C

Every where I've ever read says extruded polystyrene will not survive in the collectors and is recommended for at least the first layer near the tanks.

I'd go blueboard in a second but I need a lot of insulation. I worked out I need 8 8'x4' 3" thick boards and 8 8'x4' 2" boards to get the R values I want. This comes to $400 for the polyiso 3" for the tank, 2" for the trench.

If I was to go pink/purple extruded polystyrene it only comes in the 8'x2' boards locally and the price is $952 Blue board is basically the same just a little more dense and was the same price last time I checked but I could do it again in case it's magically half price.

I could get the same R value using that beadboard styrofoam but it's completely wrong for the job
putting this tank in ground really sucks. cost wise but is great location wise... John at build it solar put his solar shed tank in ground inside of a shed. His end conclusion was he doesn't recommend it... I can't fit mine in the basement or I would...