Polyiso between wall girts

[Mancino]

Hey GJ,

So I'm starting to think about insulation for my pole barn and I'm a bit confused about the uses for polyiso board.

First off, please correct me if this information is wrong, but from my searches on the internet, I've read a few articles that say polyiso will lose some r-value as the temperature drops, as opposed to XPS or EPS foam. One article in particular claimed the best use for polyiso is inside the heated space to get the most of it's r-value....I also know about the off gassing that comes with polyiso.

So with all that, what I was looking to do was use 1 1/2-2" foam board between the wall girts of my barn. Basically to flush out the wall and get some sort of wall insulation until I decide on spray foam or doing batts on top of that...I found a few places selling reclaimed insulation, but they keep trying to sell me on polyiso board. When I tell them what I've heard, they immediately either get defensive or say I'm totally wrong.

Has anyone used polyiso like this to insulate their pole barn? Or can anyone clear this up? Is it really as bad as I've read? Or are we talking a very marginal difference in r-value that polyiso drops in cold temperature?

 

[PWC Repair]

Well, polyiso is basically closed cell spray foam in sheet form. Its some of the best r value per inch. Im insulating my shop right now with 3" factory seconds right now. You can check it out in my build thread.

 

[toyotadriver]

I insulated my shop with 1.5 inch XPS between the purlins. I then put fiberglass batts in the walls. It works well. Polyiso does lose R value the colder it gets but it's still better than nothing. I'd use it if it's substantially cheaper than XPS or EPS.

 

[bobbyjean]

i used rigid foam board...1 inch..it left just enough space to "slip" behind posts and get into corners..then i used great stuff with the "pro" gun that i got on amazon cheap and sealed it all up...i also took roofing nails and tacked boards so they stayed secure...i am trying to load some photo's ...but i'm not computer friendly.....anyway that gave me a flush surface to work with .... the nails were tacked in to the top of girts

 

[Mancino]

I insulated my shop with 1.5 inch XPS between the purlins. I then put fiberglass batts in the walls. It works well. Polyiso does lose R value the colder it gets but it's still better than nothing. I'd use it if it's substantially cheaper than XPS or EPS.

i used rigid foam board...1 inch..it left just enough space to "slip" behind posts and get into corners..then i used great stuff with the "pro" gun that i got on amazon cheap and sealed it all up...i also took roofing nails and tacked boards so they stayed secure...i am trying to load some photo's ...but i'm not computer friendly.....anyway that gave me a flush surface to work with .... the nails were tacked in to the top of girts

Any reason you both went with XPS board over polyiso? Curious where you guys are located...I know it sells cheaper than foam board in upstate NY, so that concerns me. If it was so great, why would it be so much cheaper?

 

[OH_Varmntr]

I installed 1/4" XPS behind my wainscoting panels, foil taped the seams, then sprayed 1" closed cell foam to seal my 48x64x14 building. Link in my signature has pics of the XPS behind the wainscoting and bare steel above.

Hindsight being 20-20, I would have house-wrapped prior to siding and sprayed foam directly on housewrap.

If you're planning on spraying the foamboard for reason of being able to remove siding panels in the future if they become damaged (that's why I only installed XPS behind wainscoting), make sure to tape all of your seams since the sprayfoam will find its way through the smallest gap in the foamboard and adhere to the siding behind it.

 

[terabitdan]

I chose XPS over Polyiso for its compressive strength and moisture resistance.

I used XPS to built ventilation space under the roof deck before adding spray foam. I read that Polyiso would absorb some water vapor where XPS does not.

The other use was a floor buildup from concrete, so both the water resistance and compressive strength were important characteristics.

Neither of those seem to be important in your case. How much r-value is lost? Starting with about 30% higher r-value than XPS seems like you’d be better off even with some loss.


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[Mancino]

I installed 1/4" XPS behind my wainscoting panels, foil taped the seams, then sprayed 1" closed cell foam to seal my 48x64x14 building. Link in my signature has pics of the XPS behind the wainscoting and bare steel above.

Hindsight being 20-20, I would have house-wrapped prior to siding and sprayed foam directly on housewrap.

If you're planning on spraying the foamboard for reason of being able to remove siding panels in the future if they become damaged (that's why I only installed XPS behind wainscoting), make sure to tape all of your seams since the sprayfoam will find its way through the smallest gap in the foamboard and adhere to the siding behind it.

I too should have done the house wrap before I put up the panels. Unfortunately, I just never thought of it before I started. And yes, that was exactly my thought about spray foam. I want something between the foam and the panels.

 

[Dragfluid]

Look at my build thread for good info.

 

[toyotadriver]

I used XPS because I could get it at a good price. I couldn't get polyiso in 1.5 inch where I live without special ordering it. I didn't know about the R value loss in cold weather at the time.

I'm guessing it's too late for this but when I built my current house, we installed metal siding just like a shop. However, before we did that, we covered the purlins with 1 inch high density EPS. We then insulated with damp spray cellulose. The R value is fantastic. VERY low heating and cooling bills. The wrap of EPS helps reduce the heat gain/loss via thermal bridging. I would definitely do it again if I built another metal sided building.

 

[Mancino]

Look at my build thread for good info.

Great build Drag! Almost exactly how I plan on doing everything in my place....I definitely should've put up tyvek before I sheeted it! Do you think it's ok to spray dense pack directly on the metal?

 

[stm317]

I just picked up a bunch of foil faced polyiso sheets 1.5 inches thick to place between my girts. Found a place on Craigslist that sells it for $9/ sheet less than the box stores (who would've had to special order that size). Once the foam is up, the long term plan is to frame out the walls and insulate with r19 fiberglass. The added r value from the foam panels will only help, and it should reduce air movement and sound infiltration as well. My building is house wrapped though.

 

[toyotadriver]

Great build Drag! Almost exactly how I plan on doing everything in my place....I definitely should've put up tyvek before I sheeted it! Do you think it's ok to spray dense pack directly on the metal?

I've heard cellulose might rust metal if in direct contact. Don't know if that information is correct though. Dense pack fiberglass shouldn't be an issue.

You could also put some foam sheets against the metal between the purlins. Raise the R value and prevent direct contact of the insulation and the metal.

 

[Dragfluid]

Great build Drag! Almost exactly how I plan on doing everything in my place....I definitely should've put up tyvek before I sheeted it! Do you think it's ok to spray dense pack directly on the metal?

Thank you.
I can't answer your question. Like another member said, the acidity may be an issue on the steel. I would look into that a little more if I were you.

Something that you could do, but would be labor intensive, would be to staple sheets of Tyvek in each cavity as a barrier. Get a few buddies together and I suppose it really wouldn't take that long, once you got a system going. The material doesn't cost that much. You're only doing it once and the rewards are worth it.