Pole barn ceiling insulation with foam board?

[Lucky-Ducky]

I am in the planning phase of a new pole barn. It will have vented soffits and a ridge vent.

Is it possible to put 1.5” foam boards between the purlins and then 2” foam on top of that? Would this allow enough air flow through the ridges of the metal roofing?

Or should I attach the foam to the purlins and allow more airflow?

There will not be a ceiling installed for a long time.

 

[jack stand]

Is the ceiling level going to be your final insulated "envelope"?
I'd think about a layer (or 2) of foam there. SYP trusses can be pretty difficult to infill (crooked) and unless you cut the foam to easily fit and can foam the edges.
It's just a important to stop airflow through the ceiling as the rest of the building.
Another way is to strap the ceiling nice and straight with 2x4's and put a 1 1/2" foam under the trusses in between them and sealing that layer and then fill in between the trusses from above.
Sorry I've swerved away from you're question, but usually the space in the trusses are outside of the building insulation envelope.

 

[Lucky-Ducky]

Is the ceiling level going to be your final insulated "envelope"?
I'd think about a layer (or 2) of foam there. SYP trusses can be pretty difficult to infill (crooked) and unless you cut the foam to easily fit and can foam the edges.
It's just a important to stop airflow through the ceiling as the rest of the building.
Another way is to strap the ceiling nice and straight with 2x4's and put a 1 1/2" foam under the trusses in between them and sealing that layer and then fill in between the trusses from above.
Sorry I've swerved away from you're question, but usually the space in the trusses are outside of the building insulation envelope.

They are 6/12 scissor trusses. The bottom chord would eventually (maybe) have metal liner panels attached. I am planning to keep open trusses for a bit.

 

[jack stand]

So why would you insulate against the roof surface? Foam as I described above doesn't care when or if it's covered and would suffice as your ceiling until you decide what/if you do something later keeping out a lot of air movement. Hell with a little paint the foam could be your ceiling and insulation.

 

[Lucky-Ducky]

So why would you insulate against the roof surface? Foam as I described above doesn't care when or if it's covered and would suffice as your ceiling until you decide what/if you do something later keeping out a lot of air movement. Hell with a little paint the foam could be your ceiling and insulation.

To keep the heat in and out?

Painted foam? less work and more beer drinking project development time?

 

[CluelessPro]

Another way is to strap the ceiling nice and straight with 2x4's and put a 1 1/2" foam under the trusses in between them and sealing that layer and then fill in between the trusses from above.

This is what I would do at a minimum. But based on your other reply it seems you may end up doing a metal ceiling. If that is the case, have you worked out the cost difference to go ahead a put the ceiling up and use blown-in instead of foam boards?

 

[billconner]

I think the liner panel and blow in is the winner. Otherwise just put foam up on bottom chord and seal seams well. A really good air/vapor barrier will solve a lot of problems. (So poly under liner panel if you blow in.

 

[dcg9381]

I am in the planning phase of a new pole barn. It will have vented soffits and a ridge vent.

Is it possible to put 1.5” foam boards between the purlins and then 2” foam on top of that? Would this allow enough air flow through the ridges of the metal roofing?

Is your roof standard R panel without a roof deck? I'd have some hesitation in situations with humidity installing flat foam board against R panel as the ridges won't be insulated and "could" have condensation. Solution might be roof deck or vapor barrier.

Pole barns "usually" get trusses, so it's not like you lose space if you move to an "attic" insulation type scenario.

You have ridge vents and soffit vents. When you insulate against the roof itself you're creating a different envelope.

 

[Fordguy1964]

Only thing I can think of with an exposed foam insulation layer is the flammability factor. My old garage had foam insulation and it specifically warned against having this as the final exposed layer for just this reason. Mine got covered in cheap painted paneling for protection. YMMV

 

[racecougar]

I agree with moving the insulation to the bottom chord, and that blown insulation would be the way to go if the OP didn't have scissor trusses.

 

[Firebrick43]

Only thing I can think of with an exposed foam insulation layer is the flammability factor. My old garage had foam insulation and it specifically warned against having this as the final exposed layer for just this reason. Mine got covered in cheap painted paneling for protection. YMMV

Polyiso foam has much higher fire resistance than XPS or EPS foam.

2x4 strapping across the bottom of the trusses with 1.5 polyiso in between will be the easiest and high performance. Before putting the liner on eventually I would put a layer of 1/2” polyiso to break the thermal bridging of the strapping

 

[dcg9381]

Only thing I can think of with an exposed foam insulation layer is the flammability factor. My old garage had foam insulation and it specifically warned against having this as the final exposed layer for just this reason. Mine got covered in cheap painted paneling for protection. YMMV

There is fire rated spray foam now. If you cover it up with OSB (like I did) you're probably making things more flammable. Drywall or steel panel makes sense. Early formulations of spray foam insulation used chlorofluorocarbons (CFCs) as a blowing agent, which made it "more flammable". It'll burn, but modern spray foam you need to get it really hot to self combust, typically over the temperature that it'd take to ignite stick framing.

Don't buy the internet videos of flammability of "can foam".

 

[cpakalolo]

There is fire rated spray foam now. If you cover it up with OSB (like I did) you're probably making things more flammable. Drywall or steel panel makes sense. Early formulations of spray foam insulation used chlorofluorocarbons (CFCs) as a blowing agent, which made it "more flammable". It'll burn, but modern spray foam you need to get it really hot to self combust, typically over the temperature that it'd take to ignite stick framing.

Don't buy the internet videos of flammability of "can foam".

CFC's will put a fire out. HCFC's might burn. HFC's will burn. The new stuff will explode. Some refrigerants contain propane these days.