4" polyiso....between my rafters

[Mthomas1686]

So I went and picked up 2800sqft of polyiso yesterday. It's a glass paper faced product. I'm going to cut it to fit between my rafters tape the seams and I'll be putting up some type of ceiling.

Im curious if I should put a vapor barrier between the foam and ceiling material? I have vented soffits and ridge vent, no wrap or vapor barrier anywhere else.

The walls will get a similar treatment....polyiso fastened to the girts (leaving 1.5" airgap) then I will probably frame out the walls and put in some fiberglass batts.

 

[Kaizen]

Not sure about polyiso with water movement.
But if it were me I’d be trying to do it so all that iso is a flat plane with tape between them. The wood is a thermal break reducing effectiveness and allowing air movement. I’m stumped though at how to do it and not have to use expensive five inch fasteners for ceiling and walls


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[Bert_]

Why the foam? It will make for a lot of extra work.

 

[bkg]

also interested in the vapor barrier question

 

[Mthomas1686]

Why the foam? It will make for a lot of extra work.

I want to minimize and fiberglass or cellulose material to keep down the mice. Keep weight down, especially in the event of a roof leak. I'm not really into the spray foam thing...expensive for a building this size and if I ever need to replace anything it becomes even more of a mess.

My trusses are 4' oc so I measured in numerous spots and will be able to trim the sheets and force fit them. Was then thinking 1x3 strips on to hold in the edges (still in the air).

As for the work it's all relative....any estimate that I get for any thing I want done in here is in the thousands to 10 thousands. I was able to get this material for $0.80/ soft so I'm in it for a great price. It's nearly r24 rating so it will do a good job.

 

[Mthomas1686]

Not sure about polyiso with water movement.
But if it were me I’d be trying to do it so all that iso is a flat plane with tape between them. The wood is a thermal break reducing effectiveness and allowing air movement. I’m stumped though at how to do it and not have to use expensive five inch fasteners for ceiling and walls


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I know polyiso have a very good perm rating. The facing is not as ideal as the foil faced but this is roofing material.
I will force fit it between the rafters so I don't lose ceiling height.
The walls will be getting 1.5" and will run overtop of the girts so they will be continuous.

 

[Oregon rock crusher]

I like polyiso for ceiling insulation and put up some 2" panels over my purlins about 15 years ago in the main shop. my roof did have standard poly sheet vapor barrier under the tin. I put R-13 bats between the 24" center purlins and then put the polyiso panels over that. They were used foil faced panels that I got for about $7 a sheet. Never an issue there and the R value is pretty good.

I just finished insulating a small shop addition with 3-1/2" fiber faced polyiso panels which were about $13 each at the local bargain barn. Some of it over the thin (an inch or so) insulated vapor barrier layer they use on pole barns and some over craft faced bats. I did the same thing there just going over the top of the purlins. I think I will prime them with white Zinser and call it a finished ceiling. I previously tried the primer treatment on the same type fiber faced panels and it worked well brightening up the space.

I think the exposed polyiso does a good job of absorbing sound too making for a quieter work space. With a little effort the panels fit together so tight I doubt you could get a credit card between them anywhere in the shop. Ed.

 

[Mthomas1686]

I like polyiso for ceiling insulation and put up some 2" panels over my purlins about 15 years ago in the main shop. my roof did have standard poly sheet vapor barrier under the tin. I put R-13 bats between the 24" center purlins and then put the polyiso panels over that. They were used foil faced panels that I got for about $7 a sheet. Never an issue there and the R value is pretty good.

I just finished insulating a small shop addition with 3-1/2" fiber faced polyiso panels which were about $13 each at the local bargain barn. Some of it over the thin (an inch or so) insulated vapor barrier layer they use on pole barns and some over craft faced bats. I did the same thing there just going over the top of the purlins. I think I will prime them with white Zinser and call it a finished ceiling. I previously tried the primer treatment on the same type fiber faced panels and it worked well brightening up the space.

I think the exposed polyiso does a good job of absorbing sound too making for a quieter work space. With a little effort the panels fit together so tight I doubt you could get a credit card between them anywhere in the shop. Ed.

Ed that looks great! I'm thinking then maybe I'll just paint it and see how that looks. I wanted the foil faced but this stuff sells quick so I figured I couldn't pass it up

 

[bkg]

Ed - where did you get the washers? I'm having a hard time finding them locally.

thx!

 

[Oregon rock crusher]

Ed - where did you get the washers? I'm having a hard time finding them locally.

thx!

When I bought my first batch of polyiso panels it was from a local construction company that had removed them from a school gym that was being refurbished. They had truckloads of panels and also all of the hardware that had secured them. I picked up three or four five gallon pails with the panels. I made two trips with my PU and a trailer to get all the panels home. I see you can order the pressed fastening washers online fairly reasonable like $100 for 1000 pieces. The long 5" screws aren't that bad either in bulk but for the 2" panels I just used deck screws.

A couple pics of the ceiling and doors in my main shop too from a when I put up the foil faced panels. The wall and ceiling insulation helped but after I put 2" of polyiso in the doors the whole feel of the shop changed. That and good door seals and now it holds comfortable temps very well, summer and winter. Ed.

 

[earlybirds]

I used the same washers pictured above used with deck screws. I got them at Menards(special ordered online at Menards).

 

[bkg]

When I bought my first batch of polyiso panels it was from a local construction company that had removed them from a school gym that was being refurbished. They had truckloads of panels and also all of the hardware that had secured them. I picked up three or four five gallon pails with the panels. I made two trips with my PU and a trailer to get all the panels home. I see you can order the pressed fastening washers online fairly reasonable like $100 for 1000 pieces. The long 5" screws aren't that bad either in bulk but for the 2" panels I just used deck screws.

A couple pics of the ceiling and doors in my main shop too from a when I put up the foil faced panels. The wall and ceiling insulation helped but after I put 2" of polyiso in the doors the whole feel of the shop changed. That and good door seals and now it holds comfortable temps very well, summer and winter. Ed.

That looks nice...

I tried using construction adhesive on a couple panels last weekend - because I had it laying around - but it's not a good solution. Off to look for washers!

 

[Bretny]

I did something like the OP is doing yet I didnt use tape. I purposely left a 1/4in gap around all sides and used foam in a can to seal things up. It really helps hold the pannels after it died. This was just a 10x14 shed roof.

 

[PWC Repair]

I did something like the OP is doing yet I didnt use tape. I purposely left a 1/4in gap around all sides and used foam in a can to seal things up. It really helps hold the pannels after it died. This was just a 10x14 shed roof.

Mine is actually spray foam that came in sheets BUT,....spray foam is all that holds mine. Been a couple years now.

Pics in my build link.

 

[bkg]

QQ - what are y'all using to cut the 4" foam? I'm tempted to "borrow" the electric bread cutter from the kitchen...

 

[PWC Repair]

I just pushed a sawzall through mine with a course tooth blade.

 

[Mthomas1686]

Mine is actually spray foam that came in sheets BUT,....spray foam is all that holds mine. Been a couple years now.

Pics in my build link.

PwC I saw your build...looks great and you truly save a ton of money with those trusses! Thank you for the feedback on this issue.


I asked the guy I got it from he said sawzall as well. I was thinking circular saw though, might get a straighter cut. I'll try a nice fine blade.

 

[Jim_No_Garage]

I asked the guy I got it from he said sawzall as well. I was thinking circular saw though, might get a straighter cut. I'll try a nice fine blade.

You might do all right with a regular 24T construction blade. Try it regular. If you get a lot of ripping try reversing the blade on the saw - guys do that when they cut plastic soffits.

I fine tooth blade might get too hot and melt things.

That advice is worth exactly what you paid for it . . . .

Cheers

Jim

 

[Oregon rock crusher]

I use a guide with my circular saw. On 2" panels I can just reach through but on the thicker stuff I have to cut from both sides. I have a couple nail holes in the guide and drop a couple nails thru to hold it online for the cut. Really pretty quick to set up and if I measure careful the cuts match close enough. Sometimes I can use the 12" table saw for rip cuts but but the thick panels like to bind so it doesn't work to well on bevel cuts. The cutting dust from polyiso is like dry snow and gets everywhere. Helps to have a vac handy. Ed.

 

[Bretny]

I cut my glass paper faced polyiso with a table saw. Worked very well.

OP where did you get your polyiso from?

 

[bkg]

I use a guide with my circular saw. On 2" panels I can just reach through but on the thicker stuff I have to cut from both sides. I have a couple nail holes in the guide and drop a couple nails thru to hold it online for the cut. Really pretty quick to set up and if I measure careful the cuts match close enough. Sometimes I can use the 12" table saw for rip cuts but but the thick panels like to bind so it doesn't work to well on bevel cuts. The cutting dust from polyiso is like dry snow and gets everywhere. Helps to have a vac handy. Ed.

I cut my glass paper faced polyiso with a table saw. Worked very well.

OP where did you get your polyiso from?

excellent! Thanks!

 

[Mthomas1686]

I may even just try that....that'll be the straightest cut.

I got it from a roofer off craigslist. It was all brand new in the bundles, I had to open them to fit in the trailer.

 

[bkg]

Just a word of thanks.... the circular saw trick works phenomenally well!

 

[Mainiac Mat]

I built my house which is a timber frame with structural insulated panels made by Muris for curtain walls. Essentially isocyanate molded between 1/2" OSB layers.

The manufacturer's specs had the porosity of the iso so small that it qualified as a vapor barrier.... so I put sheet rock right up against it on the interior and skipped the poly sheet.

So I suspect your iso board is good to go.... especially as it probably has that foil skin on it to reflect radiant heat, which I suspect is also impervious to vapor.

As you mentioned, taping the seams will be an important step.

 

[Mainiac Mat]

I built my house which is a timber frame with structural insulated panels made by Muris for curtain walls. Essentially isocyanate molded between 1/2" OSB layers.

The manufacturer's specs had the porosity of the iso so small that it qualified as a vapor barrier.... so I put sheet rock right up against it on the interior and skipped the poly sheet.

So I suspect your iso board is good to go.... especially as it probably has that foil skin on it to reflect radiant heat, which I suspect is also impervious to vapor.

As you mentioned, taping the seams will be an important step.

One important consideration when using iso board extensively is that carpenter ants can and will get into it to make their nests. The only way I've found to solve this problem is to use commercial grade ant poison.... which works like a charm to kill the queen and nest.