Rigid foam under metal roofing without decking???

[hfmt93]

Hi all. I am building a 32 x 40 gambrel shop in middle Tn. I just finished pouring the slab and will be ordering wood at the new year. It has radiant tubing in floor and is "planned" to be kept at very low temp. It is a automotive/wood workspace.

I am trying to figure out the cheapest option to install the metal roof. I am considering getting half-inch poly iso- 4 x 8 rigid foam and installing it directly under the metal to keep condensation down. I was wanting to know if anybody had experience with this without using decking.

The stack up would go rafter, purlin, foam, metal. The thought is, I could get the shell up and if more insulation is needed in the future I can attach ceiling to the inner truss structure and put blow in insulation from there. Does anybody have experience with this? Does anybody see a big problem with this?

The metal manufacturer recommended closed cell spray foam, but The cost is very high for that spray foam and I can see the purlins being rotted from condensation.

Please let me know what you think and thanks in advance for any responses.

 

[theoldwizard1]

Shop for reclaimed poly-iso. Big cost savings. I would use a minimum of 2" foam board.

The real issue is the size and spacing of the purlins and the spacing of the rafters/trusses. You need enough support up there that a roofer can walk on them without falling through. 1x3, 1x4 (?) spaced 6" apart ???? If your rafters/trusses are spaced 24" O.C., then I would use 1x6s.

 

[3rdgendslmech]

HFMT,
Not sure how hot and humid it gets in TN in the summer, but I can tell you this from being in MD. We tend to get hot and humid summers EVERY summer. I just finished my build around the end of June before it got too hateful out. I added a ridge vent and vented soffits only on the eave walls and I had 0 problems with condensation even when it would cool down a good deal at night.
The only thing I kind of regret now is not putting up a radiant barrier under the purlins

 

[joe_padavano]

The problem is that you will get condensation between the rigid foam and the metal. I'd suggest spray foam to avoid this problem.

 

[stm317]

My shop has "double bubble" insulation sandwiched between the purlins and the metal. It acts more as a moisture barrier than insulation, but There's no condensation. I'll eventually finish a ceiling and insulate it properly as funding allows.

 

[Done That]

I went with "drip stop" installed with the metal roofing just for condensation control. Plus made sure of a vapor barrier under the concrete as well as having soffit and ridge vent. All good.

If I insulate in the future I'll drop a ceiling like you mention.

 

[MagKarl]

I would avoid using anything like foam that may degrade and crumble between your framing and roof.

You don't want the roofing to pucker at the screws or leak.

I'd use a thin roll product or wait and insulate from the inside after you're dried in.

I have a white/foil bubble barrier on mine and it works good in my opinion, cuts the heat and no condensation troubles. My barn is very well vented though.

 

[Captain Spaulding]

HFMT,
Not sure how hot and humid it gets in TN in the summer, but I can tell you this from being in MD. We tend to get hot and humid summers EVERY summer. I just finished my build around the end of June before it got too hateful out. I added a ridge vent and vented soffits only on the eave walls and I had 0 problems with condensation even when it would cool down a good deal at night.
The only thing I kind of regret now is not putting up a radiant barrier under the purlins

Condensation isn't normally a problem when it cools down unless you have a moisture source inside the building. The problem happens when it has been cool and it warms up. Warm, moist air getting into a cold metal building with a cold concrete floor makes it rain.

 

[hfmt93]

MagKarl, I feel that way too, but it seems to be done a lot. It does make me nervous thinking about foam being the supporting structure between the rafter and the purlin. Then years later having a roof that looks like the ocean.

One other thing the metal manufacturer recommended was 1.5" foam between the purlins. If I went that route I could put roof felt or similar over the flat purlin/foam layer as a second layer of protection from leaks. It would maintain structural strength from the purlins, and be protected from rot because of the felt underlayment. It would not insulate quite as well, but I think that would take care of my condensation issues.

Does this sound like the best route?

 

[rusty1]

...just use the insulated blankets that all the pole buiding builders use, they go directly over the purlins, with the steel on top,...never any condensation problems .
you can always add more from inside later or a ceiling.

 

[yeldogt]

Is the roof rated for no deck under it ? If it's not self supported you can't only have the foam.

People are mixing all kinds of things in this thread. Some metal roofs are subject to rust from under -- so you don't want to trap moisture. Some roofs can vent others require full VB under them.

The venting has nothing to do with venting the building -- also some roofs need special base sheets that act as slip sheets and any Ice Shield needs to be high heat.

 

[theoldwizard1]

Plus made sure of a vapor barrier under the concrete ...

Few people realize that a vapor barrier under the concrete really DOES reduce condensation inside the garage ! It is probably one of the best "upgrades" you can do when pouring a garage or basement floor.

Bonus ! The vapor barrier will slow down the concrete curing process (concrete "cures", it does not "dry") actually making it stronger !

 

[theoldwizard1]

Educate me ! For people who have placed metal roofing over purlins with no decking ...

• What rafter/truss spacing did you have ?
• What was the size and spacing of the purlins ?

 

[3rdgendslmech]

Educate me ! For people who have placed metal roofing over purlins with no decking ...

• What rafter/truss spacing did you have ?
• What was the size and spacing of the purlins ?

When I built mine trusses we're 4' o.c. purlins were 2x4 spaced 2' o.c. 4/12 pitch. Once I had it all done I felt pretty comfortable walking across with soft compound boots. There was only a slight give when I stepped in the middle of 2 purlins.

 

[RPH]

30x72’, trusses are every 4’. Purlins are 2x4, wide side flat. 24” on center.

 

[Lelandwelds]

Educate me ! For people who have placed metal roofing over purlins with no decking ...

• What rafter/truss spacing did you have ?
• What was the size and spacing of the purlins ?

Metal building. I beam or channel beams are 20 ft apart (or 15 ft. Depends on doors). 7" or 9" purlin on edge on top. Spaced 4 ft apart. I wouldnt pull an engine with it but grown men can dance on it.

 

[lakeroadster]

OP. Your likely going to have issues when trying to screw down the steel. And from a worker issue.. locating the purlins when placing the screws will be a ***** with the foam in place.

The 2" standard blankets that have the plastic covering work great. Need more insulation... place it between the purlins inside the barn.

Educate me ! For people who have placed metal roofing over purlins with no decking ...

• What rafter/truss spacing did you have ?
• What was the size and spacing of the purlins ?

Depends on the design of the barn (truss spacing, wind load, snow load, etc.)

My barn has 2 ply trusses spaced 13'-6". Purlins are 2x8's. Spacing is 18" from the ridge down for the 1st 6 feet, then 24" spacing from there down to the gutters.

Hell-For-Stout