Insulating shop roof

[shortlogger]

I'm about to build a new shop using chickenhouse trusses and I'm wondering what would be the best way to insulate the roof ? If I use the 4' rolls like they use on red iron structures I'm afraid it migh compress the insulation too much being that the purlins are only 2' feet apart . But that might be the easiest way I don't want to try and put in 24" rolls between each purlin after the roof is on that would be lots of trouble and without a solid backing It wouldn't last long .
Any ideas

[Brian_Rhoad]

I think it would be better to insulate the ceiling. It would take less insulation and you wouldn't be heating as much space.

[ScottAR]

Spray foam if your going to cover it.  Did our shop when we built it.  Bit spendy but tight as a drum now.  Heat it with a candle and cool it with an ice cube. 

[shortlogger]

there will be no ceiling just exposed trusses . So whatever I do will have to be at the purlins

[47sawdust]

I would be reluctant to insulate directly against the roof.Your building will be much more comfortable if the insulation is in the ceiling as Brian said.I would blow cellulose into the ceiling cavity it is cheap and very effective.Foam directly against the roof is not a good idea as it causes your roof to overheat and degrade.Especially true for asphalt and will cook the paint right off a metal roof.

[goose63]

I would be reluctant to insulate directly against the roof.Your building will be much more comfortable if the insulation is in the ceiling as Brian said.I would blow cellulose into the ceiling cavity it is cheap and very effective.Foam directly against the roof is not a good idea as it causes your roof to overheat and degrade.Especially true for asphalt and will cook the paint right off a metal roof.

And may start the wood on fire that stuff will get hot

[shortlogger]

The insulation will be between the wood and the sheet metal roof

[reswire]

I insulated my old shop roof like that (up against the metal and purlins), and had a terrible mold/mildew problem due to the condensation building up due to temperature change.  I opted on that building to put in ceiling joists, and nail up 2x4x8 iso-foam boards that was given to me by a friend.  Worked great, but if you have to buy the iso-foam, it gets expensive quick.  It is the next best thing to spray on foam.  I believe the spray on would eliminate the moisture from building up.  Probably best to check with builders in you area as to what they recommend. 

[Tom the Sawyer]

The rep for the company that built my metal pole barn recommended avoiding spray in polyurethane insulation for the roof.  When the foam dries it creates its own vapor barrier on the inside of the installation which means that the purlins are sandwiched between the metal and the vapor barrier.  No air movement to clear out condensation. 

The other factor he mentioned was that whenever they had to replace roof metal (rust or hail damage), it was much more difficult and expensive since the insulation is bonded to the inside metal surface and has to be cut and pried loose, plus you have to re-install the insulation.  They recommended the vinyl coated batts or ceiling insulation.

[ozarkgem]

I just did a 28 x 40 ceiling with R-30 blown in fiberglass. the local lumber yard charged $600.00 installed. A bargain

[ktowne030311]

im in the process of ding my shop as well. it has no ceiling cavity, its all exposed. i just bought 78 sheets of insulation/sheathing board for 200$ on craigslist. my plan is to make a "drop tile ceiling" with 2x6's but except instead of ceiling tiles it'll be the boarding instead.

[jwilly3879]

Insulation over the purlins and under the tin is a recipe for disaster. Where the insulation is compressed over the purlin there is basically no R-value. The purlins will be the coldest spot and moisture will condense on them, especially in colder weather with high humidity in the building. Additionally, over time as the insulation reaches its final compressed thickness the screws holding the tin will be loose and there will be water leaks around the screws, the wood the screw is threaded into eventually rots and then the tin is no longer attached.

I have seen this several times after being asked to find some leaks. To remove the tin we just picked it up and gave it a good shake and 90% of the screws came out. When we replaced the roof we fastened furring strips along the truss top chord, covered with 1/4" plywood and installed spray foam on the plywood and truss chords. The furring strips provided ventilation under the tin and no more moisture problems. This way the spray foam is not applied to the tin itself.

[shortlogger]

So I should be able to put up OSB on the bottom side of the four inch vertical lathing and spray foam on that surface leaving a four inch air gap between my sheet iron and the insulated surface if I'm understanding you right .

[jwilly3879]

That's correct.

[shortlogger]

Just got a estimate on spray foam way too high gonna have to do something else . Maybe hang chicken wire backed roll insulation on the bottom side of purlins

[petefrom bearswamp]

Wow Ozark, 600 for 1100 plus sq ft is really a bargain.
Estimate for 10" cellulose in 1600 sq ft in my house is 2100.

[thechknhwk]

I want to do something similar, and I found this document.  Thought some of you might be interested.

http://www.buildingscience.com/documents/bareports/ba-1312-application-of-spray-foam-insulation-under-plywood-and-osb-roof-sheathing