Ridged Insulation under a steel roof?

[Industrial Toys]

So my ratty (lifetime) cedar shingles in my detached two or so car garage/shop have started to leak.

Have a contractor coming tomorrow that replaced the cedar shingles on my house with steel, comes in long sheets, peak to eave, but ressembles shakes.

I have open trusses. My ceiling is very cluttered with lighting. I have stuff stored in the trusses. My garage door has zero clearance to the underside of the truses, in fact I had to notch out a truss and install a roller, for as the garage door heat up in the sun, it bows slightly and would catch the bottom of the trusses. My garage door had been raised and replaced to accomodate a ROPS.

So, I am considering putting SM under the new steel roof. Maybe 2". Any thoughts?

It's not my first choice for insulation, but since the roof needs done, is it the most logical approach? I really don't like my heat going into the upper space, although I do have a ceiling fan up there. I don't want to spray foam the underside of the roof for fire reasons and it is very unsightly and messy looking. Is there any advantage that would make me feel better about this external approach?

Maybe the only picture I have of the (cluttered) ceiling, BTW That's a 12 volt light that connects to my alarm battery for emergency illumination.

Old picture. The shingles have seen better days. Sad, I really likd the look!

Thanks

 

[herm0016]

it certainly wont hurt your wintertime temps. be sure to leave an open air gap, usually done with 1x between the insulation and the roofing. this will help with condensation on the metal in your climate. are you planning on or do you have heat in there? something with a radiant barrier would be good. reflective side in.

 

[Industrial Toys]

Thanks. I was thinking if the brown tin needs circulation to keep cooler? Atop insulation, I can't imagine the high summer temps. Strapping would raise the height even more though.

Not a building that is generally heated. Just a wood stove and consruction heater sometimes as a load on my genny.

The contractor did put my house roof steel right on the roof.

 

[deezler]

We put a full layer of 1" polyiso board atop our house roof for the extra R7 and full thermal break, when building in 2015. When the metal roofer came out to do his layout, he said he wasn't comfortable putting metal directly onto the foam insulation board. 1x strapping was considered, but materials were still cheap then, so we just went with another full layer of 1/2" OSB instead.

I assume your steel roofing material is standing seam (no exposed fasteners). The clips that hold these metal roofing panels down need to be screwed into something very sturdy and rigid.

 

[Industrial Toys]

So what is the difference between putting the steel onto SM or onto sheeting?

Our barn done in 89 had the regular steel put right on the plywood sheeting.

My 2011 driveshed was strapped over the plywood sheeting with 2x4s.

 

[herm0016]

So what is the difference between putting the steel onto SM or onto sheeting?

Our barn done in 89 had the regular steel put right on the plywood sheeting.

My 2011 driveshed was strapped over the plywood sheeting with 2x4s.

when there is condensation under the metal, it has a path to dry, also, the radiant barrier does not work well without the air gap. strapping will make the whole assembly last longer. depends on climate as well. ontario has huge temp swings and humidity.

its less important here, where we have very low humidity all year.

 

[Industrial Toys]

Just to be clear. Putting steel onto sheeting isn't a good idea either I guess.

Certainly, the place would be easier to heat with an eight foot ceiling, I'm guessing or is that not a big deal? And when I say, easier to heat, I mean the place stone cold and you want to do some work in there.

A little discouragd! Spoke with the manufacturer and they wouldn't suggest strapping or not strapping, maybe to protect their contractors that may install it wither way! I mentioned that my house roof put directly in sheeting, suffered considerable damage from walking on it, not detectible from the ground, and I asked if this wouldn't be much worse if strapped, to which they agreed.

I also wonder if there is a difference to the steel being directly on the sheeting or insulation if there is no vented attic underneith.

 

[deezler]

I also wonder if there is a difference to the steel being directly on the sheeting or insulation if there is no vented attic underneith.

This is the key thing, I think. If you have a temperature differential, this is when you get condensation on the panels. If you have very well insulated roof underneath the metal, then you dont have a temp diff on the top/bottom of the metal. Also if you seal all around the metal edges, there is no airflow and thus no air moisture reaching the bottom side of the panels.

Good point about damage from walking on it if just put onto strapping. I would put it flat over sheeting, for sure. I walk on my roof all the time and never thought about that, it would suck if I could only step above the straps.

Oh, my metal is also over a full layer of ice/water shield, too.

 

[Industrial Toys]

The design does allow for more airflow than regular steel. In fact, my house has boards (with cracks, and missing knots) on the roof and I was worried that mice and insects now have ready access to the attic, but that doesn't seem to have happened. They would hav to ake their way up two courses of water sheild though.

 

[RickB]

My dad built an unheated steel truss building with wood purlins and steel roofing for livestock in 1976. Upstate NY. To prevent condensation from dripping on cattle and everything else rigid insulation was put directly under the steel and above the purlins without any 1xs or spacing between the steel and insulation. The roof was replaced 2 years ago.