insulating unsheathed metal roof?

[umtallguy]

in the processing of trying to buy a place that has a garage with a metal roof on purlins but no sheathing, if I wish to insulate the second floor are there any alternatives to either closed cell spray foam or redoing the roof?

I assume with normal batting I would have serious condensation issues?

[Redoverfarm]

in the processing of trying to buy a place that has a garage with a metal roof on purlins but no sheathing, if I wish to insulate the second floor are there any alternatives to either closed cell spray foam or redoing the roof?

I assume with normal batting I would have serious condensation issues?

Others may have alternative solutions but IMO your only option short of removing the tin and adding sheeting/underlayment is to spray foam the rafter bays.  I have my porch the same and it is like a tropical forest at times. 

[flyingvan]

You could install a radiant barrier.  It would be very inexpensive and easy.  All you'd need is a tacker and a knife.  It wouldn't do much to keep heat in, but it would do a lot to keep heat out.  It's actually required around here now.  I like the kind that's bubble wrap with mylar laminated onto both sides.



   For a retorfit you'd staple is as close to the tin as you can.  A little sag is a good thing for the air space.  Later, your batt insulation can go in ceiling joist bays

[Redoverfarm]

Another thought came to mind.  .  Maybe you could use a rafter-mate in each bay and then foam the remaining width with sprayfoam.

https://www.google.com/search?q=rafter+mates&hl=en&qscrl=1&rlz=1T4TSNA_enUS401US438&tbm=isch&tbo=u&source=univ&sa=X&ei=0hq_UvyjHIOwsAT244HwDg&sqi=2&ved=0CDMQsAQ&biw=1093&bih=434

This would put a moisture barrier ( of sorts) between your metal and insulation.  They are normally 24" wide but can be split at the perforation to accomidate 16" OC.  Not sure what your OC is maybe the 24" portion might work.  As long as you do not make holes in them they should shed water.  Just a thought.

[umtallguy]

Is the situtation I have ( or will ife the closing ever happens)

I guess for now I could just insulate the garage ceiling trusses, and deal with the upstares space later if i decide to finish it.


(walls are already insulated, apparently the garage had a roof collapse at some point and they removed it and put new trusses on but never insulated the roof or ceiling again)

[Redoverfarm]

I think in this case the only viable option would be to spray it with closed cell foam.  It is hard to put moisture barrier up after the fact.  You would not have to spray that deep (cost wise) and fill the remaining cavity depth with fiberglass.  But then again there may not be that much cost difference just to fill it with spray foam to the desired R value and be done with it.

[MountainDon]

As John said. Spray foam on the metal underside.  OR as was said insulate the ceiling / attic floor.

[Redoverfarm]

As John said. Spray foam on the metal underside.  OR as was said insulate the ceiling / attic floor.

Don from the photographs it looks like that will be a room upstairs.  The walls are actually the trusses on the inside.  They will sweat as well so I would treat the walls the same as the ceiling.  I would throw caution in the wind that just because walls are almost verticle it will mean that the water will run down those as well . 

[umtallguy]

there is a room upstairs, but it may not be need as conditioned space, I guess simplest for now would be to strap the ceiling and insulate with bats, and leave the attic truss as effectively unfinished attic space, which would also cut by about 70% the amount of space needing to be heated, and make the upstairs easier to heat later if I did get the roof foamed.

Hmm I will have to run the numbers

[akwoodchuck]

I just finished some interior framing on a large steel building with sprayed foam....no roof deck, just foamed right up against the metal roofing....and it leaks like crazy. Gonna be a nightmare to fix that one. Any three-man crew worth their salt could pull your roofing, apply sheeting/felt and slap the metal back on in two days max....and you'll sleep a lot better at night.

[umtallguy]

open cell or closed cell phone

[Redoverfarm]

Open Cell is pourous whereas Closed Cell should seal it up without any water or air penetration.  I would check with a company that does that just to make sure. 

[umtallguy]

Yeah I thought closed cell was supposed to be effectively waterproof

[MountainDon]

So did that roof leak before the foam was sprayed?  How thick was the applied foam?  It has to be thick enough, sufficient R-value, to have the foam remain at a temperature above the dew point. If a too thin layer is applied the roof underside can still get cold enough to have water vapor condense on the underside. Might look like a leak. Sufficient R-value depends on the climate zone.  OTOH if the roof leaked beforehand I would not want to count on the foam "fixing" the leaks.

[akwoodchuck]

So did that roof leak before the foam was sprayed?  How thick was the applied foam?  It has to be thick enough, sufficient R-value, to have the foam remain at a temperature above the dew point. If a too thin layer is applied the roof underside can still get cold enough to have water vapor condense on the underside. Might look like a leak. Sufficient R-value depends on the climate zone.  OTOH if the roof leaked beforehand I would not want to count on the foam "fixing" the leaks.

Yeah I believe it's a wind-driven rain issue..... with 6 or 7 inches of urethane foam, condensation should not be the problem, but I don't know...I didn't build the thing, and haven't been hired yet to investigate the water drips coming through between the foam and wood purlins in a heavy rain....my take on it is, most metal roofs are going to have a drip drop here and there.....when you combine that with three different materials, each with it's own rate of expansion/contraction (steel, urethane, and wood), water's gonna make it's way through.  I don't like the idea of it not having an easy way to get out, or being able to come in contact with structural wood members, ever.

[MountainDon]

Just wondered... I am a believer in installing sheathing and then a weather resistant barrier and then whatever roof material you want to complete the job. Maybe doing rigid foam on top of the sheathing if the roof underneath is a cathedral ceiling.

[Redoverfarm]

Just wondered... I am a believer in installing sheathing and then a weather resistant barrier and then whatever roof material you want to complete the job. Maybe doing rigid foam on top of the sheathing if the roof underneath is a cathedral ceiling.

Yes Don when metal roof was first used it only had a purlin substrait but that was mostly barns and sheds and a small drip here and there was not a big deal.  When they started using it on houses they were probably "quick learners".   

[hickorynut7]

open cell or closed cell phone

I totally agree here ! I've sheathed roofs that size in a matter of 4-5 hours,and that's nailed off,dryed in (felt). I'm just saying that it would be so worth doing it right and then you never have to worry about it again