Can One Insulate the EXTERIOR of a Roof?

[Industrial Toys]

I realized that my "LIFETIME" cedar shingles are done after only twenty odd years.

I am trying to insulate this building as well but the ceiling poses a problem with lots of lighting and services I don't feel like moving, and the trusses are being used to store stuff like plastic pipe and conduit.

Sprayfoam is not an option as I don't like the fire issue associated with it.

So, to kill two birds with one stone, could SM be put down prior to a new roof going on? I am going with steel shakes, or rather the product that is continuous sheets from Peak to Eaves. There is no sheeting on the trusses, but 1" boards, that already have some insulation value.

Is this possible? How much SM? What about final trim and distance to the eavestroughs?

 

[AndyR2]

SM? If you want to avoid foam, there are block and panel type insulations made with wood fiber or various mineral products. Depending on the density of the product used, you could fasten through with a GRK or similar fastener. In any case the roof will end up becoming several inches thicker, depending on the product used, so you would need plan on additional trim and possibly soffit venting. Details will also depend on the snow load rating you need to work with.

 

[Industrial Toys]

The soffits are vented, but I was considering closing them as there will be no attic.

 

[RNeumann]

This is a technique that is used sometimes- more common in commercial settings. Your attic would become a “conditioned space”. You can find details on the building science website. Your attic will need to be sealed and the vertical surfaces (gable walls/ends etc) insulated.

 

[Barnbuilder]

You could put foam board down then stripping or USB on top of it using long screws to attach it to current board sheathing.

 

[Industrial Toys]

I was actually looking at the interior gable walls yesterday, wondering how to address that for insulation.

 

[EddieWalker]

It's very common to use solid sheets of foam to do this. You decide how thick the foam will be and then you install boards on edge on top of your existing roof. Cut the foam to fit between the boards and then install plywood or OSB over on top of those boards and then shingles or metal.

 

[oosik]

I built a PanAbode, double wall, cedar plank house( 1982 ). It has exposed beam cathedral ceilings. ALL the roof insulation is, obviously, outside - on top of the roof. It has a double layer of a high density foam board for insulation. Each sheet is 4' x 8' x 2" thick and is foil wrapped. The product is called - RMax. There is a total of 4" ( two layers ) up on the roof. On top of the RMax is a layer of 1/2" exterior grade plywood. This "layer cake" is all held on with long screws that go thru to the 2" x 6" T&G pine that is the exposed ceiling inside the house. There is a double layer of heavy grade tar paper on top of the plywood. The roof is now finished with 100 year asphalt shingles on top of the original asphalt shingles.

The 100 year shingles were installed ten years ago. Before the original shingles became bad and had to be removed.

I've not had a single problem with this roof. I never expected to have a problem. I'm completely satisfied.

Look beyond the Taco Wagon - the SE side of my PanAbode house.

View attachment 618648

 

[Industrial Toys]

SM is a pretty strong material. Can one not just strap it and put the steel sheets on top?

 

[Industrial Toys]

Maybe, I should ask. What is the downside to doing this? What can go wrong? lol

I only heat the shop when I have a project to do, with a wood stove or by excercising a generator. I have a ceiling fan in the peak. Despite that, will most of my heat still end up in the ceiling?