Spray insulation on underside of roof boards?

[madosta]

Hi guys.

I have a hip roof garage, with old 1x8 roof boards. I just replaced the roof this fall and want to get it insulated up before Christmas.

It had old faced fiberglass stapled from the eave to the ridge and most of it has fallen down. I can either replace with faced fiberglass at 10$/roll. No ridge vent.

But I was thinking spray foam would be cool and easy.

What is your opinion for the underside of a roof? I have heard "yea but if you get a leak, you'll never know..." etc. etc.

Suggestions?

 

[michaelg]

I'm interested in this discussion as well. I have an old home with a hip roof and a huge walk-up attic. One day I would like to finish it and I was thinking of using spray foam directly against the roof sheathing without an air channel. I've read conflicting opinions about this approach...some say it is ok others that it reduces the lifespan of the roof.

 

[gpalmer77]

If you get a leak with closed cell foam, you won't know. Plus closed cell foam is heavy. I used an open cell product in the house (converted attic space)..... Icynene. Not as heavy, and you will see a leak. Moisture travel rates through open cell foam are also minor, small enough to not use a separate vapor barrier or need for venting.

Not cheap, but well worth it. My heating bills are ridiculously low..... Icynene in the roof and gable end walls of the attic, regular R-13 insulation in the rest of the house walls. Sometimes I'm the only house with snow on the roof......

 

[Randy in Maine]

When I did my roof, I covered it all in bitchethane prior to the new ashalt shingles being installed.

5 1/2" of closed cell foam on the underside about 2 years later (valuted ceilings). Works great.

 

[porschedude996TT]

Is there a moisture problem when you do this? I too wanted to insulate with spray foam on my shop build but used conventional means and allowed air-flow under the roof sheeting. Just asking, I know that moisture in some areas of the country is a real problem...

 

[madosta]

I have seen a few websites where they show spray foam directly on the roof deck between the rafters, but I don't know yet.

The original owner had installed faced fiberglass so I picked up some more, but then I was reading differing opinions since there is no ridge vent, etc.

Then I started thinking about screwing up foam insulation boards, but then I found some of that silver bubble insulation and even only at R value of 3+ it should help. But if it makes OK sense to use open cell, then that would be awesome.

I can't install a ceiling and I don't heat it continuously and it is detached.

Hmmm..

 

[MG David]

If you use closed cell spray foam applied to the underside of the boards you will not get any condensation on the underside of the boards because it is no longer the face of a substrate.

Similarly if the other side of the boards is the face of the roof and the boards are breached you will not get a leak because the combined substrate is still waterproof. Nor will you get water working its way between the boards and the foam because you have a continuous bond.

If you have felt on top of the boards to provide the exposed face and that felt is breached there is a danger of the boards becoming waterlogged.

 

[kj_mustang]

All the foam installers I have talked to say you need a minimum of 2" of closed cell to achieve a moisture/condensation barrier. My last quote for spray foam applied to the underside of my pole barn metal roof was for 2" of closed cell and then open cell foam over that to achieve the R value required by the building officials.