To vent, or not to vent (the roof). That is the question…..
I’m doing a gut rehab on a 135 year old frame home in Chicago (climate zone 5), and in the process we’re going to vault the ceiling on the top floor and expose the collar ties, which means we’re going to have to insulate between the rafters, up against the underside of the roof sheathing.
We’ve been advised to use closed cell foam for the high R value and the water impermeability, and want to know whether I can spray directly against the underside of the roof sheathing?, or whether we need to install rafter baffles to vent the underside of the roof (as per IRC), and then spray against that.
Spray foam companies say that we don’t have to install baffles because the water impermeability of the closed cell foam will not allow vapor to condense on the inside, and that any roof leaks will vent out of the roof sheathing naturally as via evaporation.
The roof is a full hip roof with no gables, that has never had any venting installed in its life, so I would have to install soffit vents and ridge vents during the reroof if the venting is required. The sheathing is the original 135 year old 12″ – 16″ wide 1″ think planks, which is in perfect condition. Testament to how well old houses breath.
In a perfect world I would install soffit and ridge vents and use 1″ high baffles to channel airflow, but I have to watch costs as well, so I want to be sure I have to do this. The vents would also remove the extra inch of foam I could spray in the roofline, as I only have 2″ x 6″ rafters.
Thanks in advance for any advice you can give.
This detail is executed all the time with no problems. Things to consider, though.
Be sure there are no pathways (recessed lights, for example) that would allow inside air to bypass the foam and hit the roof deck.
Make sure the shingle manufacturer warrants the shingles on an unvented roof. Some do, some don't.
Be very sure the roof is water-tight. It can be devilishly hard to find a leak in a spray-foam insulated roof. Could be a minor leak never gets noticed and the roof just rots.
I intend to put the new roof in place, and then wait at least a month or more before spraying in the foam so i can be as close to possible as sure that the roof doesn't leak.
I'm avoiding putting any cans or holes in the roof line that are not skylights or plumbing vents, to minimize the chance of ice damming.Thinking of track lights hanging from the collor ties, and suspended lights from above the collar tie to add some drama.
Another spray guy, is advocating open cell foam everywhere. Will save a ton of $'s but I'm not 100% sure on what he's saying; "It's the air barrier that is most important, the R rating is secondary". You could obviously pinpoint leaks easier with open cell, and I get the air barrier, but isn't the R value just as important as well?
Is 1" of closed cell, and the rest open cell foam a good idea? The You get the water proofing, as well as lower overall cost, albeit sacrificing some R value.
He also said that ventless roofs are being approved in IRC for 2015.
Thanks for the info on the shingle warranty. I'll check that out with the roofer. Timberline HD is what we'll likely use.
A very healthy old roof - due to be ???
If you search on the net, you will read some of the horror stories, where spray foam has gone wrong. Not sticking, shrinking, cracking.
The success of spray foam is entirely down to the skill of the opperator and his ability to judge the correct mix for the time and day.
A lot has to do with the temperature of the receiving surface and the humidity of the day.
As mentioned elsewhere, using foam as indicated will make it almost impossible to track down any leaks in future years, the mere existance of the foam will hide any leak, which will then not show itself until it is of catastrophic proportion.
You have already made the point that the roof planks are in perfect condidtion after 135 years.
Of course their condition has been the result of good design based on lots of air circulation and temperature rises to dry out regular doses of condensation.
You are proposing to change all this and to move them into an unfriendly enviroment, where any water vapor that finds its way into the wood will build up and lead to wood rot and roof collapse.
Keeping in mind that water vapor is created inside the building by people and that it is programed by nature to head for the nearest cold surface, the nearest surface that is below dew point.Then that inner roof surface is target number two following any windows.
You job is to insulate the roof, while keeping the water vapor away from the roof. You can only do this by installing some form of water vapor shield, either a membrane or a form of coating that will prevent the molecules of water vapor from entering the fabric of the wood, or by providing some guarenteed ventilation that will enable the wood to dry as it has always done up to now.
You could consider placing the insulation on the outside of the roof, this will enable the wood to keep warm, above dew point, and will direct the water vapor to condense on the windows or some other cold surface, where (hopefully) it will do no harm.
Baffles it is then.
With that input I have reinforced the decision to put baffles into the rafter bays, to create an air space so the roof deck can breath, and so any leak in the roof will hopefully drain out of the soffits or evaporate out of the ridge vents.
I wish I could insulate the top of the roof deck, however the house is in a strict landmark historic district, with no exceptions for roof height or thickness.
Saftey concerns: Site-built baffles using 1" Rigid Polyiso
1" rigid insulation is often mentioned as an option for site-built baffles. Articles include;
"A Crash Course in Roof Venting" by Joseph Lstiburek
Attic-Insulation Upgrade by Mike Guertin in Fine Homebuilder
I plan to use this method but am concerned with the potential fire hazard of leaving the rigid foam exposed. My attic is not be used for storage and there is no HVAC or other equipment located in the attic. My interpretation of the code confirms that the baffles would meet code.
1" rigid foam is otherwise a great solution. I have not found any discussion on the potential fire hazard.. The black smoke created during a fire is causing me to rethink my choice of material for the eave baffles.
Comments would be greatly appreciated.