Spray Foam Under Roof Decking?

I was thinking about ways to save energy, and was wondering if spraying foam insulation under the roof decking would be an option. There's plenty of fiberglass insulation in on the (trussed) attic floor, and the attic is ventilated.

Is this being done now, as in the latest and greatest?
The ventilation pathways would have to be blocked, but there should be no condensing with the insulation.

Comments?

(I've just started to look into this.)

If the attic is not being used as living space,bedroom ect ,and the attic floor is already (sufficiently)insulated the cost of the spray foam (very expensive) would far out weigh any energy savings. I looked into spray foam last week and the price of that stuff is many times that of most other types.

If the floor of your attic is airsealed and insulated, foaming under the deck will have no effect. If it isn't, do that first, its cheaper.

I'm thinking of going that route because my attic insulation and air sealing were done so poorly. I'd have to either tear up the attic floor or the ceilings to tighten things up. Insulating and sealing the attic would be a less disruptive project and would also put my central air duct work in conditioned space.

At which point you will have a warmer attic with little effect on the house below. I agree with woodgeek ,money spent on the attic floor will have much greater payback. Fix that problem first and you wont need to do anything else.

If the attic was sealed space I wouldn't be losing heat to the outside. The AC duct work would also be in a cooler environment in the summer.

I did a little searching. Some contractor web page said they sprayed 6" thick polyurethane under the roof deck and gable ends, but all the existing insulation would have to come out: hard to navigate; absorbs odors; code. I'm not sure why the existing insulation couldn't stay, but who am I. I think if you had duct work or air handling machinery up there it would be a big plus.

Your attic will still lose heat albeit at a slower rate but your house will lose heat through your attic floor at the same rate. It s like saying your swimming pool is leaking water so instead of fixing your swimming pool leak your going to try to stop the water at your backyard gate. As i said spray foam is extremely expensive, I do blown-in cellulose and i just checked on the cost of spray foam for walls and the cost is out of sight. I can do 2 walls for about $40 in material cost with cellolose ,with injectable foam its several hundred just for the materials.

If i were in your situation i might possible attach foam sheets to the underside of the roof rafters. YOU could do this for a fraction of the cost of spray foam. If you decide to do the spray foam and have to remove all the existing insulation anyway ,i would have the foam sprayed between the attic floor joists instead of between as it would be way more effective there.

Every situation is different. if you have decking down that you can't easily pull up, then sure. I have a handler and ducts in my attic, but I have sealed and insulated them pretty well...I think I am losing <5% of my heat there. IF I had leaky ducts in my foamed attic, then I would pressurize that space, and I don't know if the heat would leak back into the house or through the gables, etc??

Generally speaking, you should define the air barrier and thermal envelope of your attic as either the attic floor or the walls/roof of the attic and just seal and insulate that one barrier/envelope.

A consideration in insulating the attic ceiling is that this can keep temperatures of the roof surface hotter and may void the warranties of the shingles in the process.

In my mind, the only thing that should be in an attic below the roof decking and above the vapour barrier & insulation at the bottom of it, is air - outside air that is just passing through.

But in my area central a/c is a foreign concept along with what it requires - that likely skews what's in my mind a bit.

Thats why i suggested foam board fastened to the underside of the roof rafters,it leaves a space for air to move from your soffit at the bottom up and out the ridge vent cooling the roof. It also (foamboard ) is so much less expensive than spray foam. Every time i price out spray foam i never see a payback.

Velvetfoot, you can do as you describe and leave the existing insulation in place. You'll need to seal off any ventilation to the attic including gable, ridge, and soffit, vents. Bringing your HVAC unit into the conditioned space is a very good idea.

Most shingle makers have backed off on the requirement for a ventilated roof deck. I don't think I'd worry about the shingles given where you live. You may find that once you do this ice damming will be minimized since the roof will stay cooler in winter.

It wouldn't be a bad idea to spend some time reading at the greenbuildingadvisor or buildingscience websites.

I used closed cell R-10 foam board to insulate my block wall for my addition last year. When the spray foam guy showed up to insulate the walls and band board of our addition he said "Why didn't you call me about insulating your basement". His price was alot cheaper ( that did not even include my labor ) than what I spent on the 4 x 8 sheets of foam board. And he guaranteed it not to leak. He tried to talk me into foaming my attic. I told him no because it was to expensive vs what I could get using blow in insulation. I got quotes for three times of what this guy charged me to do the same amount of work.

Just like anything else, you need to shop around a little, do your homework and pick the right materials for the job you are going to do.

Thanks,

Scott

I signed on tonight to post this exact topic! I had an energy audit last year and that guy recommended air sealing and insulation. I decided to do the work, but the guy flaked out and did not return my calls. I have another company scheduled for Tuesday. He does all types of insulation, but I can tell already he will be selling the foam on the attic ceiling/roof. He spent quite a while on the phone with me explaining the benefits, especially for me with central a/c.
I told him my two concerns:
1. If I get a new roof in 8-10 years and there is some bad plywood what then?
He responded that if there was any questionable plywood they would put a barrier up before spraying so the foam would stick to barrier and not plywood?
2. The expense is so much greater than other types of insulation.
He said that was true, but to do traditional added insulation would require:
air baffles at soffits
air sealing the entire attic
air sealing all my recessed lighting(and I have 28! at $50 per light)
adding plywood walkways

Very interested to follow this post. I am also hoping to use the $1500 credit I would get from my power company.

I talked to a local spray foam guy about spraying an old garage that I want to fix up.. waiting on a quote,,,how ever he did tell me that he sprays closed cell foam... He said to watch out for a cheap price that would be offered from someone that sprays open cell foam... He said that will absorb moisture..... Same thing with me on my old garage, old clap board would have to have a plastic barrier up first or I could never replace the boards without destroying the spray foam. Still thinking to go the rigid board route.Tape the seams.......

Open cell foam is fine when used in the correct places. You would not want to use open cell foam where it is going to touch water or has the possibility of getting wet. But in a wall cavity it is fine. Closed cell foam gives you more R value per inch vs open cell. Open cell is cheaper to spray because it requires less material.

The wood, concrete and tile in your home will absorb and release moisture depending on the humidity outside and inside your home. It is the nature of the beast as my dad says.

More info: http://www.icynene.com/homeowner/choosing-insulation/compare-open-and-closed-cell

Thanks,

Scott

That's why I would do it at my place too. Doing it this way solves so many little problems. ...and all the crap in my attic wouldn't freeze in the winter and might stay cooler in the summer.

How do you resolve your roof getting a lot hotter without air moving the heat out under the plywood from bottom to top and possibly causing premature failure of the roofing shingles.

The attic becomes conditioned space. The attic temps are much less extreme.

My spray foam guy said he has actually spray foams the top of the ceiling so you do not have to turn the attic into conditioned space. If I had a lot of can lights it might of been worth it. I chose to use canned spray foam and caulk for my can lights ( >10 total ) and use blown in fibreglass insulation as normal.

Thanks,

Scott

Attic ventilation is for humidity control (itself caused by lack of air sealing), and does little to reduce daytime temps below that of the sheathing. The diff in shingle temps in the two cases is negligible.

Another way to think of it, the sheathing below the shingle is R-1 to R-2...the air above the shingle is R-0.3. AS hot as your attic might be >80% of the heat is going 'up'.

I spray foamed the roof deck in my old house and I was impressed with the results. Low cost of Nat.Gas made the pay off a bit extreme. The house was very drafty and would have been a pain to insulate conventionally. The drafts stopped immediately. The AC and Furnace ran far less, and the house was much more comfortable. I'd love to do it in my new house, but it's much larger and I can't afford it.

Its good to see that you had good results. I am scared to get an estimate for this type of work. My power company is giving rebates of up to $1500 and I think it ends at the end of the year.