Need some help from the professionals: What foam and how thick should we use?? The recent article “A Crash Course in Roof Venting” caused me to question our renovation plans.
We have a standard stick-built Cape-Style built in 1992. It has 2×8 rafters with R-19 fiberglass. Not nearly enough in Zone 5. We plan to pull the fiberglass out and spray foam the roof deck. Here is where all questions come.
Author Joseph Lsiturek shows 7-1/2-inches of closed-cell foam under the roof deck. Our spray foam contractor states that 5-1/2 is sufficient and that any thicker is not worth the cost. Lstiurek also states closed-cell; our contractor says open-cell is better.
Who is right? And what is right?
I wish there was a final authority on these matters so I could make an informed and cost effective decision.
Your help is greatly appreciated.
Thank you.
Replies
Closed cell
Is your contractor recomending open cell because that is all he has available or is it strictly a cost issue?
In my area, open cell is more readily available (and cheaper), but having used both open and closed cell, I feel the closed cell product is hands down the better choice.
There is always a case of dimishing returns as you get thicker insulation. You have to weigh cost vs. return - based on how much energy savings, long you will live there, the life span of the house, etc
For what it's worth, Joe Lstiurek, by many people, is considered as close to a final authority as anyone out there.
Terry
Closed cell
Is your contractor recomending open cell because that is all he has available or is it strictly a cost issue?
In my area, open cell is more readily available (and cheaper), but having used both open and closed cell, I feel the closed cell product is hands down the better choice.
There is always a case of dimishing returns as you get thicker insulation. You have to weigh cost vs. return - based on how much energy savings, long you will live there, the life span of the house, etc
For what it's worth, Joe Lstiurek, by many people, is considered as close to a final authority as anyone out there.
Terry
"Joseph Lstiburek, Ph.D.,
"Joseph Lstiburek, Ph.D., P.Eng., ASHRAE Fellow, is a principal of Building Science Corporation. He is a building scientist who investigates building failures and is internationally recognized as an authority on moisture related building problems and indoor air quality. He is a member of ASTM and the past chairman of ASTM E241 - Increasing the Durability of Building Assemblies from Moisture Induced Damage. He is a contributor and reviewer of Chapters RSS21 and 22 of ASHRAE Fundamentals. Dr. Lstiburek is an ASHRAE fellow and a voting member of ASHRAE Standard 62 - Ventilation for Acceptable Indoor Air Quality. He is also a voting member for both the ASHRAE Technical Committee 4.3 Ventilation Requirements and Infiltration and ASHRAE Technical Committee 4.4 Building Materials and Building Envelope Performance."
Unless your contractor has better credentials than this I'm going to have to go with Joe.
Recurring theme in the spray foam threads
seems to be that you can't trust a spray foam contractor to spec the product or the application. Too many of them seem intent on pushing their own product, their own method, keeping their costs down even if that makes the installation sub-standard, etc. etc.
I would not install 5-1/2" of open cell foam in a hot roof. It could allow condensation on the underside of the roof deck and it's probably not code compliant, because the code officials know you need to use closed cell in a hot roof if they allow it at all.
As far as 5-1/2" being enough and more being a waste of money, how the hell does he know that? Has he done heat loss calcs and energy cost projections for your house and determined that extra insulation won't stop enough heat loss to ever pay off? Or is he just projecting the next 90 days, or until his taillights are no longer visible? 5-1/2" of open cell is probably about R-20, not enough in any code jurisdiction I'm aware of. 7-1/2" of closed cell will get you over R-40, which is probably code compliant and IMO is absolutely what you want. Lose the first guy and find another.
You may want to look at a combination approach. Instead of filling the whole bay with foam, you could do a couple inches of foam, then fur out the rafters and do the rest with cellulose. R value is R value, you can get there on many different vehicals. It is OK to mix and match.
Except for open cell... that just sucks compared to closed cell :)
Looking at the mass of threads, I've concluded this magazine isn'y by pros or for pros .... instead, it's devoted to holding the customers' hand.
Why would the contractor say what he did? Perhaps while he may know the correct technical answer, he also knows the inevetable response: "But that costs too much!" So, he uses his best judgement to get the job.
It's a worthy business topic ... contractors all too often encounter the customer who 'wants it done right' until they see the price tag; then the nickel&dimeing begins. They press you into shaving things as low as you dare, then take another slice off. Next thing you know, they're crying about the sub-standard job the guy did!
Or, they reject the high price, call another guy, and let on that they're on a budget. Amazingly enough, the new guy puts together a stripped-down but affordable, proposal. Now the customer tells everyone what a crook the other guy is ...
When you hire a pro, you're paying for his opinion- an opinion based upon all his experiences. Try listening.