# Insulating an Attic - Spray Foam in the Rafters v. Cellulose on the Floor



## Windows on Wash (Aug 30, 2011)

I would not convert that attic to a conditioned space. 

Blowing in insulation after you air seal the floor will be much more cost effective and safe. 

In order to put SPF on the underside of the roof, he will have to cover it with an intumescent paint and they are expensive as well as offensive to the nose/lungs. 

You don't have to necessarily removing the fiberglass on the floor as long as it does not have any feces or urine in it from animals.


----------



## Gary in WA (Mar 11, 2009)

I agree. Figure you have around R-16 there now, http://www.greenbuildingadvisor.com/sites/default/files/Compressing fiberglass.JPG

adding cellulose would bring it to (at least) code minimum of R-38; http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=Connecticut

I am amazed they would be even close to the same price... the newer com. SPF is probably $$$ with a built-in, wish I had stock in that company.....http://www.dow.com/news/press-releases/article/?id=6436; 

You'd need about 3"SPF to get to R-20 (preventing condensation), per code, then other fill to standard code; http://www.buildingscience.com/docu.../irc-faq-conditioned-attics?searchterm=attic+

Conditioned attic may require an impermeable membrane under the shingles and does reduce the asphalt shingle life-span by 10%; http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation?full_view=1

Gary
PS. Welcome to the forums!


----------



## tbeck3579 (Dec 9, 2014)

Thanks for the info Window and Gary. This is what I was looking for. From the article links you posted Gary it looks like my home would benefit from spray foam in the attic; under the roof and on the side walls. I have ducts in the attic, and "swiss cheese" ceilings  I also need to access all 4 sides of my attic to maintain the stained glass windows in the gables. The attic storage is important too. 

The roof will wear out 10% faster -- Talking (typing) out loud here -- A "typical" roof lasts 25 years, but it will only last 22.5 years with spray foam under it. Guestimating cost/benefit here is not easy. $10,000 for a new roof, that's $400.00 per year for a"typical" roof over a 25 year period. That would cost me $1,000 for that 2.5 year loss of life expectancy. Over 25 years the life expectancy loss is going to cost $40.00 per year, or should it be based on the 22.5 years? Okay, 22.5 years, but I'll figure that out later, hahaha. The energy savings clearly outweigh the shorter life span. I should easily save more than $40.00 a year in heating and cooling. In fact, keeping the ducts in a conditioned space will probably offset that $40.00 so it's a wash. I'm wondering how much R value is lost when the roof is replaced and the old nails are pulled out, and new nails put in, hmmm. Probably not much R value is lost. I would probably loose more R value over 25 years with the "fluffy" stuff going flat. Okay, now I need to research "open cell" spray foam products to find a "reliable" one


----------



## esteve999 (Apr 26, 2014)

tbeck3579 said:


> Thanks for the info Window and Gary. This is what I was looking for. From the article links you posted Gary it looks like my home would benefit from spray foam in the attic; under the roof and on the side walls. I have ducts in the attic, and "swiss cheese" ceilings  I also need to access all 4 sides of my attic to maintain the stained glass windows in the gables. The attic storage is important too.
> 
> The roof will wear out 10% faster -- Talking (typing) out loud here -- A "typical" roof lasts 25 years, but it will only last 22.5 years with spray foam under it. Guestimating cost/benefit here is not easy. $10,000 for a new roof, that's $400.00 per year for a"typical" roof over a 25 year period. That would cost me $1,000 for that 2.5 year loss of life expectancy. Over 25 years the life expectancy loss is going to cost $40.00 per year, or should it be based on the 22.5 years? Okay, 22.5 years, but I'll figure that out later, hahaha. The energy savings clearly outweigh the shorter life span. I should easily save more than $40.00 a year in heating and cooling. In fact, keeping the ducts in a conditioned space will probably offset that $40.00 so it's a wash. I'm wondering how much R value is lost when the roof is replaced and the old nails are pulled out, and new nails put in, hmmm. Probably not much R value is lost. I would probably loose more R value over 25 years with the "fluffy" stuff going flat. Okay, now I need to research "open cell" spray foam products to find a "reliable" one


Why does spray foam in the rafters under the roof cause the roof to wear out faster? Why do you think you should use "open cell" v. "closed cell"?

Steve


----------



## tbeck3579 (Dec 9, 2014)

After reading the articles Gary posted I learned the lack of air circulation coupled with the temperature of the roof being greater when spray foam is used will cause the shingles to wear faster. I also learned that the shingles wear even faster if you choose a dark shingle; even faster than with spray foam insulation. Unfortunately my house must have a dark roof to look right. I have a light grey house with a steeply pitched roof that predominantly displays itself from the curb. If I had a light colored roof it would be noticeably "off" in appearance. 

It's my understanding that closed cell foam is better for applications such as behind drywall, but much more expensive. Closed cell gives you a better R value in a smaller space. Open cell foam needs more room to expand to give the same R value as closed cell. In a crawl or attic where you have more space to allow for the greater area needed you can get the same R value with open cell and it is less expensive. 

Again, this my research and it is not based on personal experience. I'm hoping a contractor will comment on this to give you a "professional" opinion. Real life experience will give a much better overview of the topic.


----------

