Reuben's Home Inspection Blog

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Hot Roofs: A Misnomer

Foam Insulation While most attic spaces in Minnesota are insulated with fiberglass or cellulose insulation, there is a relatively new product that provides superior performance:  spray foam.  Spray foam is the best way to insulate homes, especially old one-and-a-half story homes, and I'm such a firm believer in this that I had it done on my own home a couple years ago.  Spray foamed roofs are commonly referred to as 'hot roofs'.

Why are they called 'hot roofs?'  Traditional attic spaces have insulated floors and are ventilated.  Air comes in at the soffits and leaves at the top of the roof, creating a cold attic space.    This helps to prevent ice dams, keeps the roof cooler in the summer which helps to prolong the life of the shingles, and may help to prevent the accumulation of condensation.  Spray foamed attics have foam applied directly to the roof sheathing, and the attic space isn't ventilated.  The lack of ventilation is why we call them hot roofs.

Are they really hot?  No.  Studies have shown that color differences in shingles will actually have a larger impact on the temperature of roofs than the difference between a ventilated spray-foamed.  A hot roof will typically only be a couple degrees warmer than a ventilated roof.

What are the benefits?  Spray foam has a higher insulating value (R-Value) than anything else.  Sprayed Polyurethane foam insulation has an R-Value of 6.8 per inch, while fiberglass batt insulation is about half that.  Foam insulation also makes for a perfect seal - no gaps, no air leakage, no attic bypasses.  If ductwork is located in the attic space it won't need to be insulated, elimating energy loss here, which can account for up to 10% of total energy loss.  One more benefit that I personally love is having a warm attic area for extra storage!  Note: My old house is designed in such a manner as to support extra storage in the attic, but most newer homes are not.  This might be another blog topic some day.

What are the downsides?  The only one I know of is cost.  Spray foam insulation will typically cost thousands more than fiberglass or cellulose.  I paid about $3700 to have my own attic spray-foamed with polyurethane, but I could have spent about a third of that to have fiberglass installed, along with proper vents.

Will spray foam void my shingle warranty?  No.  All the major manufacturers of shingles will still warrant their products when used with a spray-foamed attic.

If spray foam is so great, why isn't it used on walls?  It is.   I have a friend who insulated the walls in his home when he built it in 1980.  Spray foam is also used at the rim joist in almost every new construction home that I inspect.

Additional Information - technical, dry reads can be found below.

Spray-in-Place Polyurethane Foam Insulation Opion Paper, by Craig DeWitt, Ph.D., PE

Vented and Sealed Attics in Hot Climates, by Joseph W. Lstiburek, PE

Reuben Saltzman, Structure Tech Home Inspections - Email - Minneapolis Home Inspections

Reuben Saltzman, Structure Tech Home Inspections

        

Comment balloon 10 commentsReuben Saltzman • March 28 2009 07:02AM

Comments

Good stuff Reuben----while some manufacturers still have old literature that says if the roof is not properly vented the warranty will be voided, I have to think that most are on board with this newer way of looking at roof structures. 

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 9 years ago

Hey Reuben, I once worked for a manufacturer of spray foam back in the 80's. Their largest customer used it for floatation in barges on the Mississippi. It's a great product.

Posted by Tad Petersen / Home Inspector, Mpls (Safeguard Home Inspections, Inc.) over 9 years ago

Reuben, Not all spray foams have higher R values than other types of insulation. Urethane is referred to as 2 pound foam and is denser than half pound which is often referred to as Icynene. Actually cellulose has a higher per inch R value than most any other insulation except 2 pound foam. It is also cheaper and, a quality I find appealing, greener.

Insulating the roof and bringing the attic into the thermal envelope has many benfits. I would however dis-agree abount not insulating duct work.

Also the paper you cite by Joe Lstiburek is for hot climates. He has a book for every climate because there are differnet building designs required for different climates. Check out his web site Building Science for lots of good info.

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) over 9 years ago

James - very true.  I got bids for Icynene when I had my roof done, and I was surprised the bids came in at the exact same price for Icynene, but it only offered about half the insulating value.  After foam insulations, dense packed cellulose would be my insulation of choice too. 

Why would you disagree about not insulating duct work in a conditioned space?

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 9 years ago

Just because the space is conditioned does not mean there is no heating or cooling loss from the ducts. The conditioned air is designed to be expelled at the registers, not radiated from the ducts. That stuff about not insulating the ducts is, in my opinion, nothing more than marketing hype by the foam insulation companies.

If you ever have the opportunity to hear Joe Lstiburek speak, make sure you do. He is very entertaining and you will learn a lot! I own a couple of his books and find them very useful.

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) over 9 years ago

I've read about all the various materials and am prepared for finding them but so far they have not shown up in new or old construction around here.

I see James's comment above what I'm dictating. Joe Lstiburek's books and his web site are great.

Posted by Not a real person over 9 years ago

Hi Reuben, I want to pass along some new info I came across concerning spray foam insulation. Over near me on Long Island there a couple of high end homes that are having serious problems with the roof sheathing lifting. The culprit, foam insulation. I got this info directly from someone who is involved in the case so I feel it's accurate.

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) over 9 years ago

Do you happen to know what type?

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 9 years ago

I believe it was Icynene. There are insulation companies out here that flat out refuse to install any type of spray foam.

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) over 9 years ago

Thank you for the info, James!

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 9 years ago

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