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Reuben Saltzman

Hot Roofs: A Misnomer

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’.

Foam Insulation
Reuben’s Attic

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 and a spray-foamed roof.  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? Post edit 3/30/09: Probably not.  Most of the major manufacturers of shingles still warrant their products when used with a spray-foamed attic.   Owens Corning, however, does not.  See comments below – I called Owens Corning to verify this.

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 1981.  Spray foam is also used at the rim joist in almost every new construction home that I inspect.

Additional Information – technical, dry reads.

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 – EmailMinneapolis Home Inspections

No responses to “Hot Roofs: A Misnomer”

  1. Darlene
    March 30, 2009, 7:07 am

    I read your blog about the “hot roof” method of insulation and that all major shingle manufacturers will still warranty the roof shingles. My husband and I built a new home and used the “hot roof” method however, Owens Corning has stated that they will not warranty our shingles if our attic is not properly ventilated. We had our insulation company call Owens Corning but they are not budging. Just an FYI.

  2. Reuben Saltzman
    March 30, 2009, 1:07 pm

    Thank you for the info. Owens Corning… boo!

  3. jenny
    May 8, 2009, 1:46 am

    loved the post, i am brenovating an old house, and i was about to go for fiberglass, but thanks to your post now i know what to do.

  4. Mary
    October 24, 2009, 4:29 pm

    I wonder what your opinion is on open vs closed cell spray foams? I was interested in Icynene until a closed cell salesman claimed all open cell spray foams were bad. I am convinced I want spray foam on walls and roof of my renovation but cant pick a product. My home was a 1.5 story that I have converted to an almost full 2 story.

  5. Reuben Saltzman
    October 24, 2009, 6:36 pm

    Mary – nice! That’s what I did with my own home, and my neighbor just did the exact same thing. I’d go with closed cell because it has about twice the insulating value, and the cost is about the same.

  6. New Windows Are Nice... But You'll Never Get Your Money Back. | Reuben's Home Inspection Blog
    December 8, 2009, 5:02 am

    […] Attic / 2nd floor is insulated with about 3″ of closed-cell spray foam. […]

  7. Gary
    March 31, 2010, 11:40 am

    I have been told that putting asphalt shingles on a hot roof does not meet code of most communities in Minnesota?

  8. Reuben Saltzman
    April 1, 2010, 6:19 am

    Most communities will allow a hot roof as an “alternative method”. I’ve spoken with many different building officials, and haven’t yet come across one that won’t allow a hot roof.

  9. Tony
    December 22, 2010, 7:11 pm

    Good to see support for hot roof. Bought a 1920 two-story, newly renovated, 2 1/2 yrs ago. Our hired inspector was not keen to the idea of having no attic ventilation, yet city building official claimed it would be put into new code books when they came out, as a reliable and legal style of roof. All but the rim-joist are done and soon we will get them done also with an open-cell, isynene insulation. No ice dam problems and energy bills stay reasonably low all yr round. Average summer with energy efficient AC $75-$85. Winter started cold this year in MN. October to November bill $86. November to December with an average of 8 degrees colder then last years, same period $164. In the three winters here our bill has never been over $200. While friends are paying $250-$300 with fiber-glass. I had my doubts but am every year growing more confident that this stuff works great and eliminates ice dam issues. Wish I had found this blog earlier, when I had a ton of questions.

  10. Reuben Saltzman
    December 22, 2010, 9:29 pm

    Hi Tony, I’m glad to hear it. By the way, what city do you live in?

  11. Alicia
    January 22, 2011, 9:59 am

    We are considering a hot roof for our 1.5 story home in Minneapolis. I understand the difference in R-value between open and closed cell foam, but can you explain the difference in moisture control? It looks like we have a green spay foam insulation on the slants of the roof already, but we are not sure about the space above the finished ceiling. This may be part of the problem (ice daming), poor venting. Lastly, do you know if city code now require a thermal layer over the foam?

  12. Reuben Saltzman
    January 22, 2011, 10:12 pm

    Alicia – as for moisture control, closed cell foam will act as a vapor barrier; open cell won’t. If you use open cell foam, you’ll still need to install a separate vapor barrier.

    The green stuff you’re referring to is probably closed cell foam. I’m not sure what Minneapolis requires over the foam, but if you have 2×4 rafters, I would strongly recommend adding something else, like another inch of rigid foam below that.

  13. Chris
    March 28, 2011, 1:14 pm

    Great info. We are remodeling our attic, the insulation used was poor to none, as it was using broken down wool and newspaper. We have gutted the attic to get it spray foamed (as we did this with our basement remodel and it’s the nicest place in the house now (winter and summer) With pulling the permits stating we plan on spray foaming (hot roof) we got this response from our inspector:
    {The building code treats this type and other foams as hazardous and needing protection from flame as they do burn with a high toxicity, and aren’t allowed to be exposed to interior spaces and in some cases concealed spaces.} Does that mean we also have to use fiberglass on the interior walls?

  14. Reuben Saltzman
    March 28, 2011, 6:01 pm

    Chris – that just means you need to cover the foam. Half-inch drywall is fine.

  15. Drew
    March 31, 2011, 6:15 am

    “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.”

    If you get a chance, check out the experiment that Habitat for Humanity did with the two homes (one with spray foam and one with fiberglass). The spray foam home paid $500 less in electric bills in just five months. With a grand a year (minimum) savings, you’d get your money back in 4-7 years. Plus they have all those tax credits for spray foam treated homes.

    Venting homes is fine in some places, not in the Gulf South. FEMA did a study on the Florida hurricanes, turns out homes were getting ruined because the storms would destroy the soffits. Rainwater got in, then into the attic, then into the walls, and it was moldville USA. In Minnesota I imagine you’d be fine, but down here, we spray foam into the attic, and seal up the attic vents.

  16. Nate
    May 22, 2011, 8:30 pm

    Incredibly happy to have found this blog. We’re in the middle of huge brainstorming session to solve our 1.5 story house’s issues: ice dams, ventilation, bad existing insulation, etc… And a hot roof is looking like the solution to many of these issues. Regarding Chris’s comment 3/28, if we tear the drywall out of our finished half story down to the rafters, add 2x2s, spray in foam on the roof decking, can we just put drywall back on the kneewall and inner ceiling an consider the foam “covered”? Or do we need to put drywall over the foam all the way to the attic floor AND finish the kneewall? There are access doors in each kneewall, if that matters, and we’re in Minneapolis. (Also I realize this is basically asking for free advise, but I guarantee you’re my guy next time I ever need a house inspection… 🙂

  17. Reuben Saltzman
    May 23, 2011, 4:14 am

    Hey Nate, most of this blog is free advice :).

    Here is what the code says: R314.5.3 Attics. The thermal barrier specified in Section
    314.4 is not required where attic access is required by Section
    R807.1 and where the space is entered only for service
    of utilities and when the foam plastic insulation is protected
    against ignition using one of the following ignition barrier
    1. 1.5-inch-thick (38 mm) mineral fiber insulation;
    2. 0.25-inch-thick (6.4 mm) wood structural panels;
    3. 0.375-inch (9.5 mm) particleboard;
    4. 0.25-inch (6.4 mm) hardboard;
    5. 0.375-inch (9.5 mm) gypsum board; or
    6. Corrosion-resistant steel having a base metal thickness
    of 0.016 inch (0.406 mm).
    The above ignition barrier is not required where the foam
    plastic insulation has been tested in accordance with Section

    My interpretation of R304.5.3 is that you still need to have the foam covered inside the knee walls, but you can use 3/8″ drywall in those locations. If the Minneapolis permit inspector in your area has a different take on that section, please let me know.

    – Reuben

  18. Nate
    May 23, 2011, 12:33 pm

    Thanks! I think you’re reading that right… One last question – we have 2×4 rafters, and will only get ~R23 in that space. Adding 2x2s before spraying would get up to R32, but everything I’m reading says in our zone we’d need at least R38. Are you still pulling snow off your roof to prevent ice dams? I’m not clear on how anything less than R38 would work in our climate (referencing this PDF:

    I’m really leaning towards a hot roof but I need to resolve a few more questions and check our budget… Thanks!

  19. Reuben Saltzman
    May 23, 2011, 7:41 pm

    Yes, I still had to pull snow off my roof, but I only filled the 3 1/2″ rafters with foam. If I were to do it again, I would have made some type of thermal break in the wood framing, such as using sheets of rigid foam. I don’t think R38 is a magic number, it just happens to be the minimum amount that is allowed in new construction in Minnesota.

    The more the better, of course, but I’m certain that an R32 roof with closed cell foam would outperform an R38 roof with dense pack cellulose.

  20. Robert
    August 23, 2011, 4:42 pm

    I have an engineered log home built in 1979 that has cathedral ceilings & no attic space. The current “hot roof” has only about 3 1/2″ of rigid insulation & we had ice dams last winter. I am considering tearing off the shingles, adding 2×4’s on edge to the existing deck, & filling with polyurethane spray foam, then applying a new deck & shingles. Should I be concerned about thermal bridging? Do I need to add any vent space above the foam to prevent ice damming?

  21. Reuben Saltzman
    August 23, 2011, 5:32 pm

    Hi Robert,

    Yes, I would certainly be concerned about thermal bridging. As far as vent space above this, I’m really not sure if this would help. It makes sense, but you are suggesting an insulation method that I’ve never seen before. I think your best bet would be to consult with an engineer.

  22. tony
    January 3, 2012, 9:51 pm

    come back to me in 10 years when your 30 year architectural shingle is blistered beyond belief. maybe not as quick in the north, but hot roofs get REAL hot in georgia.

  23. Reuben Saltzman
    January 5, 2012, 7:08 am

    Tony – check out that link I included at the end of the post – . It’s all about how hot roofs perform in hot climates.

  24. Local Dude
    February 23, 2012, 4:16 pm

    Great info!.. I have a 1 & 1/2 in Arden Hills, previous owners have added 2x2s to the 2x6s before dry walling the ceiling in the attic, leaving a 7″ gap. Right now there is under performing fiberglass insulation with much being displaced by past squirrels. I want to tear this out and replace. I have access to many 3″x6″x8 foot pieces of Dow Highload 40 Styrofoam. I intend on placing them side by side to create a 6″(R-30) thick piece of foam to install between the rafters. The question I have, is it best to leave the 1″ air gap, or fill it with one last piece of foil faced foam and make it a “hot roof”? And if foil faced foam is suggested, do I face the foil out or in? The roof faces due south. Thanks for any information.

  25. Reuben Saltzman
    February 23, 2012, 8:25 pm

    I’m guessing your best option would be to leave a 1″ air space, because hot roofs are really supposed to be done with the foam applied directly to the roof decking so no air can possibly get in between the insulation and the roof boards. You might want to find an engineer get you a definite answer though; this is just my best guess.

  26. BadgerBoilerMN
    October 9, 2012, 10:22 am

    I design mechanical systems here in Minneapolis and specify high density (2# closed-cell foam) on a regular basis to reach the load values I need when designing new radiant floor and ceiling systems. It is especially useful when installing new radiant floors in older homes.

    There are few appropriate applications for “open” cell foam in Minnesota. You want that vapor barrier and high per-inch R-value that 2# foam offers.

  27. BadgerBoilerMN
    October 9, 2012, 12:17 pm

    I design mechanical systems here in Minneapolis and specify high density (2# closed-cell) foam on a regular basis to reach the load values I need when designing new radiant floor and ceiling systems. It is especially useful when installing new radiant floors in older homes.

    There are few appropriate applications for “open” cell foam in Minnesota. You want that vapor barrier and high per-inch R-value that 2# foam offers.

  28. Reuben Saltzman
    October 16, 2012, 4:10 am

    I have created re-directs for this blog post. All views should now be directed to this more recent post –

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