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Are ‘Hot Roofs’ allowed in Minnesota?

By In Hot Roofs On October 16, 2012

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.  On old one-and-one-half story houses, foam insulation can be an especially nice way to cut down on heat loss, air leakage, ice dams, yet maintain nearly the same headroom that already exists.

A common way of re-insulating old one-and-one-half story homes in Minnesota is to have closed-cell (2 lb) spray foam applied directly to the roof sheathing, eliminating any ventilation.  This type of spray foamed roof is commonly referred to as a ‘hot roof’.

Hot roof

Why are they called ‘hot roofs?’ A traditional attic has an insulated floor and is ventilated.  Outdoor air is allowed to come in through the soffits and leave at the top of the roof, creating a cold attic space during the winter.    This helps to prevent ice dams, keeps the roof cooler in the summer which may help to prolong the life of the shingles, may help to prevent the accumulation of condensation, and satisfies the warranty requirements of the shingle manufacturer.  When spray foam is applied directly to the roof sheathing, there is no longer an unconditioned attic space; the thermal envelope is moved right up against the underside of the roof.  Because there is no longer ventilation, these are often called hot roofs.

Are they really hot?  No, just slightly warmer.  Studies have shown that color differences in shingles will actually have a larger impact on the temperature of shingles than the difference between a ventilated and a spray-foamed roof.  A ‘hot’ roof will typically only be a couple of degrees warmer than a ventilated roof.  This has the potential of reducing the shingle life by up to 10%.

What are the benefits? Closed-cell 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, and it’s nearly impossible to install properly.  Foam insulation 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, eliminating a lot of energy loss.  Also, on old one-and-one-half story homes with no soffits or very short soffits, adding proper soffit ventilation can be difficult or impossible.

Are hot roofs allowed in Minnesota?  This is a tricky question.  The current version of the Minnesota State Building Code uses the 2006 International Residential Code, with amendments.  If we turn to section R806.4, we find a section titled “Conditioned attic assemblies.”  This section says “Unvented conditioned attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) are permitted under the following conditions:” … and then goes on to list a bunch of conditions.  So it would seem that hot roofs are allowed, as long as all of the requirements of R806.4 are met.

Not so fast though.  One of the Minnesota amendments to the IRC, section 1309.0806, deletes section R806.4 in it’s entirety.  In other words, this says that hot roofs aren’t recognized as an acceptable alternative.

Yeah but still… Minnesota Rules section 1300.0110 Subp. 13 says this:

The code is not intended to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed by the code, provided that any alternative has been approved. An alternative material, design, or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the code, and that the material, method, or work offered is, for the purpose intended, at least the equivalent of that prescribed in the code in quality, strength, effectiveness, fire resistance, durability, and safety. The details of any action granting approval of an alternate shall be recorded and entered in the files of the Department of Building Safety.

In other words, if a building official finds that an alternative method of construction is satisfactory and complies with the intent of the code (etc etc), they “shall” approve it.  In February of 2011, a letter was written on this topic by Sr. Building Code Representative Donald Sivigny to State Building Officials in Minnesota, offering commentary on code acceptance of unvented attic assemblies.  You can read the letter here – http://www.dli.mn.gov/CCLD/PDF/OpinionStaffAttic.pdf .

As mentioned in the letter, the 2012 version of the International Residential Code has expanded the section on unvented attics significantly, and they’ve also renumbered it as section R806.5.   If you’re curious about the differences, I put together a pdf showing the two code versions, which can be viewed here 2006 vs 2012 IRC on hot roofs.  Keep in mind, the 2012 IRC has not been adopted yet in Minnesota, but I expect section R806.5 won’t be deleted in the next wave of Minnesota Building Code changes, which are expected to happen in 2013.  It will probably be changed in some fashion, however.

The way it stands now, some cities in Minnesota allow hot roofs as an alternative method of construction and some don’t.  I contacted the building inspections department in twenty of the largest cities in the Twin Cities area, to ask whether hot roofs, or ‘unventilated roofs’ were allowed.  Here’s a breakdown of what some of the different cities had to say about hot roofs.  Not all of the cities responded.

No, we don’t allow hot roofs.

  • Apple Valley
  • Delano
  • Eagan
  • Eden Prairie
  • Edina (probably not)
  • Hopkins
  • Lakeville
  • Maple Grove
  • Plymouth
  • Shakopee
  • Saint Louis Park
  • Woodbury

Maybe.  We might allow hot roofs if certain requirements are met.  

  • Bloomington
  • Brooklyn Park
  • Burnsville (maybe?)
  • Chanhassen
  • Coon Rapids
  • Elk River
  • Minneapolis
  • Richfield
  • Roseville
  • Saint Paul

For most of the cities that said they would entertain the idea of allowing a hot roof, they also said that they would only allow a hot roof if the shingle manufacturer’s warranty wasn’t voided by the lack of ventilation.  For example, Owens Corning will not honor their warranty on asphalt shingles if a hot roof is used, but Certainteed does.  By coincidence (?), Owens Corning does not make a closed-cell foam insulation product, but Certainteed does.  Funny, huh?

The bottom line is that if you want to install closed-cell spray foam against your roof decking, you should check with your local building official and shingle manufacturer first.

Reuben Saltzman, Structure Tech Home Inspections


About the Author


Reuben is a second generation home inspector with a passion for his work. He grew up remodeling homes and learning about carpentry since he was old enough to hold a hammer. Reuben has worked for Structure Tech since it was purchased by Neil in 1997, and is now co-owner and President of the company.


  • Scott 4 YEARS AGO

    Oh and to follow up and my last email. I have been spraying foam all over the nation for over 15 years. I live in Blaine and think I know what I am talking about NES Spray Foam

  • Scott 4 YEARS AGO

    Wow. I just had to write as I know those are old post from people. As far as doing spray foam yourself , The biggest mistake in insulation is having your DYI stores selling it with no knowledge themselves. Also by the time you spray more than 300 board feet. That is 1 inch by 300 sq ft you would have spent what it would take for a professional installer to come do it. The average cost per board ft of close cell foam from a professional is .85 cents. When you buy a DYI kit at Menards you are paying $1.65 per board ft. Now as far as hotroofs. I did my own hotroof in a vaulted ceiling. It is by far the best overall insulation system with closed cell foam I do not think it is the best overall to hot roof a rambler type or open space attic if there is a lot of space overall. If you only have a 3 ft or less attic then great but if more than that just foam all your top plates and ceiling protrusions and make sure you have plenty of vents in the eves with wind blocks and your eves/ soffits are open for as much air flow as you can get. don't forget about roof turtle vents either and or gable vents. So as you can see it is not just add more insulation ,but to evaluate your building envelope and not just trust Menards or Homedepot personal and advise.

  • BadgerBoilerMN 4 YEARS AGO

    The very idea of a hot roof is that it is not vented. Like the old "vented" crawl space. We know better now. http://www.garlandco.com/educational/ytr/whats-driving-vapor-drive.html http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems

  • Neil 4 YEARS AGO

    Thanks so much Reuben!

  • Neil 4 YEARS AGO

    Thanks Reuben for the quick response! Yes, on the roof decking and walls inside the attic space. I look forward to your response. :) Can you give me the bullet points on why it's not a DIY in the article? Thanks! Here is where I would buy foam next time http://www.sprayfoamdirect.com/home-owners/8-mistakes-to-avoid due to cost and here is the Menards link: http://www.menards.com/main/search.html?search=froth+pak&sf_categoryHierarchy=

    • Reuben Saltzman 4 YEARS AGO

      I wouldn't recommend doing the roof decking and walls inside the attic because you'll no longer have any kind of defined thermal envelope, and your attic will no longer be vented. The moisture that escapes in to the upper portions of the attic will have nowhere to go, and you'll probably end up with moisture problems in the attic. As much as you don't want to dig through the existing insulation, it's really the only way to do it right. As for not doing spray foam yourself, a few problems mentioned in the article were improper surface temperatures, spraying the foam on dirty surfaces, having the foam mixed improperly, and spraying the foam too thick or too thin.

  • Neil 4 YEARS AGO

    Why do you not recommend installation of closed cell foam yourself? I used closed cell insulation to seal and insulation my sill plate and that space between the rafters in the basement. I bought some at Menards. It was super easy, but apply very sparingly because it expands. Just wear rubber gloves, a mask, glasses and grub clothes. The basement seemed reasonably well ventilated. My attic space is about equally ventilated. I'm planning on spraying on my own in the attic. Thoughts? I would really like to spray foam the ceiling in my attic to totally prevent ice dams. I would argue the greater risk of damage in MN in ice dams getting under roof and causing leaks and mold. I've heard of dozens of people personally that has happened to. For simplicity of not needing to remove my old insulation can I do the following: spray foam 2" layer on ceiling and walls (I'm in a rambler), Great Stuff attic bypasses and recessed lights, and add >R50 of cellulose insulation. Thoughts?

    • Reuben Saltzman 4 YEARS AGO

      Neil - I don't recommend doing closed cell foam yourself because of the amount of problems that can happen with it. The Journal of Light Construction had a great article discussing the problems that can happen with an inexperienced installer, but I think you need to have a membership to view the whole article online - http://www.jlconline.com/insulation/avoiding-problems-with-spray-foam.aspx . I had no idea Menards was selling this stuff. I'll have to check that out. I don't completely follow you when you mention insulating the ceiling and walls; where exactly would you spray the insulation? On the ceiling (roof decking) and walls inside the attic space?

  • BadgerBoilerMN 4 YEARS AGO

    Dry rot can only occur with the presence of moisture. If your roof leaks, fix it. 20 years for an asphalt roof... I say it is a good wind that blows on time.

  • john oeltjen 4 YEARS AGO

    hot roofing may void the warranty on your shingles! check with your shingle manufacturer first my boss hot roofed his new house back in about 1981. in2002 a wind storm put a tree on his house so the roof needed repair. once the contractor got on the roof to fix it he actually FELL through it!!! the roof was completely dry rot!! just some food for thought

    • Reuben Saltzman 4 YEARS AGO

      John - yes, I mentioned this in the second to last paragraph.

  • Jane Slaughter 4 YEARS AGO

    Minneapolis just told me they have a "Hot Roof Acknowledgement Form" that needs to completed at the time of permit application.

    • Reuben Saltzman 4 YEARS AGO

      Jane - very interesting. That's the first I've heard of it.

  • Dale 4 YEARS AGO

    Typical case where building departments are slow to adopt the latest trends of improved building techniques. The part that makes me laugh is the point ..... "they also said that they would only allow a hot roof if the shingle manufacturer’s warranty wasn’t voided by the lack of ventilation. " Since when is it the mandate of the building deptartment to be concerened about warranties? Sounds more like being non-committal.

  • Reuben Saltzman 4 YEARS AGO

    Reuben - yes, I think it happens exactly the way you're describing it. I spoke with several different spray-foam insulation companies when I had my own house insulated about five years ago, and none of them really had any idea whether the product was allowed by the different cities or not. None of them had any interest in pulling permits for the work either. As 'BaderBoilerMN' put it, ignorance is bliss.

  • BadgerBoilerMN 4 YEARS AGO

    Ignorance is bliss.

  • Reuben 4 YEARS AGO

    After some googling, it looks like GAF will honor the manufacturers defect warranty on hot roofs - unless it can be shown that the hot roof is directly responsible for the problems, then the warranty is void. So when spray foam contractors do jobs, they aren't trying to negotiate and get approval with the Cities for every single job, I'm sure. So... are these companies just keeping quiet and hoping nobody from building inspections notice what they're up to? Or hoping nobody cares enough to stop them?