Spray foam roof (C) D FriedmanUn-vented Roof / Hot Roof FAQs
Damage Risks, Durability, Q&As on Hot Roof Designs

  • HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS - CONTENTS: Un-Vented Roof Solutions: how to avoid condensation, leaks, attic mold, insulation mold, & structural damage to roof framing when roof venting is not possible. Hot roof designs: suggestions for un-vented or hard-to-vent building roof cavities. How to detect roof venting deficiencies, attic insulation defects, and attic condensation problems
  • POST a QUESTION or READ FAQs about un-vented cathedral ceiling under-roof spaces
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FAQs about Hot roof designs, aka dense-packed insulated sloped roofs:

Here we present questions & answers about un-vented roof designs, hot roof designs, and specifications. We discuss the durability of this design and include warnings about stumbling blocks to avoid.

This article series describes various solutions for un-vented cathedral ceilings and similar under-roof spaces, offering advice on how to avoid condensation, leaks, attic mold, & structural damage when roof venting is not possible.

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Un-Vented Roof Solutions: how to avoid condensation, leaks, damage when roof venting is not possible

Cathedral ceiling insulated with solid foam and a poly vapor barrier (C) Daniel Friedman

Recently-posted questions & answers about designs for & problems in un-vented or "hot roof" designs originally given at HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS

[Click to enlarge any image]

Photo at left: the moderator below a cathedral roof design constructed in the 1970's in New York. Details of this structure are in the HOT ROOF DESIGNS article linked just above.

On 2017-03-17 by (mod) - debating the pro's & cons of Hot Roofs: where do the risks lie

Dave: where does that claim derive?

Of course no one is saying that. Let's not be diffident.

First, soffit-vents alone won't do a thing to ventilate a roof.
Second, nobody with an ounce of sense is arguing that in heating and cooling climates the roof should be un-insulated.


Perhaps what I've not made clear is that leaks into a sealed, un-vented roof cavity is more pernicious than leaks into a vented cavity

- may not appear visible at all to building occupants - from the inside water passage is blocked by solid foam insulation or by plastic vapor barriers (the latter can hide leaks in a vented cavity too)

- water is trapped in the un-vented roof cavity - it doesn't dry out for months or longer - like the Hotel California, water can check in, but it can (almost) never leave

- wet building materials invite rot, insect attack, mold contamination - all costly problems

On 2017-03-17 by Dave

So with no soffit vents basically there should be nothing in the sloped part of the ceilings?

On 2017-03-17 by (mod) - a more-reliable roof is a smart idea over hot-roof installations

By no means would I claim that metal roofs are *required* by any code or standard for hot roof desings.

What I am pointing out is that now, with nearly 40 years of experience with hot roof designs, we know very well that leaks into a foamed (especially closed-cell foam) roofs are a disaster, so I want the most leak-resistant, long-lived roof covering I can get on the roof.

Leaks into a foamed hot roof or even a fiberglass insulated hot roof may not be detected until years of rot and decay have ensued.

"Hopefully no leaks will occur" is not enough to make me highly-confident in an individual roof design. Over the 20 to 40 or longer year life of a roof it is very common for leaks to occur:

Shingles crack, blow off, are damaged by foot traffic

Flashings crack, wear through, corrode

Branches fall and punch holes

Up north, ice dams lift roofing, push water and ice into the roof

Roof coverings have a demanding life.

On 2017-03-15 by Dave - hope that no roof leaks will occur

Metal roofing only for Sprayfoamed rafters, slopes?

Hopefully no leaks will occur but no barrier is required if Sprayfoaming the rafters or cathedrals

On 2017-03-15 by (mod)

Yes, Dave,
You are describing a "hot roof" design - that can work and makes sense for cathedral ceiling designs, but in my experience leaks into a foamed roof structure can cause catastrophic damage before they're discovered, so it is a smart design that uses the most bullet-proof roof covering - such as standing seam metal roofing.

On 2017-03-15 by Dave

Roof surface?

On 2017-03-15 by (mod)

Probably; what's going on the roof surface -

On 2017-03-15 by Dave

Regarding below, can reach me at

On 2017-03-15 by Dave - if cost isn't an issue spray-foaming ceilings with closed cell foam is best

So if cost wasn't an issue and my house never had soffit vents, (there are ridge and gable vents there),
One and a half story cape cod, sloped ceilings., I have a very reputable contractor that suggests Sprayfoam the sloped ceilings, then blown in cellulose for attic floor.

They will have the luxury of having all insulation currently in attic removed, all bedroom ceilings removed as house is being renovated.
So Sprayfoam the short exterior walls in closets (they're about 3 feet high, the slope part of ceilings, replace new drywall, then cellulose in attic.

Using closed cell.
Ps. After all old insulation has been removed in attic, they will seal all air leaks.
Does this sound okay?

On 2017-01-29 by Robert Fentie

In the spring water coming threw drywall

On 2016-10-25 by (mod) Why is there not one site on the internet that shows or tells how to properly vent an existing A-Frame style home?

Excellent question, Di.

I suspect the reason you don't see more about venting A-frame home roofs and ceilings is that most of these homes were built with no roof cavity whatsoever and no roof venting. The roof decking is often plank and is exposed on the home's interior as a wood ceiling.

Adding ANY venting in the A-frame roof in a cooling climate such as Wisconsin would result in pouring building heat outdoors in winter and an astronomical heating bill.

Therefore I take a different approach:

1. Find and remove indoor sources of abnormal moisture
2. I install and make intermittent use of one or two gable-end fans to provide outlet venting when needed. With a removable insulating cover I can remove or hinge-open the cover and run the fans on occasion.

We do not make use of passive venting if it's going to just dump indoor heat outside.

Please use the page top or bottom CONTACT link to send me some sharp photos of the home from outside showing the whole structure from front and side, and from inside showign the area near the ridge and the areas of damage and I will comment further.

On 2016-10-25 by Di Haman

Why is there not one site on the internet that shows or tells how to properly vent an existing A-Frame style home? Many in my neighborhood (central WI) that were built back in the mid 80's to early 90's have no ventilation at all so moisture has no way to escape and rot begins to eat at rafters and decking. I have images for your review to show the extent of hidden rot and decay.


On 2016-06-08 by Tom - best option for a hot roof design in Maine

My house has a main section in hexagonal shape with 2 wings. The has a hot roof with 8 inches fiberglass bats and 3 in rigid sheets insulation.

Ceiling is 3/4 inch v-match pine boards cut at angles between 6 large beams at the corners of the hex and extending up to the peak where they meet at a large fieldstone chimney.

Condensation appeared two years ago. Building inspector and green contractor suggested taking out ceiling around chimney, where condensation was biggest problem, removing insulation, drying out, spry with foam insulation and put everything back.

Also added range hood, used bathroom fans religiously and kept humidity from 25-50% all winter. Now we are back with the same problem in several sections of the ceiling.

Recommendations are tear down whole ceiling, dry out, spray foam etc like we did around chimney, or caulk all ceiling boards as well as seams at beams, buy a whole house dehumidifier, build a new ceiling with steeper pitch so it is like an attic and vent normally.

I live in Maine and we don't have too many experts in hot roofs so am looking for guidance on which option(s) I should choose.

None are very appealing and all seem costly but I know I have to do something. Thanks for any advice you can give. Tom

On 2016-01-06 by (mod) - would a metal roof be better insurance against leaks and future problems in a hot roof building?

6 mil poly is fine. As I said it's the cuts, tears, or openings or penetrations such as at light fixtures that will dominate any problem of indoor air or moisture leaking into the roof.

For a DIY insulating project fiberglass, neatly placed to avoid voids and air bypass leaks, is fine. For a professional job I prefer spray foam insulation.

And yes absolutely, a metal roof is in my opinion a superior choice for a well-sealed hot roof design as it reduces the risk of a perforation leak in the roof surface in the future. That's what I put on my own office. Keep the roof penetrations such as plumbing vents to as few as you can and be sure those are perfectly installed and sealed.

On 2016-01-06 by Fred

Hello again, We are doing the work as I correspond with you. So your prompt response has been very valuable. I am going to do everything possible to prevent any problem as much as possible.

So the vapor barrier will be installed. What if any specific material would you suggest? Any particular brand and type or alternative would be very helpful. Also I have un-ventable eves/ soffit. What type of insulation should be used in that area? Would loose cellulose or pink fiberglass be exceptable?

This is a post and beam home built in 1850. So you can imaging the challenges I've been having to overcome. And finally would a metal roof be better insurance against leaks and future problems? Is there any way I could send you pics of the area in question?

If so please explain and I would be very happy to do so. Thanks so much once again. Fred

On 2016-01-05 by (mod)

In my OPINION, because a hot roof design can trap moisture and damage the building, having the best vapor barrier you can achieve is worth the small extra expense, but don't forget that an opening anywhere is able to overwhelm even a perfectly-placed foil vapor barrier; And if the roof leaks from above, that water entering the roof cavity is going to stay therein and be a problem.

I don't think the voids you describe argue for or against the vapor barrier, as they are not what will cause water or moisture to be trapped in the roof cavity.

On 2016-01-05 by Fred Grassia

Thanks for all that helpful info.... Since I am going to have some voids between the drywall ceiling and the foam board. Do think it wise to create a perfect vapor barrier? The room will be used in the summer and air conditioning will be in use. And in the winter the space will be heated on a minimal basis and left a cold space with screened windows crack for ventilation and circulation of air.

On 2016-01-04 by (mod) - defining "perfect vapor barrier"

Fred, thanks for asking.

Perfect vapor barrier is my term for a vapor barrier that has a perm rating of zero AND has no punctures, tears, leaks over the surface to which it has been applied. Metal foils have a perm rating at or close to zero. Plastics and paper are more permeable.

Keep in mind that since the mid 1980's research has shown that the vapor barrier, or even its absence, in a wall or ceiling covered with a continuous material such as drywall, is less important than penetrations in that surface that permit air (and moisture) to leak into the building cavitiy.

The air pockets you describe help you they don't hurt. It's penetrations in the ceiling that would be a concern.

On 2016-01-04 by Fred Grassia

In reading Hot Roof design.. I see you mention "Perfect Vapor Barrier" however I haven't been able to read anything on the subject and what it is actually referring to. I have a non vent able situation. In an sloped roof design.

Using closed cell 4' foam board in between the rafters. their will still be minimal air pockets 1"-2" wide and 1/2" deep on the underside of the roof deck and 1/2"-1" x 12" square areas between the drywall and foam board.

Do I need to fill these shallow areas as best as possible and where and how to I install a "Perfect Vapor Barrier"? Thanks for all your help.. Fred Grassia

On 2015-07-17 by (mod) - options for an A-Frame timber framed home


I don't think you mean that the roof edges actually extend into ground contact - which would be a rather iffy roof design inviting rot and insect damage. So we hope that there's reasonable ground clearance.

Your options are to install a starter vent at the lower edge of the roof decking and a ridge vent at the roof top, or to go to a hot roof design.
If there is no actual roof cavity space thorugh which air could pass you're going to be looking at hot un-vented roof options.

On 2015-07-13 by Di

VENTILATION DILEMMA: Timber A frame style home. A frame roof goes from ground to peek, no soffit vents as the walls are one huge gable filled with insulation. How can these A frame style homes be properly ventilated to avoid dry/wet rot or decay when there is no attic as the gable itself is living quarters?

Much appreciated if you have answers


Continue reading at HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




Or see ROOF FRAMING TIES & BEAMS for a discussion of proper framing of a cathedral ceiling

Or see ROOF VENTILATION SPECIFICATIONS for additional information about the pros and cons of hot roof designs

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