Reader & Moderator review the risks of hot roof designs, aka dense-packed insulated sloped roofs and suggest a "best design".

Here along with reader Dave we discuss hot, un-vented roofs and what steps we might take to avoid trouble.

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.

Un-Vented Roof Risks & Solutions with Dave

Recently-posted questions & answers about designs for & problems in un-vented or "hot roof" designs originally given at HOT ROOF DESIGN PROBLEMS - you will enjoy reviewing the advice given there.

Photo at page top: open celled foam insulation being installed in a hot roof

Photo just above: the author (DF) below a well-ventilated cathedral roof he built in the 1970s.

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 2017-03-15 by Dave - You can reach me at davemax21@hotmail.com

Reply by (mod)

Probably, as I like the waterproof nature of closed cell foam. But I warn that ANY dense-packed or foam-insulated roof, either with closed cell or open celled foam, can become a disaster if there are leaks into the roof cavity.

That risk can be greater with closed cell foam that holds water and doesn't let it show up as a leak indoors until the roof structure has become badly rotted.

SO: what's going on your roof surface ?

Followup by Dave

Roof surface?

Reply 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.

Followup 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

Reply 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.

See my hot roof design detail suggestions

at HOT ROOFS vs VENTING in COLD CLIMATES

Followup by Dave

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

Reply 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.

See inspectapedia.com/ventilation/Roof_Ventilation_Specs.php ROOF VENTILATION SPECIFICATIONS

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

Followup by Dave

Sorry but the diagram keeps getting deleted .
It's:
I.stack.imgur.com/0tHCk.jph

Click on link shows my second floor, easier without the small knee walls

Just a technical note, this is a small house, only 900 square feet. Has a ridge vent, and gable vents.

Meant what is one to do?

No not being difficult , just gets frustrating cause I really don't want to put in soffit vents but want a warmer ceiling. What does one with existing roof shingles only 10 years old and assumed they never leaked

Your articles makes sense, vents need to draw.
Great article, we need to go with the unvented though

Reply by (mod) - A hot roof design can work, but has the risks we've suggested.

Yep.

So a hot roof design can work, but has the risks we've suggested. And a vented roof doesn't vent unless the vents are properly sized, located, and un-blocked.

Improper venting increases heating and cooling costs and can also invite its own moisture problems.

Our expert Steven Bliss commented to offer a final word on Hot Roof Designs:

I agree with you that, in the real world, this is not such a good idea. If there's a flashing leak or other roof leak, you could have a pretty soggy mess that stays wet for a long time and could cause structural decay.

Plenty of people are building hot roofs, but I wouldn't -- except maybe one with spray urethane which won't absorb much water like cellulose would.

If you need to go un-vented, do all you can to bulllet-proof the installation.

See our summary of the difference between the ideal and what actually happens in "real life" at

CONCLUSIONS & RECOMMENDATIONS for HOT ROOFS

 

 

 

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Continue reading at HOT ROOF DESIGN PROBLEMS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see HOT ROOF UN-VENTED ROOF FAQs-3 - more-recently posted questionsand answers about un-vented roofs and cathedral ceiling ventilation - or not.

Or see this

Article Series Contents

• CATHEDRAL CEILING VENTILATION
• HOT ROOF DESIGN PROBLEMS
• HOT UN-VENTED ROOF DESIGN SOLUTIONS
• HARD to VENT ROOF SOLUTIONS for HIPS & CATHEDRAL CEILINGS
• PROBLEMS with PARTIAL ROOF VENTILATION
• ROOF VENTILATION SPECIFICATIONS pros and cons of hot roof designs

Suggested citation for this web page

HOT ROOF UN-VENTED ROOF FAQs-2 at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING VENTILATION

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