Hot roof designs, aka "dense-packed" insulated sloped roofs for warm or hot dry or hot humid climates:

This article describes various warm or hot or hot-humid climate hot roof designs, benefits, and risks. The text discusses solutions for un-vented cathedral ceilings and similar under-roof spaces in hot humid climates and offers advice on how to avoid condensation, leaks, attic mold, & structural damage when roof venting is not possible.

This article series about roof and ceiling ventilation describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems in buildings.

Hot Roofs vs Vented Roofs in Hot Humid Southern Climates - Building Heat Gain

Our photo (left) shows an extensive Penicillium sp. mold contamination in an attic where moisture condensation had gone unnoticed

The identification of this mold as Penicillium happened in the lab, of course, not based on just the visual inspection.

TenWolde and Rose pointed out in 1999 that

No scientific claims have ever been made that attic ventilation is needed for moisture control in warm, humid climates.

In these climates, the outside air is much more humid than the inside air which is cooled and dehumidified by air conditioning.

In such climates, attic venting tends to increase rather than decrease moistur4e levels in the attic .... [and] may therefore increase the danger of condensation on [the] HVAC ducts [as well]. [7]

Our photo (left) shows stains on the upper building walls under an inadequately-vented soffit. This New York Home suffered recurrent ice dam leaks sending moisture through the building walls, with the expected problems that result.

Joe Lstiburek's view is that particularly in the hot humid South (of the U.S. - and not the local of the buildings we are illustrating here) there is not much gained in venting a roof (2-3% reduction in heat transfer in a vented attic).

And quite properly Dr. Lstiburek continues to point out that once A/C ducts are placed into the vented attic heat transfer to the occupied space goes up to 5-7% (for tight insulated ducts) compared to routing those ducts through conditioned space of the home, and a much worse 25% heat gain to the home if the HVAC ducts are leaky.

And his argument that air movement through the attic under the roof will not flush heat being radiated towards the ceiling below is both interesting and compelling as well.

Joe also argues that the moisture from warm humid outside air run through an attic to ventilate it can move through the attic insulation where it can condense on the cool building ceilings below. [5]

When does warm air move "down" and when might cool air move "up" in a structure?

Those data are compelling, though I'm not entirely convinced that that nasty warm southern air my respected acquaintance describes actually moves "down" to the cool ceiling in the attic. In roofs I've worked-on and examined with air tests and smoke guns, warm air rising in the hot attic went zooming out at the ridge vent, drawing cooler (nasty moist Southern) air in at the eaves.

The incoming air followed the underside of the roof upwards on the air currents exiting at the ridge. without however, recapping construction costs, or considerations for an existing home rather than new construction.

I have found surprising down-currents in indoor air inside a few buildings where upper floor air was being cooled to a temperature and density greater than warmer air downstairs. In that circumstance cooler upstairs air flowed down stairwells to lower floors and on occasion that down-draft could de-pressurize an attic or cathedral ceiling causing warm moist outdoor to enter those areas.

Impact of venting on roof temperatures

Joe pegs the impact of venting on roof temperatures at 5%[5][6].

Really? I actually measured the temperature drop in a hot, un-vented attic in New York from over 145 degrees to 95 degrees after we cut in soffit and ridge venting

I was measuring air temperatures in the attic, not roof surface temperatures, and not radiated heat effects. That was a 35% drop in air temperature in the attic! This single "real world" case found a roof cavity temperature drop of seven times Dr. Joe's estimate.

Clearly we have seen some actual building conditions that don't behave as the scientists expected.

Watch out for real world snafus, damage, leaks in roofs. In sum, I'm left unsure about the gap between new construction designs and a perfect world where roofs never leak and the roofs I and more importantly, repair and renovation roof contractors have found when we inspected, tore apart, and repaired leaky roofs of both insulated cathedral ceiling homes, and homes with vented roof cavities.

• In hot humid climates an un-vented roof is a defensible approach that may avoid both condensation and heat transfer problems.
• In both warm climates and in cool northern climates an un-vented roof is a defensible approach that avoids risk of chimney effects affecting building heating and cooling costs
• Moisture problems originating outside the building such as due to damage or poor workmanship involve an element of luck and appear to have been excluded by experts and studies in the field, notwithstanding the experience of repair-roofers and home inspectors that those leaks appear on a great many homes, can cause severe rot damage, costly mold damage, and that leaks eventually happen on a great many if not most homes that remain standing for 20 years or more.
• Attic or roof cavity condensation due to indoor moisture sources vary widely as individual building moisture conditions vary (we start our roof inspection by looking for evidence of basement or crawl space flooding) as well as by quality of construction and the number of penetrations in the building ceilings and walls, not to mention the effects of air pressure differences within the building.
  
  Delivering a durable hot dry roof is sound in theory but hard to deliver in practice.
  
  Samuelson (1995) pointed out that "... to guarantee no indoor air movement into the attic, the ceiling has to be airtight and the pressure of the attic needs to be higher than that of the indoor air (i.e. pressurized attic or depressurized living space)"[7] - not normal conditions in most climates.
• An under-roof cathedral ceiling or attic ventilation system, to help the building in any way, needs to be properly designed and installed with adequate intake venting at the eaves, outlet venting at the ridge, and with careful sealing of air leaks from the occupied space into the attic or roof cavity (to avoid heat losses or increases in both moisture movement into the roof cavity and increased heating or cooling costs).
• The packed un-vented hot roof system can work very well in the climates where recommended if everything is perfect in construction and if no leaks ever occur in the roof system over its life  - that is, if nothing ever goes wrong. If leaks do occur, from either direction, damage is likely to be more severe than in the older vented roof cavity approach.

 

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: options for venting or conditioning a cathedral-ceiling vs attic space used for HVAC in Louisiana

2020/02/27 Mike said:

My cathedral ceiling is insulated with some thick type of fiberboard. My roof/ceiling for my 1957 home stacks up as follows: architectural shingles/double tar paper/OSB/gravel and tar/5inches of fiberboard/paint.

This exists throughout the house. At some point a small attic was constructed by placing a flat ceiling in about 1/5 of the house.

This attic houses the AC flex ducting. It currently has soffit vents and a whirlybird [turbine vent - Ed.]

[Click to enlarge any image]

After reading about ducting and the benefits of sealed and conditioned attics, could I seal this attic and give it a little AC? It has the exact same insulation as the rest of the house. This is in central Louisiana. Thanks for any advice.

This Q&A were posted originally at FIBERBOARD SHEATHING

Reply:

Mike

Thanks for the question. First help me get this right.

Your roof - is it a flat roof? Low slope?
Do you really have a mutli-layer roof of shingles over tar and gravel?

About the high attic space running ductwork, those spaces are hard to vent effectively if the lower pitched roof is un-vented (the lower roof is a "hot roof" design by your description).

Do you really have soffit vents letting air into your upper attic ? Soffit vents are installed at the lower roof edges or "eaves" - not part-way up a roof.

Before I continue explaining what may be pointless arm waving, please
draw a sketch and use the "add image" button to show us what's there.

Reader follow-up:

Mike said:
Hey ... thank you for the quick response.

The roof is a 3/12 pitch. As I understand it, originally the roof was tar and gravel. At some point a metal roof was put directly on top of this tar and gravel, which remained for something like 20 years.

Soon after I bought the house, a tornado peeled the metal off. The roofing contractor nailed down OSB directly onto the tar & gravel, then a double layer of paper (due to the shallow pitch). I think i painted an incorrect picture with my description: I only have 1 attic. The roof is layered as described, with zero air gap between the layers.

The soffit vents are in the lower edge of the roof overhang. I'm attaching a sketch of a slice through my house, please let me know if you would like any additional information.

Moderator reply:

So you could vent the half attic- properly by continuous vents along the soffit and a continuous half ridge vent at the ridge.

The present venting system won't be as effective.

Just where insulation is places is relevant.

Also about that "fiberboard ceiling" - in a modern home meeting current fire standards I'd expect your local building inspector to want that covered by drywall.

Reader follow-up:

Mike said:

Is it possible or advisable to seal the existing venting and condition that space? I will look into the fire standards for my state.

Moderator reply:

Yes Mike.
The reason I asked about insulation is that when converting the utility-space-attic containing HVAC equipment/ductwork into a conditioned space (generally something I like), we need to consider the costs and effectiveness of doing so.

If your insulation is under the roof and at the gable end walls then cooling the attic space will be far less costly and much easier than if the insulation is in the attic floor. (You can of course add insulation).

Another consideration is that while "hot roof" designs can work well in buildings, I prefer that the roof covering be absolutely the most-bullet-proof possible. That's why I like a standing seam metal roof if slope permits, or a high quality membrane roof otherwise.

Shingles on a low slope roof are leak prone. Leaks into a hot roof enclosed cavity tend to go un-discovered for long enough that very serious rot and mold problems can develop.

Also see our articles on this topic beginning at HOT UN-VENTED ROOF DESIGN SOLUTIONS

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Continue reading at HOT ROOFS vs VENTING in COLD CLIMATES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• CATHEDRAL CEILING VENTILATION
• CAPE COD ROOF & KNEE WALL INSULATION & VENTILATION
• CATHEDRAL CEILING INSULATION
• HOT ROOF DESIGN PROBLEMS
• HOT UN-VENTED ROOF DESIGN SOLUTIONS
• HOT ROOFS in HOT HUMID CLIMATES
• HOT ROOFS vs VENTING in COLD CLIMATES
• HOT ROOF DESIGN vs LEAK RISKS
• HARD to VENT ROOF SOLUTIONS for HIPS & CATHEDRAL CEILINGS
• PROBLEMS with PARTIAL ROOF VENTILATION
• ROOF FRAMING TIES & BEAMS proper framing of a cathedral ceiling
• ROOF NOISE TRANSMISSION
• ROOF VENTILATION SPECIFICATIONS pros and cons of hot roof designs

Suggested citation for this web page

HOT ROOFS in HOT HUMID CLIMATES at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING VENTILATION

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