Ice dam leak stains (C) D Friedman Hot Roof Designs for Hot Humid Climates
Moisture, Venting & Cooling Effects

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Hot roof designs, aka "dense-packed" insulated sloped roofs for warm 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.

Green links show where you are. © Copyright 2017, All Rights Reserved.

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

Penicillium mold in cape cod attic (C) D FriedmanOur 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.

Ice dam leak stains (C) D FriedmanAnd 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.


Continue reading at HOT ROOFS vs VENTING in COLD CLIMATES 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

Suggested citation for this web page

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


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman