Convective Loops, Air Leaks & Heat Loss Analysis for buildings InspectAPedia® -
How to find & diagnose convection loop heat losses in buildings
Heat loss due to convection loops in fire walls, party walls, & interior partition walls
Heat loss due to convective loops in bathrooms & plumbing chaseways
Heat loss due to cold air leaks into return ducts routed through building framing chaseways
How convection loops pump heat out of a building even with no air leaks
Where convection loops most often occur in buildings
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This article series discusses how to find and correct convective loop heat losses in buildings, a step in how to find points of heat loss and air leaks in buildings using a variety of tools and inspection methods including infra red, smoke tests, visual inspection, and tests. This detailed article accompanies a building weatherization and energy-savings company through a detailed building inspection for heat loss points and air leaks. The author accompanied Princeton Energy Partners as they used the blower door, thermal imaging, smoke guns, and visual inspection to pinpoint building air leaks, convective loops, heat loss points, air infiltration and air exfiltration on a building. Readers should also see AIR BYPASS LEAKS as well as HEAT LOSS in buildings and HEAT LOSS INDICATORS.
Our page top photo shows an uninsulated interior partition wall forming a convective heat loop as well as a thermal bypass loop in a building. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
How to Find Points of Convective Heat Loop - Heat Loss in buildings
This article series explains how to survey a building for air and heat loss or gain points and how to correct them.
As we introduced above, in this article the author, Steven Bliss, accompanies a building weatherization and energy-savings company through a detailed building inspection for heat loss points, convective loops, and air leaks. The author accompanies Princeton Energy Partners as they use thermal imaging, smoke guns, and visual inspection to pinpoint building air leaks, heat loss points, air infiltration and air exfiltration on a building. The importance of setting priorities for sealing these points of energy wasted is emphasized and discussed, and sketches as well as photographs of common points of building heat loss, or unwanted heat gain, and air leaks are provided.
The text below paraphrases, quotes-from, updates, and comments an original article, "House Doctors with Better Medicine, Princeton Energy Partners use the latest diagnostic tools to comb a house for the major causes of heat loss. Their findings are often astonishing. Their strong prescriptions bring results', Steven Bliss, (see links just above) from Solar Age Magazine and written by Steven Bliss.
Thermal Bypasses, Air Leaks, & Convective Heat Loops in Building Partitions
The energy contractors assert that their work consistently saves the homeowner considerably more money than is predicted by the reductions in air infiltration alone. This seeming magic is attributed to the elimination of "thermal bypasses", that is, the myriad ways heat gets out of the building other than through wall and ceiling components following the rules and regulations of ASHRAE.
Prominent among these heat leaks are convective loops - wall and ceiling cavities that act as room-sized heat exchangers, relentlessly pumping heat out of a building even if there is no direct air leakage from indoors to outdoors.
The convective loop shown in the sketch at left depicts heat movement in an interior hollow core masonry block partition wall in a building. In many cases PEP (the energy consulting and weatherizing firm in the original article) finds interior partitions colder than outside walls.
Since the cooling effect of a convective loop is only partly due to air leakage, even a completely closed wall cavity such as a (hollow core block) masonry party wall can cause problems.
In multi-unit dwellings energy retrofitters often encounter concrete-block party walls between adjoining living spaces and extending into the building attic. Warm air rises through the masonry block hollow cores, is cooled in the un-heated attic, and falls again.
Regardless of whether or not the block wall is capped with solid blocks in the attic (or outside above the rooftop), this convective heat loop chills the party wall and robs heat from the building. A related problem might be a reduction in building wall R-values if air flows through fiberglass insulation. While this phenomenon has been reported many times in the research literature, it is generally overlooked since it defies accurate prediction.
One cure in the case of a masonry party wall is a solid course of block at the ceiling level at each floor level, isolating the airflow within the heated space. Of course this is most easily accomplished during new construction. Sealing a block party wall against convection loops as a retrofit is a challenge and may be costly. An example might include drilling every block at ceiling level to inject fire-block-rated foam.
Convective loop heat losses occur in buildings within uninsulated, wood-framed or metal stud interior partition walls, not just in hollow core masonry walls, as we discuss below at Convection Loops at Interior Partition Walls.
Stacked Bathrooms & Plumbing Chases & Return Ducts Run Through Framing Form Convective Loops
In multifamily and even single family buildings it is common for bathrooms to be "stacked" one on top of the other, forming a giant convective loop.
The sketch at left shows warm air and heat leaking into the ceiling around a bathroom light and exhaust fan (the red arrows).
The blue arrows in the sketch depict cold air falling from an un-heated attic into the heating cool-air return duct, leaking through wire holes and gaps in framing, allowing un-wanted cold air to be drawn into the heating system duct work.
Also, since this is a bathroom wall, moist air is likely to be drawn into the framing as well. The solution is to seal off the duct space (run through framing members in this building) so that it functions properly.
Convective Loop Heat Losses at Kitchen Cabinets & Soffits
As we cited earlier at Living Space Heat Loss, air leaks through kitchen cabinets and interior soffits can be very significant heat loss points in buildings. Soffited ceilings are popular in kitchens and baths, and make a handy home for recessed lights. But improper insulation installation or openings cut for recessed lights, wiring, or plumbing can permit air leaks into the ceiling cavity or directly into the attic. Such leaks can be extensive.
An infrared scan or invasive inspection can identify this energy loss location.
Sealing the connection between the cabinet soffit and the building's exterior wall reduced building air leakage by 20 percent at a townhouse investigated by the experts in the original article given here.
The sketch at left demonstrates how a kitchen soffit can become a heat pump, moving warm air (red arrows) and heat into a cold attic space while bringing cold air down into the living area.
A small soffit over a kitchen cabinet may not look like much until you discover that it is drawing air from all kinds of wall and ceiling spaces.
Convection Loops at Interior Partition Walls & Party Walls or Fire Walls
As our photo at the top of this page shows an open, uninsulated interior partition wall top, viewed from a building attic. In an older wood frame building an interior partition wall such as this one may extend from the attic down through two floors in the building, finding a stop only at a sill plate or "shoe" where the partition wall begins on the first floor.
Our photo of a fire-rated drywall party wall (left) shown extending into the attic of a multi-unit dwelling, describes a tall, often uninsulated interior partition wall that can form a convective heating loop. Party walls between building units are required to slow the spread of fire from one building section to another.
(A catastrophic fire occurred in an older section of the Fox Hill condominiums in Poughkeepsie NY (ca 1980) as fire spread rapidly through an attic where these fire barriers had been omitted.) The party wall either extends through and above the roof, or the roof sheathing may be protected (typically for four feet) on either side of the party wall using fire retardant treated plywood or fire-rated drywall covering.
The hollow chimney created by an interior partition wall, regardless of whether or not it extends up through the attic, forms a heat pump, moving building warmth into the cold attic during the heating season.
Simply sealing the wall top in the attic will help alot, as would fire-blocking (now required in many jurisdictions) at each floor and ceiling level in such a wall. Fire-blocking retrofit that can also interrupt this thermal convection loop may be permitted in your jurisdiction using a fire-rated foam - check with your local building department.
When we observe thermal tracking stains on the upper section of an interior partition wall, one possible explanation is that convective loops within the wall, even if it is not leaking badly into the attic, are cooling the upper part of the wall. See THERMAL TRACKING & HEAT LOSS for details.
Estimating convective loop heat losses
In addition to their effect on energy bills, the impact of convective loops in buildings can be gauged by measuring attic temperatures before and after sealing off thermal bypasses such as chases and wall partitions, or it can be roughly modeled from the temperature difference and the height of the convective loop.
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Solar Age Magazine was the official publication of the American Solar Energy Society. The contemporary solar energy magazine associated with the Society is Solar Today. "Established in 1954, the nonprofit American Solar Energy Society (ASES) is the nation's leading association of solar professionals & advocates. Our mission is to inspire an era of energy innovation and speed the transition to a sustainable energy economy. We advance education, research and policy. Leading for more than 50 years.
ASES leads national efforts to increase the use of solar energy, energy efficiency and other sustainable technologies in the U.S. We publish the award-winning SOLAR TODAY magazine, organize and present the ASES National Solar Conference and lead the ASES National Solar Tour – the largest grassroots solar event in the world."
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
Excerpts with updates and annotations expanding the original Best Practices Guide text can be found in the online review and book summary at BEST CONSTRUCTION PRACTICES GUIDE and also at DECK & PORCH CONSTRUCTION, at INDOOR AIR QUALITY IMPROVEMENT GUIDE, and in other articles found at InspectAPedia.com such as HOUSEWRAP AIR & VAPOR BARRIERS, SOUND CONTROL in buildings, and other topics.
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