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
• Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices

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.

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.

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.

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.

Contact Daniel Friedman for website content suggestions or for fee-paid consulting