Insulating Heated Garage Plenums in Multifamily Buildings

In cold climates, the freezing of pipes that run through garage spaces is a common concern. Occupant comfort in dwelling units directly above the garage is also a concern because the slab between the garage and the conditioned space above can get cold even with adequate insulation. Heated recirculation plenum spaces can be installed in the garage to keep pipes warm and heat the underside of the slab between the garage and the conditioned space above. 

Heated recirculation plenum spaces in garages are insulated dropped ceiling spaces that are located between the garage and conditioned space above, and are equipped with plenum heaters that provide both heat and air circulation to the plenum space. The plenum heaters are typically equipped with controls and will shut down when the temperature inside the garage plenum space exceeds 40°F.

Unlike supply and return plenum spaces, recirculation plenum spaces do not transport air that is supplied to or exhausted from spaces; instead, air is recirculated within the plenum space. As a result, there is usually little to no pressure difference between the plenum space and adjacent spaces, leading to limited or insignificant air leakage into and out of the plenum space. However, it should be noted that it is not recommended to install heated garage plenums in IECC Climate Zones 3 and lower due to concerns of upward vapor drive during cooling seasons. 

In buildings with heated garage plenums, the floor between the garage and conditioned occupied space above must be insulated per IECC requirements. 

Note that with both approaches, the following should also be ensured: 

• The sides of the plenum space are insulated, and an air barrier is fully aligned with the insulation along the sides. 
• Any service penetrations through the garage slab should be air sealed. 
• There is an opening to access the interior of the plenum space for any necessary repairs or maintenance. This can be achieved using lay-in ceiling tiles, which are typically insulated with fiberglass or mineral wool batt insulation (Figure 3). 

There are two approaches to meeting ENERGY STAR Multifamily New Construction requirements for heated garage plenums. 

• Insulate the bottom of the plenum space (Figure 1). In this case, the insulation at the bottom of the plenum must meet or exceed IECC insulation requirements and should be fully aligned with an air barrier. 
• Insulate both the bottom and the top of the plenum space (Figure 2). In this case, the insulation at the top of the plenum space must meet or exceed IECC insulation requirements and should be fully aligned with an air barrier. With this approach, ENERGY STAR Multifamily New Construction requires that the insulation at the bottom of the plenum space is R-13 or greater. 

This heated garage plenum is insulated along the bottom and sides, and the insulation is fully aligned with an air barrier. The insulation at the bottom of the plenum space meets IECC insulation requirements.

View Image Download

Source

Courtesy Of

BSC

This heated garage plenum space is insulated along its top, bottom and sides, and the insulation at the top of the plenum meets IECC requirements. The insulation along the top and sides is fully aligned with an air barrier.

View Image Download

Source

Courtesy Of

BSC

Drop-in ceiling assemblies consisting of suspended ceiling tiles (common in multifamily buildings) are often insulated with fiberglass or mineral wool batt insulation.

View Image Download

Source

Courtesy Of

BSC

Air gaps within walls are a concern due to the large vertical height that can lead to significant convection forces. However, airflow is generally not a concern within garage plenums, as the convection forces are small due to a limited vertical distance within the dropped ceiling (Figure 4).

Airflow within drop-in ceiling assemblies is generally not a concern due to the limited vertical height. However, airflow within air gaps in walls is a concern due to the much larger vertical height.

View Image Download

Source

Courtesy Of

BSC

In cases where a garage plenum space is adjacent to a separately cast garage slab without a plenum space, adding rigid insulation as a thermal break between the garage slabs can reduce thermal bridging and energy loss (Figure 5). For more information, see the guide on Reducing Thermal Bridging in Mass and Steel Framed Multifamily Buildings. 

Rigid insulation can be used as a thermal break between a garage slab with a heated plenum space and a separately cast garage slab without a heated plenum space.

View Image Download

Source

Courtesy Of

BSC