ASHRAE Journal - October 2012 - 70

building sciences
DP Attic Insulation Bathtub

Crawlspace Insulation Bathtub

Figure 10: Wind. Air blowing into the bathtub from the top or bottom respectively is not typically an issue due to the absence of wind in attics and crawlspaces and due to the density of the “fluffy stuff.” Yes, attics and crawlspaces are vented and there is airflow, but they are sheltered from the wind. Get a grip here folks. Note that we are talking about assemblies sheltered from the wind, attics, crawlspaces and garages, and assemblies where air cannot enter at perimeters. I remind everyone again that the sides of the bathtub are sealed. We are not talking about a cantilevered floor exposed to the wind with a perforated vented soffit. Let’s examine two bathtubs both filled with insulation: an “attic” bathtub and a “crawlspace” bathtub (Figure 9). From a code perspective and an EPA ENERGY STAR/LEED perspective we are all “tickety boo.”# Air can’t get into the insulation from the sides or through the bottom in an attic due to the bottom of the bathtub and the sides of the bathtub, or through the top or sides when the bathtub is inverted in the crawlspace case. One issue to deal with is air blowing into the bathtub from the top or bottom respectively (Figure 10). This is not typically an issue due to the absence of wind in attics and crawlspaces and due to the density of the “fluffy stuff.” Yes, attics and crawlspaces are vented, and there is airflow, but they are sheltered from the wind. Get a grip here folks. Note that we are talking about assemblies sheltered from the wind, attics, crawlspaces and garages, and assemblies where air cannot enter at perimeters. I remind everyone again that the sides of the bathtub are sealed. We are not talking about a cantilevered floor exposed to the wind with a perforated vented soffit. So what happens when there is an air space between the insulation and the bottom or top of the top, respectively (Figure 11)? Not a heck of a lot because there is, for all practical purposes, no air pressure difference across the insulation. This is not a wall. No stack effect. No real convection. You could get some convection in ceiling insulation or floor insulation if there is a huge temperature difference, and the insulation is of real low density. But let me remind you once again that there is a big difference

Figure 11: Air space. Not a lot happens when there is an air space between the insulation and the bottom or top of the top respectively because there is, for all practical purposes, no air pressure difference across the insulation—the “blue” DP. This is not a wall. No stack effect. No real convection. The big pressure difference is the “red” DP. between a wall and a ceiling or floor because the stack pressure depends not only on the temperature difference but also on the height of the cavity (Figure 12). Wall tall. Floor short. Ceiling short. Lie the wall down and the stack effect goes away. You still have to worry about air blowing into the stuff from the top or bottom (Figure 13). But this issue is not made worse in any practical sense with the air space. It is the same problem as in Figure 10, alignment or not. The flow path resistance in Figure 13 can be increased

Figure 12: Wall vs. Ceiling/Floor. There is a big difference between a wall and a ceiling or floor because the stack pressure depends not only on the temperature difference but also on the height of the cavity. Wall tall. Ceiling and floor short. Lie the wall down and the stack effect goes away.

Figure 13: Wind Redux. You still have to worry about air blowing into the stuff from the top or bottom. But this issue is not made worse in any practical sense with the air space. It is the same problem as in Figure 10, alignment or not. The flow path resistance can be increased significantly if the fluffy stuff is batt insulation that is faced on one side, such as a kraft-faced batt insulation.

# Canadian expression meaning “OK.” Canadians picked it up from the Brits in the RAF when Canucks were on the other side of the pond fighting the Hun. The Brits brought it from India. According to Michael Quinion, the British Army in India used the Hindi phrase “tikai babu,” which means “it’s alright, sir.” www. worldwidewords.org/qa/qa-tic1.htm.

ASHRAE Journal

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October 2012

ASHRAE Journal - October 2012

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