I have a proposed underfloor air distribution system (UFAD) for a one level office in a temperate climate. Typically the demand for heating and cooling in this climate is about even for offices. However the building is well insulated and has a fair amount of (shaded) low U-value glazing on the sun facing elevation.

Now the question boils down to whether the thermal mass of the slab will be beneficial overall or not. At the moment I tend to have insulation within the plenum, i.e. excluding the impact of the thermal mass to create a more controlled supply, instead of having the insulation under the slab.

Based on some handwavy assumptions though, I could see some benefits of including the mass of the slab in order to reduce cooling loads (which I expect to be somewhat dominant due to the glazing and amounts of insulation).

However what would happen once the slab is fully heated, i.e. runs out of capacity? Would I night purge it? Can someone share their thoughts/experiences? When I insulate the plenum, should I include or exclude the slab?


1 Answer 1


It depends — can you run air through the plenum without simultaneously running the active heating/cooling system? If not, you'll never get the benefit of its thermal storage capacity.

For example, when heating, the plenum will always be hotter than the slab, putting heat into it. In order to extract heat from the slab, you need to be able to put building air that's cooler than the slab directly into the plenum. Yes, there will be some transfer of heat directly from the slab to the rooms through the floor, but that would only occur near the plenum.

The same thing applies, with the heat flow reversed, when cooling.

There are secondary issues, too. The transient response of the system (i.e., the ability to change the temperature of the building interior rapidly) will be slowed down by the thermal mass of the slab if the plenum is not insulated. This could be a problem in some situations.

Also, when cooling, there could be condensation issues in the plenum. With active cooling, excess moisture in the air is removed at the expansion coils, and the system is designed to handle it. However, when doing passive cooling from the slab, the condensation will occur inside the plenum, which could cause maintenance and reliability issues.

  • $\begingroup$ I can run untreated air through the plenum and the idea was to capture some of the cooling benefits by keeping track of the supply temperature. The transient response was something we considered. Thanks though for pointing out condensation, which definitely could become quite problematic in this scenario. $\endgroup$
    – pandita
    Feb 9, 2015 at 10:45

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