Thermal Mass in Hot Humid Climates

This is a visual of the effect thermal mass has on your home.

This is a visual of the effect thermal mass has on your home.

  Here in College Station and Bryan we have a very hot and humid climate, which presents a unique set of design and construction challenges.  For one, traditional approaches to thermal mass walls are out for us. However, this potentially beautiful design approach is not necessarily a bad idea; if we modify its application from what is traditional.

Thermal mass is thick, highly dense material that includes stone, adobe and structural brick.  The idea is that through the hot day, heat does not penetrate the full depth of the thermal mass. Then, at night the outside temperature cools down enough to pull the heat out of the wall so that the process can start over the next day.  Yet, because our humidity is high, we do not cool down enough at night to pull the heat out of the wall. Consequently, over time the wall serves to heat the building.

It is odd that brick is the preferred veneer through most of the hot and humid gulf coast south.  Even though this is not usually structural brick, there is an air space between the brick and the insulated wall. Nonetheless, the brick is a very affective solar collector that creates a hot surface next to the house.

On my own house that was built in 1950 and once had a 100% brick veneer, I removed all of the brick that was serving to heat my home. I replaced the brick with a second insulated wall and sided it with fiber cement siding.   In doing this, I was able to more than double the insulation, create a better air seal and reduce the heat next to the house.  I also happen to think that it looks a lot better.

Okay, so now that we understand the process of eliminating thermal mass, how can we use it to positively affect the Brazos Valley? As we pointed out, thermal mass works well in hot, dry climates that cool down at night.  In a hot, humid climate we can bring the thermal mass inside, away from the source of heat; and have it work for us.  When this is done, the mass assumes the ambient inside temperature.  Consequently, as temperatures rise through the hot part of the day, it helps to reduce the cooling load required to keep the house comfortable.  It does this by absorbing some of the heat in the air.  Although the same amount of energy is needed throughout the day, there is less of a peak load required for the hottest part of the day; which allows us to reduce the size of equipment needed to keep the house cool.

Unfortunately, more research is needed to quantify this effect.  Currently, there is nothing in the program that calculates equipment size that takes this into account.  Good air conditioning companies are reluctant to deviate from the standard sizing of equipment.  We love our AC mechanic and have a huge amount of trust in him. However, we frequently push him to reduce equipment size because we design in ways that reduce load but that are not considered in load calculations.

Next time I will talk about using interior thermal mass in home design.