Helpful? 1

Thermal Mass in Hot Humid Climates

I am looking for information on thermal mass in hot humid climates as it relates to two different questions. First, is the foam block in ICFs enough to keep the concrete core from heating up when nocturnal temperatures do not dip below that of inside temperatures for three months?

Second, dose anyone know of research done on the efficacy of placing thermal mass inside conditioned spaces as a means of moderating AC loads?

Asked by Hugh Stearns
Posted Mon, 10/18/2010 - 09:00

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15 Answers

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1.
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I've heard that thermal mass (which acts like a thermal flywheel to store thermal 'momentum') inside the insulation envelope and is always a good thing, particularly if it can be achieved cheaply....If that entails a slab on grade you have to worry some about foam kept 8" above grade due to termite infestation...turns out the little critters love an insulated home as much as we do. You could also protect the foam with a fine stainless steel mesh and/or use borate treated EPS foam like that available on Polysteel ICFs. I've also heard that in hot climates it is very moderating to keep insulation (masonry, concrete or adobe) on the exterior of the insulation envelope.

Answered by D. Duffe
Posted Mon, 10/18/2010 - 09:37

2.
Helpful? 0

oops!! I meant to say 'thermal mass' whereIi wrote 'insulation' in the last sentence.

Answered by D. Duffe
Posted Mon, 10/18/2010 - 09:38

3.
Helpful? 0

Hugh,

Where are you if night-time temperatures remain above room temperature for three consecutive months?

But the answer to your first question is NO. Not only will the small amount of external insulation in ICFs not inhibit the concrete cores from equilibrating with ambient temperatures, but the interior insulation will almost eliminate any potential value of the thermal mass. ICFs are among the worst of all insulated concrete wall systems in terms of thermal mass advantage.

Placing thermal mass inside air conditioned spaces will do nothing except increase the thermal inertia and make it difficult to change interior temperature if any set-back is used.

Exterior thermal mass walls are most effective in climates like the US desert southwest, in which ambient temperatures swing above and below indoor temperatures on a daily cycle. This provides a thermal dampening and a thermal lag - it both reduces and delays impacts on interior conditions. If the mass is thick enough, before the daytime heat can penetrate, the nighttime cool begins to reverse any thermal accumulation in the walls.

For hot climates, however, exterior wall mass is most effective if all the insulation is on the exterior or midline to the wall, as in the ThermoMass system. Such walls have almost double the dynamic mass benefit of ICF walls in climates such as Miami and Phoenix.

Answered by Riversong
Posted Mon, 10/18/2010 - 09:40

4.
Helpful? 0

I'm in College Station , Texas. While the average lows show are in the mid to low 70's through the three months that I am speaking of, we frequently go through this period with the lows in the upper 70's.

I think that increased thermal inertia my be the impact that I am wondering about. Can this lower the AC tonnage necessary by essentially spreading the the cooling load over a longer period of time? Basically reducing peak demand, which are what AC loads are based on. I took a look at the ThermoMass system. It makes sense to create a thermal break in the concrete. But this also leaves you with exposed thermal mass to heat up all day long. Will this created enough pressure to "jump" the thermal break?

Answered by Hugh Stearns
Posted Mon, 10/18/2010 - 13:07

5.
Helpful? 0

Hugh, it's possible that your idea is accurate and that you could reduce peak demand by adding interior thermal mass. You could also adjust the AC such that interior temperatures drop at night, drawing heat out of the mass, and then let the temperature rise during the day. It's basically the reverse of thermostat setbacks in a heating climate, where the system is inactive or less active when you're asleep. There's also a more complex strategy called the ice battery, where energy is used during off-peak hours to allow reduced power demand for daytime AC.

However this still isn't necessarily a good idea. As Riversong states the real value of massive exterior walls is in desert climates with a wide diurnal swing, including chilly nights in the summer. This way the heat of the day never gets all the way through the wall before the sun sets and it starts going back out to the exterior. Chances are you'd be better off using those inches of wall thickness for extra R-value, because the heat is always flowing in the same direction (outside-in).

Your home could also be a good candidate for solar power and/or water heating. PV works in a cooling climate because peak demand corresponds to peak supply. And using gas to heat water when your roof is searing hot seems wasteful.

Answered by TJ Elder
Posted Mon, 10/18/2010 - 21:55

6.
Helpful? 1

I would be leary of excess interior thermal mass. The most effective place for thermal mass in a hot climate is in the exterior walls. Concrete walls with insulation either on the outside or mid-wall are both very effective at reducing AC costs in climates such as Miami FL and Bakersfield CA. Your summer climate is not that different.

The best way to reduce costs is to raise the thermostat setting.

Answered by Riversong
Posted Mon, 10/18/2010 - 22:14

7.
Helpful? 0

Thomas Jefferson nailed it: "Chances are you'd be better off using those inches of wall thickness for extra R-value, because the heat is always flowing in the same direction (outside-in)."

Answered by Martin Holladay, GBA Advisor
Posted Tue, 10/19/2010 - 02:55

8.
Helpful? 1

Hugh,
I live near Dallas (Hot Humid/Mixed Humid)
I agree with Thomas Jefferson (and Martin).

The "conventional Texas wisdom" has been that we do not "need" high R-value(especially in walls).

After building my "half-assive" two years ago...I realized that "conventional Texas wisdom" was wrong.

I believe that "super insulation" works in Hot Humid climates.

Answered by John Brooks
Posted Tue, 10/19/2010 - 06:17

9.
Helpful? -1

I meant to say "Half-Assive House"
my house is HERS 51

Answered by John Brooks
Posted Tue, 10/19/2010 - 06:31

10.
Helpful? 0

No amount of thermal mass will store free ambient 'coolth' over a three-month period and if it did you'd have condensation problems. In a hot humid climate much of your a/c energy is devoted to dehumidification, so impeccable air-sealing is vastly more important. Wall systems that include mass precast concrete as RR suggested are probably helpful with air-sealing and may have a small contribution to make in the diurnal cycle by taking advantage of off-peak power - but at what cost?

As several commenters have already advised, high thermal mass structure is well-suited for those hot dry climates which typically have much colder nights - elsewhere not so much. It always pays to look at those construction traditions for a particular area which date from our low-energy past. Traditional buildings in hot humid climates almost invariably rely on ultra-low thermal mass for fast overnight relief from the heat, big roof overhangs to shade the walls and keep the rain out, and all the ventilation you can get. Adding A/C to a well-insulated, well-sealed enclosure is a boon in the very hottest months but the traditional strategies will still work very well for you in the shoulder seasons. Adding thermal mass will provide no benefit and the traditional knowledge of centuries suggests it could be deleterious.

Answered by James Morgan
Posted Tue, 10/19/2010 - 07:49

11.
Helpful? 1

This is great information. Thanks to all for helping me think about this. Does anyone know of systematic studies looking at the use of thermal mass in hot humid climates? In our office we have had the pleasure of employing design interns from tropical countries where AC is not typical in residential construction. Concrete block is often used, which seems counter-intuitive. On my own house I recently removed much of the brick veneer under the assumption that it was working as a large solar collector. On the brick ledge I built a second insulated wall. Unfortunately, from a data collection stand point, I also did a lot of other things that will have impacted the effectiveness of the home's envelope, so I am not really going to know the full impact of the reduced heat load and added insulation.

The ICF project on which we are just starting design provides a great opportunity to raise more questions on thermal mass. This issue of how to place the thermal break seems big. I'm guessing that a system like ThermoMass is more expensive to place. With more exact information on the thermodynamics in this climate one could begin to think about ROI. Unfortunately that seems to be an issue for my clients :-)

Answered by Hugh Stearns
Posted Tue, 10/19/2010 - 08:02

12.
Helpful? 0

"Concrete block is often used, which seems counter-intuitive."
Yes, but it's cheap and available, and fire and termite-proof.

Answered by James Morgan
Posted Tue, 10/19/2010 - 09:03

13.
Helpful? 0

This issue of thermal mass within the insulated envelope vs outside the insulated envelope is misunderstood. There is nothing about thermal mass that stops the insulation from working. There is nothing about thermal mass that causes more of the sun's heat to get into your house. If you put the thermal mass on the outside, the insulation will be working with an "external" environment that swings from maybe 85 degrees in the late afternoon to maybe 65 degrees in the morning. These temps are fairly easy to insulate against. On the other hand, with the mass on the outside there is little mass inside the insulation and so the small amounts of heat transfer will still cause some room temperature fluctuation.

If the mass is inside, then the insulation may see external temps of maybe 115 degrees to 35 degrees, causing much higher rates of heat flow into and out of the living space. Countering this is the inside thermal mass that is available to absorb the heat flow without much change in temperature. In some respects it is a bit of a wash whether the thermal mass is inside or outside. I know I am leaving something out because it is widely considered better to have the mass inside but my point is that it is only a matter of degree. Having thermal mass outside the insulation still helps some.

Also thermal mass is not insulation. With lots of thermal mass and no insulation, if the temp is 50 during the day and 0 at night, your house will eventually be hovering around 25 unless you have built in a truly staggering amount of mass. Believe it or not it is possible to do so. It is called passive annual heat storage (PAHS). Basically you cover a small hill with insulation and waterproof pond membrane. You can plant stuff on top if you like. You build an underground house within this hill. You also build in some "earth tubes" which are PVC tubes that allow air circulation to warm the dirt of the hill during the summer and cool it during the winter. Other solar design features are employed such as south facing glass and overhangs. In a perfect world your house would be 75 degrees year round. What you really get is a house that is 68 degrees in the winter and 82 degress in the summer. Whether this is success is subjective. You could easily survive both summer and winter with no utility bills so perhaps it is a resounding success. If however you can't stand 68 in the winter you are totally screwed because the massive amount of thermal mass would soak up the heat from your wood stove like it was never there so maybe it is a total failure.

Answered by Anonymous
Posted Sun, 12/05/2010 - 15:09

14.
Helpful? 0

This issue of thermal mass within the insulated envelope vs outside the insulated envelope is misunderstood. There is nothing about thermal mass that stops the insulation from working. There is nothing about thermal mass that causes more of the sun's heat to get into your house. If you put the thermal mass on the outside, the insulation will be working with an "external" environment that swings from maybe 85 degrees in the late afternoon to maybe 65 degrees in the morning. These temps are fairly easy to insulate against. On the other hand, with the mass on the outside there is little mass inside the insulation and so the small amounts of heat transfer will still cause some room temperature fluctuation.

If the mass is inside, then the insulation may see external temps of maybe 115 degrees to 35 degrees, causing much higher rates of heat flow into and out of the living space. Countering this is the inside thermal mass that is available to absorb the heat flow without much change in temperature. In some respects it is a bit of a wash whether the thermal mass is inside or outside. I know I am leaving something out because it is widely considered better to have the mass inside but my point is that it is only a matter of degree. Having thermal mass outside the insulation still helps some.

Also thermal mass is not insulation. With lots of thermal mass and no insulation, if the temp is 50 during the day and 0 at night, your house will eventually be hovering around 25 unless you have built in a truly staggering amount of mass. Believe it or not it is possible to do so. It is called passive annual heat storage (PAHS). Basically you cover a small hill with insulation and waterproof pond membrane. You can plant stuff on top if you like. You build an underground house within this hill. You also build in some "earth tubes" which are PVC tubes that allow air circulation to warm the dirt of the hill during the summer and cool it during the winter. Other solar design features are employed such as south facing glass and overhangs. In a perfect world your house would be 75 degrees year round. What you really get is a house that is 68 degrees in the winter and 82 degress in the summer. Whether this is success is subjective. You could easily survive both summer and winter with no utility bills so perhaps it is a resounding success. If however you can't stand 68 in the winter you are totally screwed because the massive amount of thermal mass would soak up the heat from your wood stove like it was never there so maybe it is a total failure.

Answered by Anonymous
Posted Sun, 12/05/2010 - 15:09

15.
Helpful? 0

Anonymous,

It's apparent why you wouldn't want to put your name to your statements because they are patently false and based on a complete misunderstanding of the importance of thermal mass location in an insulated structure.

You admit you are "leaving something out because it is widely considered better to have the mass inside". What you are leaving out is all the research that's been done on the relative benefits of exterior versus midline vs interior thermal mass in various climate zones, as well as a fundamental understanding of the principles of heat flux. And your temperature profiles are irrelevant without a climatic context (and largely mythical).

Answered by Riversong
Posted Sun, 12/05/2010 - 15:47

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