4" was a direct quote.

"On" is what I meant by "running". If you condition the interior to a fixed temperature, interior thermal mass doesn't help you.

'4" EPS on walls' would have been a direct quote. Wall does not equal roof.

Also, thermal mass only makes sense for constant interior temperatures, especially if the outside temperature is fluctuating quickly.

I don't understand why you think that thermal mass doesn't help maintain constant room temperature? That's exactly why you want mass.

Posted By onesojourner on 22 Jan 2013 09:05 AM
The construction industry is afraid of anything new. I think it really comes down to laziness.

Not necessarily. When I was getting ready to build my ICF house I spent I don't know how many hours learning about thermal efficiency, window ratings, various heat systems and how they compare, proper ventilation, good insulation practices, the differences between various foams, and on and on. I had retired not too long before that so I had the time to do all that learning. I got to too thinking that it's no wonder contractors are sometime quite slow to change. It takes time to learn new methods and practices and approaches. When you're busting your rear using your time to produce income, it can be hard to find the time to learn. I don't rack it up to laziness but rather to ignorance of the importance of keeping up with the times.

Obviously, the contractor who doesn't keep up with the times will eventually lose out. But keeping up can be hard. So, instead of being hard on them, let's empathize with them as we watch them fade away from the construction scene.

Posted By galore on 25 Jan 2013 04:25 PM
'4" EPS on walls' would have been a direct quote. Wall does not equal roof.

Also, thermal mass only makes sense for constant interior temperatures, especially if the outside temperature is fluctuating quickly.

I don't understand why you think that thermal mass doesn't help maintain constant room temperature? That's exactly why you want mass.

You are correct, thermal mass DOES HELP maintain constant room temperature. I think he misspoke.

If you use the HVAC system to create and hold a specific temperature, then the interior thermal mass isn't absorbing or releasing any energy. Interior thermal mass only helps when you allow the temperature to vary (albeit less than the outdoor temperature). This is well established basic thermodynamics.

Posted By jonr on 25 Jan 2013 09:30 PM
If you use the HVAC system to create and hold a specific temperature, then the interior thermal mass isn't absorbing or releasing any energy. Interior thermal mass only helps when you allow the temperature to vary (albeit less than the outdoor temperature). This is well established basic thermodynamics.

Not quite the way you have it. The internal thermal mass comes into play, not when you vary the indoor temperature, but rather when you vary the heating or cooling input into the space. If, for whatever reason, you want to be able to allow the indoor temp to swing up and down the interior thermal mass works against you. I found that out fairly soon after moving into my ICF house. The interior mass, coupled with a heat pump for heating, caused the house to take up to four hours to heat up by 5° in the winter. During heating season we keep the temp constant.

This past summer I switched to time of day metering so cut the AC back in the daytime. Because of the mass of the ICF walls and the concrete rat slab in the crawl space the house would rise from 72°F at noon to about, at most, 76° at 9 PM.

A few days ago we had a power outage from 5 AM to 8 AM with the outdoor temp being about -5°F. The house dropped from 72° to 69°.

Wrong (the last jonr post).

The thermal mass is buffering the temperature thus making the HVAC work less to maintain a constant temperature. Because if you would not have mass and the outside temperature changes drastically, any heat transfer through the insulation would go directly to the interior, thus making the HVAC kick in. With thermal mass, this heat transfer goes into/comes from the mass.

For example, if it is cold at night and you have lots of thermal mass, the heat transfer to the outside through the insulation will get its energy from the thermal mass (which if very high will release quite a bit of energy but not lose a lot of temperature because it has all that energy stored in its mass), not the furnace. Likewise, if it's hot outside the following day, the energy that was just lost during the night from the mass, will be replenished, slightly heating up the mass but not the room, so the AC doesn't have to kick in.

If you would not have that mass, the furnace would have to provide the energy flowing out during the night (which costs $$) and the AC would have to sink the heat that comes in during the day (which costs $$), should you want to have a constant interior temperature. So with thermal mass, you save $$$$ in this scenario.

And this is a correct version of basic thermodynamics.

Let's keep our thinking and terminology straight here. The thermal mass of the outside walls absolutely buffers the outdoor temp swings as you say. The thermal mass inside the house has limited value for buffering the temperature unless you are in a heating dominant climate and the interior mass captures solar heating during the day to provide heat during the night. That's why your 6" interior concrete walls, not your outside walls, are being questioned. If you keep your house at a constant temperature then your interior concrete walls have limited thermal value.

I think it is safe to say the generally accepted view here is that the thermal mass of the outside walls has most effect and value in those climate areas where the daily temperature swings are significantly above and below the desired indoor temp, like in the SW. In areas of the north, or in the deep south, where the daily temperature remains below, or above, the desired house temperature for weeks and months on end, thermal mass looses its impact.

Some of you mathematically trying to prove that solid concrete walls will not keep the heat inside. But physically there are some structures that are built with solid concrete walls in somewhere of the planet called earth. Physically I know them well they are not keeping the heat inside or outside. People insulating them with EIFS and stucco. So I am voting on ICF for DIYers.

However I have a point "big constructions companies still are not using ICF".

But next time I will try to use solid concrete slabs instead of engineered slabs (like hambro or comslab).

Posted By dmaceld on 25 Jan 2013 10:49 PM

Not quite the way you have it. The internal thermal mass comes into play, not when you vary the indoor temperature, but rather when you vary the heating or cooling input into the space. If, for whatever reason, you want to be able to allow the indoor temp to swing up and down the interior thermal mass works against you. I found that out fairly soon after moving into my ICF house. The interior mass, coupled with a heat pump for heating, caused the house to take up to four hours to heat up by 5° in the winter. During heating season we keep the temp constant.

This past summer I switched to time of day metering so cut the AC back in the daytime. Because of the mass of the ICF walls and the concrete rat slab in the crawl space the house would rise from 72°F at noon to about, at most, 76° at 9 PM.

A few days ago we had a power outage from 5 AM to 8 AM with the outdoor temp being about -5°F. The house dropped from 72° to 69°.

One thing with thermal mass during winter in climates like the SW. Sometimes during the day due to passive solar and high thermal mass, the home may "heat up" quite a bit, even to the point of seeming too hot. Some might refer to this as "overheating" but that is relative depending on the occupants of the home. A 80F interior temp during a 30F exterior temp might seem cozy and nice to some. Point being is that even at 80F peak interior temps, let's say by 5PM. Your HVAC system SHOULD NOT be programmed to turn on and try and cool down the home during winter, as that is counter productive and a waste of energy. After the sun sets, it will take a while for the home to begin to cool down. By morning the home might be down to 70F but as long as it's sunny again, the process starts all over.

For instance, in a climate like mine where in winter it can get to 55F in the daytime by 4PM and then 25F by 7AM, that is a perfect climate for thermal mass. In the summer in can get to 95F by 7PM and then see 55F by 6AM, thermal mass helps to even out these diurnal swings. During the summer one would want to open some windows at night to cool down the home.


Why do you call it a "rat slab"? Do you get pack rats burrowing underground where you are at?

We currently live in an uninsulated CMU house in FL. There are about 6 months of the year that we don't use heat or A/C. The thermal mass is a big help smoothing out the highs and lows, keeping the house reasonably comfortable unless it is hot or cold for a long spell. We are probably different than the average American, being happy between 60 and 80 degrees. In our case, the thermal mass is a big help. We are currently building an ICF house in upstate SC, and again, much of the year, the weather is what we call comfortable. In the middle of winter, or the middle of summer, the HVAC will be maintaining the interior temperature. But during the shoulder seasons, we should be able to skate by with little conditioning, and once again, I think the mass of the walls will be an effective shock absorber.

I also think that there is a small amount of 'geothermal' help, as the concrete in the walls is thermally connected to the ground through the footing. In SC, the deep soil temperature falls into a pretty comfortable range. I think that's a part of why ICF functions so well in a mild climate, and starts to lose out in a severely cold climate. This is only a guess on my part, though. Time will tell once the house is done.

Posted By dmaceld on 25 Jan 2013 11:09 PM
Let's keep our thinking and terminology straight here. The thermal mass of the outside walls absolutely buffers the outdoor temp swings as you say. The thermal mass inside the house has limited value for buffering the temperature unless you are in a heating dominant climate and the interior mass captures solar heating during the day to provide heat during the night. That's why your 6" interior concrete walls, not your outside walls, are being questioned. If you keep your house at a constant temperature then your interior concrete walls have limited thermal value.

You have it exactly backwards. The thermal mass is most effective if it's on the inside, not the outside because the MASS is what keeps things temperate. For example: You have a beverage that you want to keep cool. If you put it in a styrofoam cup, would you put the ice on the outside (thermal mass on the outside) or the inside (thermal mass on the inside). The ice keeps the beverage cold (like a cold concrete wall keeps the air in the house cold) if it's in the drink, not if it's outside the styrofoam cup. That's also the reason, why insulation on the inside (ICF) works against the mass benefits of the concrete and it's the reason why the international energy code doesn't consider thermal mass, if the insulation in on the inside ("The insulation must be at least 50% on the exterior or integral to the wall to count. Otherwise you're back in the wood frame wall insulation requirements. In other words, brick veneer or log siding don't count."). Some references: http://blog.srmi.biz/energy-saving-tips/insulation-air-sealing/high-thermal-mass-walls/ "The best way to employ thermal mass is to maximize the surface area of the thermal mass facing the interior of a home by making the interior walls massive in addition to the exterior walls." http://www.ornl.gov/sci/buildings/2012/1998%20B7%20papers/070.pdf Kossecka, E. and Kosny, J. - “Effect of Insulation and Mass Distribution in Exterior Walls on the Dynamic Thermal Performance of a Whole Buildings” - DOE, ASHRAE, ORNL Conference - Thermal Envelopes VII, Clearwater, FL - Dec. 1998. "This data shows that the most effective wall assemblies were walls with thermal mass(concrete) being in good contact with the interior of the building (Intmass and CIC). Walls where the insulation material is concentrated on the interior side (Extmass) were the worst performing wall assemblies. Wall configurations with the concrete wall core and insulation placed on both sides of the wall (ICI) performed slightly better than Extmassconfigurations."

Wow.

Jonr assumes that his house has no temperature variation whatsoever when in fact a typical Tstat set point is plus or minus 2 degrees-- potential hours that the AC isn't running in a high mass house. And there is still a diurnal swing in internal temps -- a big one in the case of a large family living in a house with poorly placed windows; small in the case of DINKS who in restaurants and send out their laundry. As DMACELD points out, thermal lag allows set-ahead strategies for summer and setback strategies for winter that narrow the temperature difference between inside and outside and save energy.

Lbear is half right. Thermal mass is a terrific strategy for AZ in summer. But if the winter amplitude is 25 to 55 degrees, heat is traveling in only one direction -- out.

Jdebree is repeating the ICF industry's Holy Grail -- earth coupling that enhances ICF performance. It was put to the test in a joint ICF/ORNLtest house near Knoxville. ORNL's conclusion: No magic chalice in sight.

One caution for the OP: The AC runs right along in the typical leaky, poorly insulated Fla home. Ramp up the insulation and reduce air inflitration to a pittance and AC may not run enough to handle humidity.

In defense of traditionalist tradesmen, without an HRV/ERV, there is a limit on a how tight a home can be and remain healthy.

Posted By galore on 26 Jan 2013 08:18 AM

Posted By dmaceld on 25 Jan 2013 11:09 PM
Let's keep our thinking and terminology straight here. The thermal mass of the outside walls absolutely buffers the outdoor temp swings as you say.

You have it exactly backwards. The thermal mass is most effective if it's on the inside, not the outside because the MASS is what keeps things temperate

Uh, if you read closely you'll see I said "outside wall", not "on the outside of the wall." I'm not saying anything at all about the insulation being on the interior or exterior surface of the outside wall. The impact of its placement is the subject of another discussion, and the comparative value of the inner vs outer thickness is not clear cut when climate is factored in. This has been the subject of many discussions here the past several years.

Posted By toddm on 26 Jan 2013 08:48 AM

In defense of traditionalist tradesmen, without an HRV/ERV, there is a limit on a how tight a home can be and remain healthy.

That's why the mantra now is, "Build it tight and ventilate it right!"

We're talking code here rather than mantra. I have to say I don't know if 2012 IRC requires an HRV or sets MINIMUM infiltration rates. I rather doubt it. Kinda like radon code around here. You need underslab ventilation but don't have to test or add a fan if necessary. (Seven of 10 homes here have radon issues.) An overzealous builder+the wrong homeowner+the wrong building site = ?

Anything inside adds to thermal mass so of course interior concrete walls count as well. If you don't understand this I suggest researching this topic a bit more.

Posted By Lbear on 26 Jan 2013 02:00 AM

Why do you call it a "rat slab"? Do you get pack rats burrowing underground where you are at?

That's the colloquial term that is used to refer to a concrete slab in a crawl space. I think it actually does come from its use in some areas to prevent rats from burrowing into the crawl space. Where, why, when, I do not know!

In my case I placed about a 2" thick concrete slab over 3/4" blue board primarily to help keep the crawl space atmosphere clean and secondarily for thermal mass. I use the crawl space as my supply plenum for heating and cooling.

Posted By galore on 26 Jan 2013 01:17 PM
Anything inside adds to thermal mass so of course interior concrete walls count as well. If you don't understand this I suggest researching this topic a bit more.

I don't say they don't count, just that they don't count enough to justify the expense of installing them for that reason. There's no question mass in the interior moderates temperature swing, but the key word is swing. If your heating/cooling system is set so that the interior temperature does not swing more than a degree or two then there is minimal benefit from the interior mass. I get a benefit from my rat slab but that's mostly because in the summer I let the temp in the house swing +/- 2° to 3° from ideal, and because I use a heat pump that has no excess capacity above the maximum heating and cooling load on the house. But I don't let the temperature swing in the winter. It's uncomfortable waiting for the house to come to temp after letting it cool down. I said earlier, the internal mass both helps and hinders the heating/cooling process.

Keep in mind the purpose of the heating/cooling system is to replace or extract heat that leaves or enters the building envelope. Thermal mass in the building envelope plays a significant part in moderating the flow of heat into and out of the envelope. The only reason it's better to have the insulation on the outside of the thermal mass is solar absorption. If the primary driver for the heat flow through the wall is air temperature, which is pretty much the case on north walls, the heat flow through the wall will be the same regardless of whether the insulation is on the outside or inside of the thermal mass.

Not true, dmaceld. Thermal mass works because of thermal lag -- the combination of high heat capacity and low conductance that slows transfer through massive walls. Ideally, the flux of heat and cold through the 24 hour cycle never reaches the inside surface, and maintains comfort insde as long as the average daily temperature is comfortable. Exterior insulation increases thermal lag obviously but it isn't required (e.g. adobe walls in the SW and rubble filled masonry walls in the Mediterranean.) By contrast, exposing mass to the interior harnesses thermal lag inside the house. Walls that are slow to heat and slow to cool buffer temperature spikes inside the home. The more variation you are willing to tolerate, the more energy you save.And I'm betting even Jonr sweats a bit in spring and shivers a bit in fall before turning the HVAC on. With thermal lag, the honeymoon lasts longer.