It seems that thermal mass is a big negative excepting the few climates where there are huge daily temperature ranges that average a comfortable 72 degree. In normal climates, where mean daily temperature is not 72 degrees thermal mass would only increase costs. For example, if mean temperature range is 55 degrees, then the only thing the thermal mass did is require your heating system to heat up a huge useless mass And, if the next day it is hot, you now have to spend money on your air conditioning to cool down that thermal mass you just heated up. What a waste of energy!

It seems like a far superior solution would be to have as little thermal mass as possible. And, have as air tight and insulated house as possible. In this way, the heating / cooling loads are extremely small. And, most importantly, heating / cooling is very responsive!

This is like radiant heating where people put the tubes in their slab. They waste tons of money heating up their slabs. Then, the sun comes out and shines through the window and heats up the house too much come 2PM. They cannot "turn down" the radiant heating because the slab is already hot. It will take hours and hours to cool. So, they have to open the window and hope the heat escapes while they are sweating and fanning themselves. All that escaped heat was wasted energy they spent unnecessarily heating their slabs.

On the other hand, if they had installed radiant heating in the sub-floor and put insulation over their concrete, the system would be responsive. It would only heat exactly when they needed it to and it would turn off exactly when it was too hot. The little thermal mass would mean it would cool down very rapidly and heat up very radiply. Net-effect, sun comes out, heater stops heating and the sun heats. Since there is littel thermal mass, once the heater stops heating it can cool at a relatively fast pace. Window never needs to be opened. No heat escape. Everything more efficient.

I have been reading this site (especially in the ICF forum) and other green sites and everyone talks about thermal mass like a positive thing. When, based on the above analysis, the rule is that it does not help you. The exception is it does (only for very specific climates that small % of people would be). Is my analysis correct or am I missing something very obvious?

If you want to sort out exactly how ICF (or any other wall construct) will perform in your actual seasonal climates, we developed this software several years ago:

Borst ICF Performance Calculator

The actual season climate and location of the thermal mass determines how much of a benefit the thermal mass will provide in terms of effective R-value. For standard ICF construction using our Rogue River, Oregon outdoor temperature profiles, this is R21.38 in Spring, R66.15 in Summer, R63.42 in Fall, and R21.07 in Winter. So this is an average annual ICF performance of R43.07 or 1.86 higher performance than the conventional R23.10 R-value. This does NOT include the reduced air infiltration benefit that ICF (or any other wall construct) can also provide. As a result, we don’t require or use any AC to address our 90F+ average highs including the several 115F days that we get each year. The interior thermal mass and high effective R-valve of the ICF walls keeps the interior of house from never exceeding 70F.

So I would say that proper application of thermal mass is never bad. Improper application of thermal mass for a given design and location can certainly result in problems. One needs to accomplish an integrated design that fully considers passive solar design, thermal mass design and HVAC design for the given location to achieve great results.

Yes, thermal mass is great when you have a mean temperature of 72 Fahrenheit, which Rogue River Oregon does in the summer. However, for the remaining 9 months of the year, Rogue River has a mean temperature of about 55 degrees. Thermal mass is a two edged blade. If it helps you summer, then it necessarily would harm you any other time of the year with different conditions.

So, 3 months out of the year you avoided using your AC. However, for the remaining 9 months you are now spending much more on heating. How does that help anyone? Just from a resource perspective, you have a big loss. And, if you assume ICF has thermal mass (I don't think it does), then you also lost the responsiveness of your AC / heating.

Reduced air infiltration is important, which is why I said insulation + air tightness is the best. There are two theories in ICF from what I can see: 1) ICF has thermal mass due to the concrete 2) ICF does not have thermal mass due to being sandwiched between two insulated layers. Personally, #2 is most likely correct, IMO. Otherwise, if thermal mass works with insulating materials between it, then every house has enormous thermal mass with the earth right below it and with the mountain separated by 500 miles of air to the sides. However, even if #1 is correct, most people would be harmed by that thermal mass. #2 is actually the optimal reality for people.

Studies that I have seen on ICF (sponsored by cement companies I might add) are always comparing apples to oranges. They will compare a 3000 sqf wood house with a 1000 sqf ICF house. Or, even more common, they will compare houses with dissimilar air infiltration rates.

It seems to me that ICF does not offer any thermal mass advantage, and in fact, perhaps even a disadvantage (neutral to negative). It DOES offer an air infiltration advantage compared to a normal stick built home, which is where most of the gains of ICF come from.

However, the question becomes, is ICF cheaper than the equivalent stick built home that has low air infiltration? Looking at passive house designs, they are basically all wood construction.

I am not knocking ICF. I plan to build with it due to its better performance against rodents, insects, and increased strength.

I don't really want to make this about ICF because I don't believe ICF has thermal mass in any case. I am talking more in general about thermal mass. For example, in a radiant heating system, most people put the pex in the slab. That seems like a huge energy waste due the issue I described in the original post: You spend tons of energy heating a slab, 2PM comes by and the sun warms the interior via the windows, and now you are hot and have to open the windows resulting in wasted energy. It would have been far better to not spend so much energy heating the slab, let the sun heat the house, and never opening a window.

So, my question is, why is there an obsession of thermal mass on this forum? Shouldn't the focus be on responsive heating / cooling, which would mean as little thermal mass as possible?

I totally get situations where thermal mass is great. You can run a rocket mass heater at 8PM for 1.5 hours and the mass keeps your house warm all night. No need to stoke the fire or add fire wood. That is a great use of thermal mass. But, that is far different than not isolating a slab in the house that will work against you.

If you properly design thermal mass, it never hurts you. It either increases the performance significantly higher than the conventional R-value or it doesn’t change the conventional R-value performance at all. It never decreases the performance lower than the conventional R-value. In Rogue River, the ICF thermal mass increases the conventional R-value performace by factor of almost 3 in the summer and fall months. It doesn’t do anything in the spring or winter months, you just get the conventional R-value performance. My heating bill isn't any higher as a result of this. It's exactly the same as it would be without the thermal mass.

If you have a well-sealed and well-insulated building that incorporates interior thermal mass, the indoor temperature of the building will tend to stay rock stable...pun intended. If you have a hydronic radiant floor slab using thermostats with indoor temperature and slab temperature feedback and a heat source using outdoor temperature feedback (outdoor reset), you are not going to see any significant indoor temperature variation year-round. You don't need a very responsive heating system if the interior of the building is insensitive to what happens outside. Lots of folks live this way including myself. My house is ICF and is passive solar heated/cooled. It is also heated via hydronic radiant slab heating and a backup wood burning masonry heater. It has a whole house fan to chill the thermal mass at night if desired and no AC. Works wonderful. Without thermal mass, it wouldn’t work nearly as well in Summer and Fall and AC would be required.

The aforementioned software will tell you precisely how ICF will perform for any outdoor temperature profile. So there is absolutely nothing to debate with regard to how ICF will perform in a given location. Please be sure to read the instructions carefully along with the analysis conclusions at the end with regard to how thermal mass and insulation location affects performance.

I don’t think people on this forum are obsessed with thermal mass or anything else. There are just some people who know how to wisely use thermal mass and other green building technology. And there are other people who seem to want to debate most everything. Nothing wrong with that, but I am short on time right now. If you are unfortunate to live someplace where thermal mass doesn’t significantly benefit your integrated green building design, simply don’t use it...or make building location selection part of your integrated green building design criteria.

BuildNewb: consider that in a well insulated building, a large radiator might never go above 75F (depends on your load, inside temp and floor coverings and of course there are temperature variations within it). So if for any reason, the building gets too hot (say > 75F), a large, high mass radiator automatically stops heating the building. The radiator does not require cooling down with AC and radiator heat isn't flowing out a window opened to reduce room temperature to 75F. Similar concepts apply to interior mass that isn't a radiator (it's more like 70F).

On the other hand, high mass interferes with thermostat setback strategies - which I like for both comfort and cost reasons. And high mass may cost more.

Actually, in the winter months on the coldest days (low teens), our slab has never gotten above 72F. We keep most of our thermostats set to 68F because with hydronic radiant floor heating that is a very comfortable temperature. We have never seen more than a 1 degree indoor temperature variation from this heating set point. You definitely do NOT want to use thermostat setback strategies with high mass hydronic radiant emitters in typical residential buildings. This can be done in larger commercial/residential buildings, but more sophisticated HVAC control is required. We just keep our master bedroom set at 64F all the time. About 40% of our heat gain comes from passive solar and we are not in a crazy cold climate, so our annual heating bill is a couple hundred dollars and can be near zero if we fire up the masonry heater with wood from our property.

In the summer months on the hottest days (typically 90F+ highs and a couple days of 115F highs), our slab has never gotten above 66F and the indoor temperature is typically 0 to 4 degrees higher than the slab. We see an indoor temperature range between 64 to 70F without the use of AC or any active temperature control. This works because of our low humidity diurnal summer climate where the night time lows are in the 40s or 50s even if the high was 115F. This would not work in a high humidity climate where it is hotter than perhaps 75F all day and night. But this is why one needs to accomplish an integrated design that fully considers passive solar design, thermal mass design and HVAC design for the given location to achieve great results.

I don't find thermal mass to be a negative - but neither is it the silver bullet often touted. I think many of the ICF claims of inflated R due to thermal mass are unfounded or based on very specific climates with diurnal temps which swing both above and below the conditioned space temp short-term (thereby reversing the heat flow through the wall fairly frequently) That said, I have an ICF foundation in a southern New England island climate (think fog, highs in high 80's lows in low teens) and I love it. I may be the only house of 1200 on this island without a musty basement! My first floor is a slab and second floor is gypcrete on wood so 22 tons of concrete and about 5 tons of gyp. 1950 SF living space, passive solar (sort of), radiant heat, no A/C, R37 walls, R60 roof. In winter house stays nailed at 68, slab only a few degrees above that (75 tops). I did not expect, or get, "warm toasty" floors. I did get efficient heat and use a wood stove at night to add a few degrees of coziness ("hygge"). Even during a winter storm - winds gusting 50-60 and temps in the teens the temp stays nailed at 68 (air sealing was raised to an obsession on this house - but i don't have final blower door results - I would guess around .8 ACH50 as low setting on range hood pulls smoke down the chimney). Bottom line, thermal mass is not a bad thing unless you want to do large day/night setbacks.

Birdman,

Glad to hear your house is working well. How do you like mainland grid power?

Thermal mass can be very good if designed and used properly. Your example of using a heated floor with Passive solar is a prime example of why the technologies are not compatible, But here is an example: A thermal mass floor that is insulated but not directly heated, coupled with Passive solar. The Mass floor is close to but a little lower than the ambient air temperature in most cases due to air stratification in the room. During the winter days the sun directly heats the floor. During the night that heat bleeds back out of the floor helping to heat the space. A fireplace or stove has similar results. During the summer it actually works in reverse. A ceiling fan pushes the air to the floor where the Mass floor can absorb some of the heat. We have measured a 2 degree difference in room temperature at waist level 20 minutes after turning on a ceiling fan with exposed concrete floors.

I see ICF as great building concept.

So heat moves through a wall. It is slowed by insulation. If the sun heats the outside of a wall it takes a certain amount of time to get through the wall. If this time it takes to get through the wall delayed(4 to 12 hours?) it will lessen the temp swings in a house thus reducing your heating/cooling bill. I hope one day to chart this throughout the day and see how the heat moves through the wall.

In the dead of winter it doesn't help in this respect because heats is always moving out of a house but it helps in another way. In the winter electric is 1/3 the cost at night. Our house is geo thermal and we only heat at night when the rates are low. The mass of the house keeps it warm with the help of the sun all day long.

We also installed high solar gain on all the southerly facing windows. This keeps the house warm during the day and the mass absorbs that heat all day. If we didn't have 1-1/2 and 6 inch concrete floors to absorb that heat the room temp would sky rocket during the day and way over shoot. Right now at 9:30 am till 5pm that room temp raises from 69 to 73 degrees(if sun is out), the floors absorb that heat and I run our radiant pumps 24/7 and it redistributes that heat to the rest of the house instead of over heating the house.

I can't see how mass doesn't help. With a good thermostat like an ecobee you don't get extreme overshoot anymore (I used to in my old house). You just have to think how to use the mass to your advantage.

Chris J - Yes, the house has exceeded my expectations and has been a real treat to live in. With a really great ocean view to the north and east I compromised the passive solar bit for glass in that direction but it still performs well overall. Being on the mainland grid has been good at stabilizing our rates - but we still have pretty high total rates (about $0.28/kWh) but they are down from the highs of $0.68 we saw with diesel generation!! Technically we purchase power from the mainland grid but when there is any wind at all 100% of our electrons come from the offshore wind farm. The local furor over the farm has subsided quite a lot and the side benefits are emerging (folks coming out to see it, charter boats providing tours, incredible fishing around the foundations!) The quality of our power has improved immensely - voltage and frequency fluctuations are pretty much a thing of the past - motors and digital controls may actually last a while now! The nicest part is we're not burning 1,000,000 gallons of diesel every year and now in the spring around the power company all we hear are peepers.

One related article I read recently on radiant floor heating (in slab hydronic) argued that those pictures you see of bare feet and babies walking on a warm tile/concrete floors are pure marketing.  To make a slab feel warm to the touch takes an enormous amount of energy and to maintain such a temperature with large mass one cannot hope to keep up with fluctuations of passive solar heat gain.  The sun comes out and the house will over heat, and the thermal cycle will be a mismatch. 

The author argued when going passive solar best to keep the slab cold (no hydronic) and let the sun heat it up, then use a wood stove or gas fireplace with blower to supplement the air temperature.

Like I said, that is an aside topic, but related, yes?

Another aside example came about a discussion with a friend at a cigar lounge recently. 

We we sitting inside with our cigars near a large expanse of storefront glass.  I explained that there is this thing called Delta-T (triangle symbol and capital "T").  The rate of energy flow and the direction has much to do with our comfort levels.  That it wasn't just the poor R-value of the glazing system and all of the air leaks from the door affecting us. It was the rate of the heat in our 98.8 degree bodies leaving us, leaving us "in the cold" - which determined our comfort level.

Where mass comes into this is, if that mass is closer to our own body temperature the less of an affect it will have on us because it will be a lower Delta-T.  No mass in that window system to speak of, and very uncomfortable.

Big temperature between a cave or basement in the hot summer, walk into a cold cave or basement and instantly feel cool.  This is very different than having cool air blown on to your exposed skin, right? 

I have an old heavy mass oven in my kitchen, nothing heats that room up like turning that baby on, and it isn't all because it vents heat into the room.  The room stays warm long after the gas is turned off because of the mass of the old oven, the radiant heat.

Mass is great for affecting your comfort levels, trick is getting it into the right range and at the right time.

Wikipedia to the rescue - link below.

Thermal Comfort

Kach - Not to put too fine a point on it but reread the part in your link (which is good) about Mean Radiant Temperature. MRT is where a big part of our perceived comfort comes from. While you were enjoying your cigar you were next to a sheet of glass whose surface temp was less than 98.6 - that big surface was a good portion of the MRT around you and therefor a big factor in your perceived comfort. The delta T you cite relates not to your comfort so much as to maintaining the air temperature within the space - IOW, the delta T determines how rapidly heat will flow through the wall or glass. An insulated wall will have a higher indoor surface temp than a plate glass storefront by a lot so the wall surface will have a higher MRT therefor you "feel" warmer next to it. This is why not many folks replicated Phllip Johnson's Glass House - regardless of how high the inside air temp is the low MRT will make you feel cold.

If, for some reason you have to build with lots of glass and want it to feel comfortable, consider:

high R windows
room-side low-e coating
radiant heat, preferably under the windows, to help balance out the MRT (and drafts).

If experience counts .... Frank Lloyd Wright fan here living in a house with lots of south facing glass in a climate that's poorly suited for passive solar.

* Balancing passive solar and slab-based radiant heat is not difficult at all if the mass is oversized. Nothing happens quickly in a high-mass house; my wood stove boiler warms the slab at about a degree an hour. In an unusually sunny stretch for Pennsylvania in late Nov. it coasted for three days on passive solar with a touch of heat pump in the morning.

* The guy recommending wood stove-backed passive solar is way off base in any climate that isn't the desert Southwest. Once a slab begins cooling, radiant heat isn't going to bring it back any time soon at air temps considered comfortable, and a slab below 65 degrees pretty much insures discomfort regardless of air temp. Been there with a slab inside ICF frost walls and sitting on 4 inches of xps.

* Radiant floors are not inefficient, even in my case where occasionally I fire up the wood stove boiler to add six or seven degrees to the slab. Yes it takes hours, but then again, it took many more hours for the slab to cool that much.

* A radiant slab is ultra comfortable: a nonfactor above 65 (Your feet may vary) Noticeably warm at 75, which is my default stat setting, and bliss at 80, which is my reset when the temps dip into single digits.

* Coated windows aren't going to get it done if you live in a greenhouse. The problem is convection -- cold glass creating a reverse chimney effect that drops cold air into the room -- and the answer is window treatments that seal them. Wright put radiators beneath the glass to counter MRT. He was death on anything covering his glass, pulled down drapes if he discovered a homeowner in flagrante delicto. Myself, I rather not heat the great outdoors. My windows are r7ish with cellular and nuclear winter shades lowered. My uncoated windows are SHGC 62.

Thermal mass is a tool, and a necessary one in my case. It has a bad rep because it is poorly understood and often misused.

Yes indeed, MRT gets into the finer scientific details and aspects of why a HR floor causes one to feel more comfortable at lower air temperatures than would otherwise be the case. Birdman/Toddm, you raise a very good point about MRT and sitting close to windows. This is a common problem that often doesn’t get properly addressed in a house design. For example, if you have a dinning room or kitchen nook in close proximity to much window surface in a cold climate, you should zone these areas (i.e., have dedicated HR circuit(s) in these areas) so a higher floor surface temperature can be achieved even though they are not isolated rooms. The warmer floor MRT will offset the lower window MRT. This normally falls out of a proper design if you place invisible walls around these areas to cause them to be treated as separate rooms by the design software when accomplishing the room-by-room heat loss analysis and HR design. Anyhow, I thought this was a very good point worth mentioning and expanding the discussion. Birdman, it sounds like you have a very nicely done home...congratulations!

Todd,
When you say your house coasted for 3 days, you're saying it stayed within a degree of setpoint or did you let it dip a little?

I had considerable variation over those three days. The first day, the sun had heated the house to 76 by late afternoon, which is the highest temp passive solar has achieved to date. The next morning it was 68 IIRC. 74 the next afternoon and 66 in the morning. Didn't check the temp the third afternoon but it was comfortable; much less so at 63 on the third morning. When I turned on the radiant that evening , 63 was the temp of the concrete. Over the three days the floor itself had cooled from 75 to 63. The ambient temp ranged from 50ish to just freezing. We lowered cellular shades at night over 240 sf of uncoated double pane.
None of this added up to discomfort during waking hours thanks to a two-ton mini split. (Necessary for humidity in the summer, over which thermal mass has zero effect.) Except for a half hour or so each morning, the house was in a band of 70-76. The missus seldom complains that it is too cozy. As for me, passive solar cuts my workload considerably by taking over pretty much every afternoon that the sun shines. I heat the floor (and a 1600 sf house) with about three cords of wood a season, spinning two circulators and two 6 inch duct fans 8 hours a day at combined load of maybe 250 watts.

To the people actually living with concrete slab floors, and some passive solar design helping out, would you attempt a ground floor slab without hydronic radiant slab heating?

My latest notes on my schematic design has a lot of south facing windows under an overhang (insulating shades at night), well insulated concrete slab, open plan with gas fireplace and blower, Master Bedroom and other zones with split mini Heat/A/C eliminating duct-work and creating several "zones".

I am worried that eliminating the hydronic radiant slab heating system of my earlier concept is going to create some MRT issues.

Also concerned that just blowing hot air into the great room (open plan w/kitchen and dining) isn't going to get the heat up under the glass where it belongs. Maybe I can duct under slab the gas fireplace hot air to vents under the windows? Is there such a thing?

Maybe some supplemental mini type heating system for just the windows that go down to the slab?