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Creating Thermal Mass with ICF

Last Post 31 Jan 2011 09:38 PM by SolarOH. 30 Replies.

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Delta-Form User is Offline

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Posts:15

24 Oct 2010 12:27 PM
I have a question; hopefully someone can provide some insight. A huge part of our design/build business includes the design and construction of super-energy efficient and "off the grid" homes. We rely on passive heating and cooling to minimize the energy burden of each home. Passive solar requires an adequately designed thermal mass to store and release solar heat; it is an essential design component. In my opinion ICF would be a logical choice for walls however the EPS foam on the interior wall virtually eliminates the effective use of the concrete as a thermal mass. Does anyone know if there is a Single Sided ICF block system that leaves the concrete exposed on the inside wall surface? I have read that some builders will apply two layers of drywall to create some thermal mass. Any thoughts?

TexasICF

Advanced Member

Posts:622

24 Oct 2010 09:50 PM
Posted By Delta-Form on 24 Oct 2010 12:27 PM I have a question; hopefully someone can provide some insight. A huge part of our design/build business includes the design and construction of super-energy efficient and "off the grid" homes. We rely on passive heating and cooling to minimize the energy burden of each home. Passive solar requires an adequately designed thermal mass to store and release solar heat; it is an essential design component. In my opinion ICF would be a logical choice for walls however the EPS foam on the interior wall virtually eliminates the effective use of the concrete as a thermal mass. Does anyone know if there is a Single Sided ICF block system that leaves the concrete exposed on the inside wall surface? I have read that some builders will apply two layers of drywall to create some thermal mass. Any thoughts? Delta-Form, Here are a couple of thoughts: The idea that EPS foam on the interior wall virturally eliminates the effective use of concrete as a thermal mass is not correct. From a thermal mass view, it is true that concrete performs statistically better on the inside surface -- IF you provide the same thickness of foam on the outside (you would need to move the inside 2 5/8" from the inside to the outside) -- (e.g. outside EPS is 5.5 or so inches thick and concrete remains the same -- perhaps 6"). Statistically, the improvement is very very small (see ORNL) and the fact that you lose the flexibility of simple electrical and plumbing runs makes it generally less effective than ICF. That said, one or two ICFs allow you to run one side of foam and the other side concrete. Remember, double the foam in order to get to the statistical equal. Then the builder will need to determine how to best provide elec/plumbing. I suppose some builders might apply 2 layers of sheetrock to increase conventioal stick thermal mass but if they do this with ICF contruction with the same intention they are smoking illegal stuff. Regards.

arkie6

Veteran Member

Posts:1453

24 Oct 2010 11:46 PM
"...EPS foam on the interior wall virtually eliminates the effective use of the concrete as a thermal mass." I disagree. It does negate it somewhat, but does not eliminate it. Note also that some ICF manufacturers (Quadlock and TF come to mind) offer a "lopsided" ICF where there is less foam thickness on the inside vs. the outside. These are typically offered with 2" of Type IX EPS foam on the inside (~R8.5) and 3 to 6 inches of Type II EPS foam on the outside (~R12 to ~R24).

adi43d

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Posts:87

25 Oct 2010 09:34 AM
here is another idea: http://www.integraspec.com/documents/IntegraExposed2.pdf

http://torontonetzerohouse.blogspot.com/

renangle

Basic Member

Posts:304

25 Oct 2010 01:50 PM
Here is a great case study that was done some years ago, but they study the thermal performance of several different wall systems, including an ICF wall. According to some of the engineers that I work with, the math and calculations used in the case study are VERY advanced. http://faculty.virginia.edu/ribando/modules/projects/ISEC2004-65022%20web.pdf renangle

dmaceld

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25 Oct 2010 03:49 PM
Posted By renangle on 25 Oct 2010 01:50 PM Here is a great case study that was done some years ago, but they study the thermal performance of several different wall systems, including an ICF wall. According to some of the engineers that I work with, the math and calculations used in the case study are VERY advanced. http://faculty.virginia.edu/ribando/modules/projects/ISEC2004-65022%20web.pdf renangle Very interesting paper. After reading it and seeing what they show for the performance difference between frame wall and ICF, which I already had a pretty good sense about, I almost wish I would have gone 100% ICF and put an ICF roof on my house rather than the R40 spray foam underneath a conventional truss roof. Just looking back at some the HVAC calcs on my house, ceilings, hence roof, have twice the heat gain of the walls, and about 70% of the heat loss of the walls.

Even a retired engineer can build a house successfully w/ GBT help!

BrucePolycrete User is Offline

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Posts:524

25 Oct 2010 07:01 PM
Good stuff. Ren. Thanks for sharing this.

arkie6

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Posts:1453

26 Oct 2010 07:23 AM
Another option for having the concrete on the inside with all of the insulation on the outside is Lite-Form strippable forms. These allow you to form and pour the wall, then you can come back and strip the insulation off the inside if desired. You will have plastic ties protruding from the concrete every 8" on center, but these can be cut flush with the face of the concrete. You would then likely need to apply a plaster or stucco like finish to the concrete to get the desired look. The foam insulation used for the strippable forms is generally 2" thick XPS since it doesn't stick to the concrete like EPS foam does. Now what you do with the foam that you stripped off depends on your situation. It could be moved to the outside and attached to the exterior foam depending on whether the wall is above grade or below and what you will be using for the exterior finish.

Eric Anderson User is Offline

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Posts:441

26 Oct 2010 09:44 AM
My take on the paper: I took an excerpt of the Paper. They compare 2 houses of different construction types. One house is 3 times larger then the other so qute different surface area to square footage. They use a R30 ICF form which is not the norm, most are around R22 vs a 2X4 stick built house with R13 Presumably fiberglass Batt insulation. "EXPERIMENTAL DESCRIPTION Energy usage data for two different residential building was collected between August 1988 and January 2003. This data is normalized using floor square footage and published degree-day weather data. A conventional stick-built frame wall construction residence was completed in August 1988. This building has nominal R-15 (2.642 m2-C/W) insulated walls and R-30 (5.283 m2-C/W) attic insulation. The building consists of 1200 square feet (34 square meters) of living space over an unheated crawl space. The floor has nominal R-19 (3.346 m2-C/W) insulation. This building has natural gas heating and hot water. The heating system was a nominal 30,000 BTUH (8.79 kW) gas furnace. The cooling system was a nominal 2.5-ton (8.79 kW) central air unit. An insulated concrete form (ICF) residential building was built in 1997. The walls of the structure were nominal R-30 (5.283 m2-C/W), and the ceiling was nominal R-40 (7.044 m2- C/W). The building consisted of 4800 square feet (136 square meters) of living space. The primary heating system consisted of a high-efficiency gas furnace converted to burn propane. The design heating load for the building was calculated to be 32,000 BTUH (9.38 kW), but the installed gas furnace capacity is 45,000 BTUH (13.19 kW). The larger size unit was installed in order to get a larger fan needed to move more air around the building. The backup heat and primary cooling system is a 3- ton (10.55 kW) air-source heat pump." If I was a reviewer I would have rejected this paper outright. 1. No Mention of surface area to square footage corrections in experimental data 2. No mention of the types of houses so it is hard to know what they are comparing. 3. No mention of windows types or numbers, doors, large chimneys etc. 4. No blower door test to determine leakage rates 5. No mention of the climate the experimental houses were in 6. No mention of comparison of the relative efficiencies of the HVAC system 7. No accounting for how they determined the design load of the buildings or if they even did it for the stick built house. 8. No mention of what type of insulation was used in the stick built house. 9. How have they separated the fraction of gas used for hot water vs heating. 10. No mention of how many people live in each house. 11. No calibration for thermostat settings or listing what temperature settings are used for modeling 12. No modeling of internal heat gain from pets, electronics etc. \ 13. They are modeling using a daily temperature swing from -3°C to 23°C, in other words a 46 degree F temp change between day and night with a daytime temperature of 73 degrees. Desert southwest maybe? This is a totally irrelevant comparison of two very different houses in a temperate climate with huge temp swings. It is clearly biased to show that ICF is more efficient. In this narrow range of conditions, they are correct. It would however be quite easy to build a stick built house that would be zero energy usage under these same conditions. Cheers, Eric

Think Energy CT, LLC Comprehensive Home Performance Energy Auditing

JakeG

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Posts:55

27 Oct 2010 09:34 AM
Hi Delta, Durisol provides a lop sided block as you have asked. The higher the thermal resistance block you require, the more the resulting insulation is towards the outside. I have not completed any off the grid homes, but I am aware of a number of Durisol off the grid homes (Texas & Ontario). If you are going all the way to passive house standards, that is more difficult to achieve. Good luck.

slenzen

Basic Member

Posts:434

27 Oct 2010 11:59 AM
Some use regular poured walls and removable forms w/ foam on outside. Not sure how that stacks up w/ icf as far as cost. Whether attaching foam after the pour or putting the foam in the form before hand. thermomass and or western forms emaxx, comfort wall

Dana1

Senior Member

Posts:6991

27 Oct 2010 04:13 PM
I'm with eric- it's a crappy paper, which missed or made no attempt to measure a whole slew of relevant factors. (As for the VERY advanced mathematical modeling, I beg to differ with renagle's engineers. Approximating a simple ordinary differential equation using finite-difference numerical method isn't particularly high-math- I know some high-school kids capable of pulling that off. But what do I know? My degrees are in math & physics, not engineering. ;-) ) The no-math-required answer requires only a bit of physical-intuition. The basic principles of ICF are sound the thermal mass DOES participate, even with half the insulation on the interior- it's basically buffering the heat transfers through the wall in both directions, damping the peak heating & cooling loads. In a structure where most of the heat gains & losses are from conducted heat through the wall the effect is very good for the efficiencies of the mechanical systems, since they can be right-sized for the average heat load on design day rather than for the actual peak that would have occurred without the thermal mass. But in real homes air infiltration and window gains/losses often equal or exceed conductive heat transfer through the walls. The mass still participates, but not as effectively as could were it fully within the insulation rather than thermally isolated from the interior.

dbollermann User is Offline

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Posts:20

27 Oct 2010 08:27 PM
Interesting discussion. The empirical data from the field at some point has to be matched to some kind of model that explains the observed results. That is where modeling becomes useful, as a way to predict changes in observable data which should result from specific changes to the modeled system. Further observations can then change the model, and, thus we have the foundation of the scientific method. That being said I have a pretty much purely physical intuitive thought on ICFs. Thermal mass has all of the previously described benefits and many builders, like the originator of this thread, are seeking to maximize interior thermal mass in a building, that is, thermal mass that is coupled only to the conditioned interior space. But the mass in an ICF wall, as we all know, is not really interior mass because it is not directly coupled to the interior conditioned space of the building. I'm starting to think maybe this is not a bad thing, for the following reason. Concrete has, on average, 10-15 percent of the r value of wood. It's not an insulator and in fact, what does not insulate conducts, so we would call concrete a thermal conductor. If then we look at a typical ICF wall cross section, including a footing below frost line, and we posit that the concrete core of the wall is a heat conductor, and if we acknowledge that while "heat rises" in gaseous or liquid systems, but that heat conduction in solids pretty much doesn't care about up and down, then it seems reasonable to posit that heat can flow through the concrete core of the wall and into and out of the soil below and adjacent to the footing(s). If the concrete core of the wall, then, were a perfect heat conductor, and the soil in the area of the footings had idealized heat transfer as well, then we could expect to maintain the concrete core of the ICF wall at exactly ground temperature all the time. The delta T from the wall core to the conditioned interior space would always be about 15-20 degrees F, with a small amount of heat being continually transferred through the insulation layer separating the interior space from the concrete wall core. With, of course, varying outdoor temps., the wall core in this model is then conducting heat into the soil from outside air if outside temperature exceeds soil temperature and out of the soil to the outside air when outside temperatures fall below soil temperatures, with this heat transfer being mediated by the outer insulation layer separating the concrete wall core from the outdoor temps. Add thermal mass and actual thermal conductivity of both the wall core and the soil to arrive at a close to real model. The more I think about this the more I start to see ICF walls as a "ground coupled thermal mass shell". Interior thermal mass? Thicker floor slab (thermally broken at openings), interior plaster, sand bed storage under slab? In summary, how thermally coupled to the soil is the ICF wall concrete core? What are the effects of this coupling and how much, if any, coupling of interior thermal mass to the soil is advisable? Under slab insulation has become de rigueur. Why would the coupling of interior wall thermal mass to the soil be advisable? Perhaps a standard ICF wall system with insulation on both sides is already a fairly well idealized system, regardless of the relative thicknesses of interior and exterior insulation layers. I would like to see some data comparing the performance of ICF walls to concrete walls insulated only on the outside.

ICFBdr

Basic Member

Posts:238

28 Oct 2010 05:47 PM
I think there are a couple important things to keep in mind that have not been mentioned: climate and thermal bridging. If your design is to gain maximum use of the thermal mass of concrete, it is important to eliminate any thermal bridge, which can transfer cold through the concrete. This was touched on by dbollermann talking of cold in the ground transferring though the footing and into the ICF foundation. Also important is above grade. For example, is the sill plate at the top of the wall properly insulated? If not, the cold in the attic space can easily pass through the sill plate, then transfer down the wall - keeping the concrete warm will increase the efficiency. Also, from many articles I have read there is no one arrangement that is right for thermal mass in ICF. Rather it depends on the area you are building. For instance, I am in an area that can experience extreme hot and extreme cold (-40 to +40 Celsius through the year). The best configuration in this case is to have 2/3 of the insulation outside the concrete and 1/3 inside the concrete. Other areas require a 50/50 split and yet others call for 100% insulation to the exterior. Fully eliminating the insulation on the inside will be great for winter heating, but in summer, there is no protection from the heat in the concrete, generated from the sun. This brings up other factors, such as color and type of exterior finish - a dark finish will absorb more heat from the sun, which is great in the winter, but not so good in the summer when air conditioning is needed. Even through over 5" of EPS, I have seen concrete heated to 48 degress Celsius (dark green tin exterior). Of course, the biggest factor is the orientation of the building and windows to increase the passive solar gain in the winter, and eliminate and sun during the hotter months (with larger overhang, window shades, etc.) Using tile floor, stone fireplace, etc. can help capture this heat during the day and keep the home warm as it cools at night. Not sure where this building is being constructed. I realize these thoughts are more aimed at a climate that has both cold and hot seasons and may not be of any help, but just thought I would share my experiences.

dbollermann User is Offline

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29 Oct 2010 02:42 PM
ICFBdr is right on the money as far as what I've seen and taking theory to real world steps is where I think we are all going. I also reside in an extreme climate (approx. 100 f plus summers and -20 f winters). Orientation and glazing positioning is key when managing thermal inputs. This is a big subject, often woefully overlooked, though it has been studied and documented extensively. I have seen some houses with very little solar consideration built with good ICF walls, good attention to detail and good roof systems perform very well anyway. A related example to the sill detail mentioned by ICFBdr is the following. An ICF house is being built in the southwestern style with "flat" roofs and "parapet" walls extending above the roof line on all sides. The roof is suspended from the ICF walls about 18" below the tops of the ICF walls. In this case, pouring the walls and screeding off the concrete about 2" below the top if the ICF blocks rather than flush with the tops of the ICF blocks allows strips of 2" foam board to be subsequently set into the tops of the ICF blocks and stuccoed over. Alternatively, if a lot of block scrap were generated on the job, this would be an easy place to use it. This creates a tighter thermal envelope for the concrete wall core and also makes final truing of the wall top preparatory to stucco much easier. It seems clear that treating the concrete ICF wall core like the "thermal battery" that it is means insulating its perimeter wherever possible. This means tops of walls, exposed ends of walls that extend beyond building corners, walls that are connected to outside retaining walls, edges of floor slabs, and, ideally, all window and door openings. We have all seen ICF buildings which, even if these details are overlooked, perform very well. It will be exciting to see just how well they work as more of these details are understood and attended to.

Dana1

Senior Member

Posts:6991

29 Oct 2010 04:04 PM
dbollerman: The thermal conductivity of concrete isn't anywhere near that of a refractory metal, it's QUITE insulating if thick enough, with a K value of about R1 per FOOT. The notion that heat losses to the ground occur is correct, but the internal temp of the concrete 4' above grade will have far more to do with diurnal swings in outdoor temperature than the delta between ground temp and conditioned space or exterior temp. Even if it were a refractory materal, the difference between ground temp and conditioned space varies a LOT more than 15-20F range. In some areas like FL it could be +/-5, and in northern MN would usually be more like 30-35F in winter. Ground conductivity is a secondary effect, just one of many deficiencies in the model of the prior-referenced paper. Treating the concrete as a "thermal battery" is ridiculous, "thermal capacitor" would be more appropriate. Even with 3 foot thick walls you'd be hard pressed to get more than couple of days of "heating" out of it at anything less than superinsulation levels on the exterior. ICFBdr: With all of the insulation on the outside, you are are BETTER protected from the peak cooling loads, since a large fraction of the solar gains in a building are from windows, not the highly-insulated walls. With all of the mass inside the thermal envelope you get better moderation of those window/roof/door gains by letting the concrete soak them up during sunny hours, delivering them back night. And higher R on the exterior the heat gained by the concrete from the exterior is proportionally less. In heating OR cooling mode there is no real down side to having the thermal mass inside of conditioned space- it averages the load over the course of a day much better.

dbollermann User is Offline

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29 Oct 2010 10:53 PM
Dude, an R (not k) value of R=1 per foot is not what I would call "QUITE insulating", not when styrene foam has an R value of (on average) R=62 per foot. I'm sure you've helped a lot of folks on the forum and I have all due respect for your number of posts and all that, but I do believe you've slightly missed the point I was making in favor of making, er, what point exactly? That concrete is "QUITE insulating"? Okay, go for it. That "capacitor" would be a better term than "battery"? Point taken! That delta Ts in northern MN will be more than in Florida? Really? No kidding? Sorry, but I find this to be ridiculous. So sorry to have attempted to have some fun with the concepts involved and the practical implications of such rather than engaging in what seems to be some kind of pissing match between, uh, what and, uh, what, exactly? Oh, secondary effects versus primary effects! Who needs 'em? Good point. Oh. You mean, like thermal mass? Oh well. I guess that is a secondary effect! Your (thermal) loss, or gain. Cheers

Bruce Frey User is Offline

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Posts:429

30 Oct 2010 06:48 AM
I always enjoy these thermal mass discussions! Thermodynamics/heat transfer was never my favorite subject (actually, I hated it), so I leave the math to others. What I have taken away from the numerous studies I have read is that: 1. for a continuously conditioned interior space, thermal mass generally belongs on the inside 2. for a passive solar house, thermal mass is the inside is needed 3. if your climate has hot days and cool nights and you like to open windows to "cool soak" at night, thermal mass belongs on the inside How much thermal mass do you need? I have read that a double layer of 5/8 drywall is generally enough to carry through a dinural cycle. While ICF may be "less good" in this situation because of its' interior insulation, it would not deter me from using them, although an assymetrical form would be a better solution. 4. if your climate has hot days and cool nights and you keep things closed up, the qualities of an ICF (good R, low infiltration and some thermal mass) are ideal, but the R and infiltration are of higher importance than thermal mass Also see point 1. 5. if you live in a cold climate where exterior temps are low for long periods of time, thermal mass of an ICF has little or no value 6. the idea that IFC magically "wicks" heat to/from the soil is BS 7. if you plan to use thermal mass to buffer the inside (e.g. earth shelter) you need feet, not inches, of mass for it to be effective I think that ICF have many excellent qualities and I hope to use them (assymetrical) in my hot/humid climate build, but thermal mass is not a significant driver in my decision making process. my €0.02 Bruce

Dana1

Senior Member

Posts:6991

01 Nov 2010 06:20 PM
Posted By dbollermann on 29 Oct 2010 10:53 PM Dude, an R (not k) value of R=1 per foot is not what I would call "QUITE insulating", not when styrene foam has an R value of (on average) R=62 per foot. I'm sure you've helped a lot of folks on the forum and I have all due respect for your number of posts and all that, but I do believe you've slightly missed the point I was making in favor of making, er, what point exactly? That concrete is "QUITE insulating"? Okay, go for it. That "capacitor" would be a better term than "battery"? Point taken! That delta Ts in northern MN will be more than in Florida? Really? No kidding? Sorry, but I find this to be ridiculous. So sorry to have attempted to have some fun with the concepts involved and the practical implications of such rather than engaging in what seems to be some kind of pissing match between, uh, what and, uh, what, exactly? Oh, secondary effects versus primary effects! Who needs 'em? Good point. Oh. You mean, like thermal mass? Oh well. I guess that is a secondary effect! Your (thermal) loss, or gain. Cheers Concrete's thermal conductivity IS far closer to that of styrene than it to any refractory material, and as such that conductivity to the subsoil through the footing doesn't much affect the behavior of the wall once you're more than a few feet above the footing, making 2-dimensional modeling of how that thermal mass behaves reasonably accurate without factoring in the ground losses/gains was the point. So, what's your point, exactly?

wes

Advanced Member

Posts:810

02 Nov 2010 07:47 AM
Man, this conversation reminds me of the old tale of the two professors debating the relative viscosity of the quick sand they are standing in. Definitely more than this old builder can keep up with.

Wes Shelby<br>Design Systems Group<br>Murray KY<br>[email protected]