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Q&A Spotlight

Does R-Value Trump Thermal Mass?

Choosing between insulated concrete forms and wood-framed walls

This video is only available to GBA prime members

Jesse Lizer’s plans for a new house in Climate Zone 6 call for a 60-foot long walkout basement wall on the north side. The three below-grade foundation walls will be built with insulated concrete forms (ICFs) with an R-value of roughly R-25.

Lizer’s question, posted in GBA’s Q&A forum, concerns the exposed north wall of the foundation. Should it be built from ICFs like the rest of the foundation, or would he be better off framing it as a double-stud wall with an R-value of 40, like the rest of the house?

With the wood-framed wall, he not only gets better thermal performance, but wood framing would also make it easier to put in windows and doors. “Plus,” he adds, “if a rough opening gets framed wrong or something changes, we are fixing wood and not concrete.”

“Would it still make a better/stronger basement system if all the walls were ICF?” he asks. “I realize the length is quite long, which is why I am curious if it would even help make any difference or not.”

The discussion is the topic for this week’s Q&A Spotlight.

Thermal mass vs. higher R-value

With a core of concrete, the ICF wall has more mass than a wood-framed wall, and Mike Collignon thinks this benefit is worth considering. “This often doesn’t show up in R-value discussions,” he writes, “but it [is] quantifiable and does make a difference in reducing air infiltration.”

Added thermal mass, however, may not have much benefit in Lizer’s climate zone, Dick Russell says.

“Bear in mind that the ‘thermal mass’ effect of concrete is most useful in climates where there is a lot of outside temperature swing above and below the inside temperature,” Russell writes. “In prolonged cold weather, the…


  1. Expert Member
    Dana Dorsett | | #1

    Yes, yes, and maybe
    Thermal mass is more useful when fully inside the thermal envelope, where it can allow a higher south facing glazing fraction (or solar gain factor) for a higher degree of passive-solar space heating, and for soaking up peak sensible cooling loads from window gains. Isolated from the interior by even R5 of insulation in an ICF renders it useless for that purpose, although it will continue to take the edges off peak heating & cooling loads related to the heat transfer through walls. DOE2 (and other better energy use simulation tools) do a reasonable job of simulating those mass effects.

    And at R25 whole wall values (not a huge upgrade from the ICF minimum), glazing (even pretty-good U0.30 goods) will usually dominate the heat gain/loss of the exterior walls, rendering the thermal mass isolated by the interior EPS largely moot. Whether the window is modeled as R1 or R 3.3, it's still a huge hole in an R25 wall and an even bigger hole in an R40 wall in relative terms. A U0.30 window has about an order of magnitude more heat loss per square foot than the wall, and an even higher heat gain for windows that get direct sun.

    According to 3rd party test results summarized by JM, ( - see graphs on p8) new school micro-fine fiberglass at 1.8lbs density has about the same air-retardency as dense-packed cellulose. But if one projects the air-retardency graphs to 1.0lb density (which is also a recommended installed density) it's air-redardency at low density will be under that of open-blow 1.5lb cellulose. Between 1 & 1.8lb density it's not much better than 2-hole method cellulose. Decent stuff, not necessarily worth paying a premium for.

    Also absent from the fiberglass vs. cellulose comparison is the substantial moisture buffering capacity of cellulose put up against the relatively rapid drying capacity of Spider & Optima. The crossover where rapid drying becomes more important to the long term resiliancy of the structure than hygric buffering depends a lot on the wall stackup and climate. It doesn't take much exterior foam to render the rapid-drying toward the exterior nearly irrelevant, (as does rainscreen-vented siding, for most US climate zones (including zone-6, with ~0.5-1 perm interior-side vapor retardency- it takes more than standard paints, but doesn't need to be poly). Moisture buffering only has downside when it masks bulk-water leakage over long periods of time.

  2. dickrussell | | #2

    "Optimizing" glazing losses
    While windows and doors are indeed "thermal holes" in a high-R wall assembly, I would hesitate to use the word "dominate" when comparing total heat loss through windows/doors vs through walls in a properly design shell. If cheap, high-U windows are used or there is far too much glazing in the first place, then of course total heat loss through them will dominate.

    One of the uses of a good energy model for a house design is to show the distribution of losses through the various aspects of the shell. There is always the tradeoff between what can be done to reduce heat loss through some part of the shell vs how easily and inexpensively that can be done. Heaping loose insulation in the attic is easy to recommend, because usually there is the space to do it and the material is cheap. After that, it makes sense to address the biggest slices of the heat loss pie, then recalculate the distribution. At some point, you have both "low" total heat loss and "reasonable" distribution of those losses.

    In the case of my own house, the final model showed that about 20% of the total loss is through the walls and about 34% through windows and doors. While the latter certainly is the largest subtotal in the list, it's still only a third of the total.

  3. user-716970 | | #3

    What about the gains?
    Is it not true that very good windows can be net energy gainers ?? No opague wall no matter how well it is constructed can make that claim...

  4. Expert Member
    Dana Dorsett | | #4

    Sure, passive solar and solar-tempering works
    Selection of window type & size is indeed an important aspect of designing for minimum energy use, and vertical south facing glass is almost always going to be strong net-gain in zone-6 rather than a net loss, particularly if you keep it to double-panes and skip the low-e coatings for maximum solar gain. For low-e triple panes that effect is largely blunted. Simulating the gains and providing for sufficient interior thermal mass to limit overheating counts for a lot when looking at solar tempering at standard glazing fractions. It becomes critical when the south facing glazed area exceeds ~5% of the total floor area (rule of thumb.)

    Dick, in high-R buildings it's pretty common for windows & doors to dominate the heat loss figures for the whole house, but the house under discussion I was only referring to the walls, at typical glazing fractions (and at-code U values, not be$t-available.) Isolating the thermal mass with R8+ of EPS renders that mass relatively useless for moderating the more significant gains & losses from windows. Sure that mass does something and averages out the very modest peak gains & losses through walls, but going to 3/4" or 1" wallboard throughout the house can sometimes have as much or greater effect even though it's a fraction of the thermal mass of the concrete between the EPS in the exterior walls. To be maximally useful the thermal mass has to be "available", in thermal contact with the conditioned space.

    A low mass R40 wall will outperform an R25 wall of any mass or construction in US zone-6. But on the cellulose/fiberglass front, R40 of 3.5lb cellulose is going to have roughly the same thermal mass as 3/4" of gypsum, whereas the fiberglass would be much less. This is probably measurable in a lab, but probably not so much in a house (not enough to be a deciding factor.) The thermal mass of the cellulose will also vary significantly with seasonal moisture cycling, and will be affected by things like siding type/ventilation and vapor retardency of the interior & exterior wall materials & finishes.

  5. user-716970 | | #5

    Common wisdom
    The common wisdom around here has been that triple glazed,argon filled, high SHGC, low-e, will "net" more energy gain than clear double pane glazing and by a rather wide margin...
    Is there something I am missing here?

  6. Ted Clifton | | #6

    Let's get real.
    Regarding windows, on a recently completed 2,900 sf (HERS 42 w/o PV) home with 25% glazing area, we have 3.8 MM Btu/yr heat loss from the walls, 5.5 MM Btu/yr from the floor and ceiling (including foundation walls below the floor), and just 1.2 MM Btu/yr from the windows. The windows used are triple-glazed, with an SHGC of .43 on the south-facing windows, and .31 on the east and west. The U-values are around .20, depending on the configuration of the window. This house was designed to take full advantage of passive solar heat gain, in a Marine Zone 4 (we have zero cooling degree days, over 5,500 heating degree days). During the heating season, we are only able to gain heat for about eight hours per day, whereas we are losing heat for about sixteen hours per day. On sunny days, it doesn't take long for the solar gain to make up for the overnight loss from the windows, but when we have several dark, cloudy days in a row, the temperature drop over time is severe.
    Dana is right, thermal mass that is not in direct contact with the inside of the living space is useless, furthermore, thermal mass that contacts the outside is worse than useless in a heating climate, whether you are in Marine 4, zone 5, or any zone colder than that. We are employing 3" of concrete on the second floor of our homes. This thermal mass is contained entirely inside the building envelope, and it radiates in both directions, up and down. Without any heating system in operation, we are experiencing about one degree of temperature variation from day to night, when the outside temperature is varying by more than thirty degrees.
    For too many reasons to enumerate here, I would only build with ICFs in a hot climate, and if I were also in a hurricane or tornado area, I would build with nothing else. For the climate discussed in this article, I would recommend only SIPS for the north basement wall. A 10" SIPS wall with 5/8" gwb on the inside will get you about R-40, at a fraction of the cost of a double-stud wall. It is far easier to remodel than a stud wall. You will experience zero problems with moisture or condensation in the cavity, because there is no cavity. The ICFs will perform well for the buried foundation walls, but 4" of XPS foam on the outside of conventional concrete foundation walls will perform better, especially in a cold climate.
    For a decorative touch, try installing poly sheeting on the inside of your concrete forms before pouring the walls, you will get a very smooth surface. Then acid-dye the concrete to whatever color you want after you strip the forms. No studs, no drywall, no fuss, no muss! Just don't forget the 4" of XPS on the outside!

  7. Expert Member
    Dana Dorsett | | #7

    Thanks Ted!!
    Nothing like a real design to make the point. At 25% glazing fraction and additional interior mass to deal with it, that's a true passive solar design, not mere solar tempering. SHGC of 0.43 is pretty good for triple panes. I'm not sure how that's even possible in a U0.20 window, but I'll take you at your word. With fairly vanilla U0.30 code-max double panes you can get SHGC that high or higher.

    I like the R40 SIP option too, but I'm not convinced it'll always be a fraction of the cost of double studwalls. A lot of pencil on paper analysis has driven many to conclude the converse (usually without calculating in the "time is money" factor.)

  8. user-946029 | | #8

    Glad to see the discussion continue
    Didn't know this would get spotlight treatment, but that's fine. These types of discussions should occur in the open. What a great way to learn from each other.

    Wanted to pass along the latest from the engineer I referenced before. Anything in quotes are his words, not mine:

    "With ICFs, you don’t need to work hard to air seal; the concrete does it for you. This air seal also doesn’t degrade with time. If one builds a wood frame wall and is relying on a product such as Tyvek for an air barrier, the tape on the joints doesn’t last forever. Similarly, if someone is putting plastic behind the drywall (which drywallers absolutely hate), the airsealing at outlets and penetrations has to be perfect, and the taping at the wall/floor and wall/ceiling interface also has to be perfect and permanent. This doesn’t always happen … or if it does, is it permanent?

    The dirty little secret of the window ratings is that the window’s U-factor is measured at the center of the glass. This doesn’t account for frames (which, at most, are R-1 per inch of thickness) and air leakage between the frame and operable glass. They also test the windows at room temperature for leakage. If they tested at a range of temperatures, you would see vastly different air leakage values. The window rating doesn’t account for spacers between the layers of glass (they are typically metal, even if there is a 1 mm thick thermal break) and between-pane mullions.

    Here is my favorite window story. I went to look at a school in Washington state about 10 years ago. I was there for some non-window issues, but we got to talking about energy use. They proudly showed me triple pane high efficiency windows that they paid a fortune for. They claimed the R-value of the window was R-8.5. It was a cold day (maybe 10 °F outside), and I measured the temperature of the aluminum frame to be 38 °F. So much for the high R-value of the glass; all of the heat was lost through the frame! The glass may have been R-8.5 but the window sure wasn’t…"

    Looking forward to further discussion, everyone.

    P.S. Nice to cross paths again, Ted. It's been a while.

  9. user-869393 | | #9

    Does R Value trump Thermal Mass. Put mass inside!
    I live in Colorado, mostly a zone 6 but with a lot of sunshine and here in the mountains very cool nights. Passive Solar makes a lot of sense as does natural ventilation.
    With Passive Solar you need mass that is inside the building to absorb the heat so the ICFs isloated that mass with the insulation on both sides. Sirewalls and Concrete walls with the insulation in the middle of the wall section makes more sense. It exposes the mass to the inside yet also has a very durable surface on the exterior. Sirewalls are compacted soils which operate much as a concrete wall would. The exterior walls in my climate need to be heavily insulated but do not really need the mass if built in a durable fashion. Thus Sips or framed walls would make sense if the r values or close to R-40. The other benefit of the mass on the interior is for temperature swing moderation and humidity control. The mass can act as both a heat sink and a humidity sink.
    Michael Sanner, Architect

  10. user-946105 | | #10

    Lessons from the Great White North
    Up here in the great white north, some of us are starting to get away from ICFs in such a case. The solar heat effect on the wall doesn't really make a great impact when the air temperature outside is -30 F outside, the air strips heat P.D.Q. The benefits of the air sealing capabilities are great, but most of us would argue not better than wood frame.

    We like to use a system of double stud walls (2X6 exterior and 2X4 interior) or stagger studded 2X8 on 2X8 or 2X10 plates. Studs and sheathing are 'sandwich' caulked with Sika 15LM or 1a to create single 'bays' and then additionally we caulk the stud to sheating union from the inside all the way around. Bottom plates are caulked or spray sealed and all sheating joints are taped with Vycor flashed with Super 77 or sheating tape.

    With a setup like this I would argue that you would get a decent performance value out of glass insulation. That said, I wouldn't personally use it. Mineral wool is the better choice for installation and fire rating, but we almost always use spray foam or blow in. With the air sealing techniques, you eliminate infiltration. With double studding, you eliminate thermal bridge. If your drywally contractors are up to snuff and use the proper air sealing detalings, the only problem spots are openings to the exterior (e.g. lights) which can can be sealed with Hilti putty and spray foam, and windows.

    Lately, one of my engineers has been talking about his own renovation about to take place and how we in the residential world don't fully utlilise fenstration properly (he's a commerical enginner). He is always singing the praises of commercial windows and how much better insulated they are and available in larger sizes. I don't know much about the topic myself, but could commerical windows solve the one problem in the setup? Or at least eliminate part of the 'hole in the bucket' idea?Thoughts?

  11. GBA Editor
    Martin Holladay | | #11

    Response to Mike Collignon
    You wrote, "The dirty little secret of the window ratings is that the window’s U-factor is measured at the center of the glass. This doesn’t account for frames."

    Actually, the NFRC label that appears on most windows includes a U-factor rating that accounts for the frame. NFRC U-factors are NOT glazing-only numbers; they are whole-window numbers.

  12. Mike Eliason | | #12

    @Michael Haluschak

    @Michael Haluschak

    having used commercial windows on residential and institutional projects - i can unequivocally say the thermal performance is as bad, or worse, than residential non-high performance windows. you can get large windows for decent cost - but the performance isn't better than residential. also, there are curtain wall systems that make importing passivhaus-certified wood windows look affordable.

  13. watercop | | #13

    I 2nd the motion that ICF works well in hot humid climates
    ICF all the way to the roof does several good things in a hot humid hurricane climate.

    1) Termites don't eat ICF or concrete

    2) The insulation-surrounded thermal mass is really helpful in spring and fall when cool nights let walls chill the house during warm afternoons - no AC needed on days with 55 lows, 85-ish highs

    3) The thermal mass helps in high summer by delaying afternoon peak loads...ICF houses operate with smaller HVAC systems. Heat of the day is carried deep into the night, giving HVAC some sensible load to work with late at night, allowing late night latent-only load to be met without separate dehumidification.

    4) Reduced infiltration helps with humidity control.

    5) Thermal mass helps with the few "cold snaps" we do get - the mass carries a house through the few hours at 20*F late at night, allowing a smaller HVAC to carry the heating load without excessive strip use.

  14. user-1012653 | | #14

    Mike C-
    I do not think anyone on here will disagree with your engineer's findings on a typical commercial full aluminum window. Some of them we still use on schools on our projects are not even thermally broken. However for most homes, and especially in this topic, the windows would be fiberglass or possibly wood (or aluminum clad).
    "perminate" air sealing is a good point for ICF, assuming it was installed without any voids in the concrete. However with today's glues, foams, tapes and caulks I would still question the price vs full ICF or similar performing walls (again, in heating dominate climates only). It would be interesting to jump in a time machine and go 20 years later and rip apart a passive house and see how it is doing in terms of tightness.

  15. RMGheHhbJx | | #15

    As cost has not been a stated consideration, take the ICF all the way around and all the way to the top. The durability and reduced air infiltration along with the sound deadening characteristics have been universally appreciated by the scores of home owners I have built for.
    Also the statement," and air sealing is relatively easy to accomplish with wood framed walls" does not address the human factor unless you are planning on performing all of the work yourself. The importance of air sealing and "grade one" one level insulation installation is rarely achieved without reinspections and call backs to the trade contractor. Usually I have to get my spray foam and caulk out to touch up even after calling the trade contractor back in. Next, be sure to look at actual R-Value when comparing systems. For example, a standard 2x6 wall performs at some where between an R-12 and an R-14 after accounting for the poor R-Value of the wood. When you say R-40, are you stating the assembly R-Value or the R-Value of the thickness of the insulation.
    Lastly, water management details are another critical step when building near air tight. Plan on this and other details requiring in-detail understanding on your part and constant inspection for quality if you have someone else do the work. Avoiding moisture related damage to wood is paramount to our success in offering efficient structures to the point where all major efficiency programs are requiring the aspect to be verified before homes are being approved.

  16. user-755799 | | #16

    Response to Tyson
    You wrote:

    ...Also the statement," and air sealing is relatively easy to accomplish with wood framed walls" does not address the human factor unless you are planning on performing all of the work yourself. ...

    The same is true for ICF Walls. If you don't have a good trade contractor assembling the ICF's to your spec, you will have problems as well, some of them catostrophic--Think blowouts. I have been fortunate to have used a very experienced ICF contractor when building with ICF's. So the caveat works in all areas of construction.

    Water management is as true for ICF as it is for double stud or conventional framing. Leaking water on window bucks is as bad in ICF as it is for wood construction. Don't assume because the construction is concrete, water can't hurt the building.

    We acheive Level 1 insulation installation consistently--ask our energy rater.

    My caveat is there is no "best". It all depends on project needs, desires and budgets.

  17. RMGheHhbJx | | #17

    Response to John
    Point well taken. As to water damage, my focusing on the blocks and not the windows and doors didn't cover the whole issue. Also it sounds like your insulation contractors may be from the planet unusually good. Mine are from earth where insulation contractors mostly follow the industry standard of very poor quality control. It take relatively little research to confirm the overall chronic state of the insulation industry which is even echoed by their trade association of all things..
    I have many examples as an Energy Star verifier. I have to call back the insulators on 75% of the jobs to re-install part of their work. It used to be 95%(not making this up) but by repeated relentless insistence on achieving the program requirements for something approaching level one quality, they have improved somewhat. Equally as bad is that the inspectors WILL NOT inspect for the quality of the installation or that code requirements for air sealing are met. As a member of our local HBA, I am getting close to giving up on soft selling these issues and considering openly calling out the all the prominent builders and building officials who have their head stuck in the sand. The old argument that efficiency is driving up the cost of housing holds little truth anymore when by simply modeling your efforts around ROI you can almost universally lower your monthly cost of ownership.
    One last comment on air sealing ICF vs stick frame. We pour with Logix ICF blocks. As with many of the newer blocks, they are extremely strong and have the added internal corner supports. I haven't lost concrete on a pour for as long as I can remember and our blocks have insignificant lateral blockage to the flow of concrete. While pouring, the lateral movement of the concrete in the walls moves unrestricted making it easy(for me) to achieve superior air sealing through the ICF itself. This leaves me only the windows and doors as you pointed out; foam and caulk on the inside and flash and wrap on the outside. Much easier in our area than trying to get air sealing accomplished in two by four walls that are drilled by the next three trades, and that doesn't count the holes left by the electrical boxes and other penetrations through the sheetrock. I am sure it would help if you would pease beam me over some of your insulators and I will go to a happy place.
    All this said and my next house is at the city with the plans showing 2x4 walls with R-13 insulation and 2" of foam on the outside. You caveat rings true about no "best" in my world too. Our model here is to achieve superior performance for the same cost as standard construction. In this case we don't need the "Best", we just need to take control of the shell sufficiently to downsizing the HVAC with ducts in conditioned space. the off-setting savings cannot be achieved with the added cost of the ICF system.
    By the way, thank you so much for your reply. As you can see, it was stimulating.

  18. smalld | | #18

    Keep on topic, PLEASE!
    HELLO!!! This is a NORTH WALL in the NORTHERN HEMISPHERE. The most expedient form of wall he might well utilize would be a double stud with woven; rock wool, or possibly a Larsen Truss Wall; again with Rock Wool. Given this I would also recommend that he drastically limit the amount of glazing in this non solar-gain (north facing) wall and stick to triple glazing with all the argon and filming he can possibly acquire. And stay away from any kind of aluminum frames for the windows. I would suggest to those debating the cellulose vs fiberglass to openly investigate the option of rock wool tried and true for some 50 years or more in many parts of the more technically sophisticated areas of the globe.
    Yes, Solar storage should be inside an inhabitable structure, we were having these debates about it and empirically experimenting with them not just in the 1960's and 1970's but as far back as 1000 BC and they proved their worth even then.
    I would also suggest that residential designers and consultants SPECIFY not only the structural members/components but also the specify the standards of construction and details: which should include the due diligence (standards/expectations) of energy efficiency 'detailing' for the contractors

    "when astrologers eventually discover the center of the universe, there are going to be a lot of people that are going to be extremely disappointed it is not them"


  19. 7u8zfUR5vB | | #19

    Heating with Firewood is free for me, what is the ROI of ICF?lol
    People that have bad things to say about the insulation or construction industry need to put the keyboard down. You cant insult a working man from your PC, it makes you look worse than them. If you think the quality is that important on wall insulation aka @ 5% of the least important surface of your envelope for r-value, try learning more and stop being condesending. Funny how the insulators are actually the smart ones in this argument, so many tools getting their BPI or HERS or the worst LEED, worthless licenses and they are all being outsmarted by 7$/hour insulators that know time wasted on perfectly installing batts, is money wasted. Those gaps might look pretty on a thermal camera, but they don't cost you enough in energy to pay 20 more dollars for properly installed batting. Insulation inspectors are 99% a waste of air. Too many "book" nerds think they can get into construction or build homes and that they are going to reinvent the home energy "wheel". How long ago did we send a man to the moon? Building science has been EXCELLENT ever since then, and if you think it is WAY better now, with new cool products we didn't have then, your head has been in the sand for fifty years. They were smart enough then to know you base your requirements on COST, and RETURN. Back then energy was cheaper and requirements were lower. Triple pane windows, foam insulations, radiant barrier foils, supertight designs were all there fifty years ago, why hasen't home building changed much? Nobody wants to spend a dollar to save a dime! ICF, double wood walls, ARE spending a dollar to save a dime. Use Remdesign or talk to somebody that can do a Manual J, people with energy auditing licenses are usually fools you should stay away from, and if they have Dr. before their name, trust them the least. They will have never touched a house during construction phases, everything they "know" came from a book, and when they get that peice of paper they become gods in their own minds. I would take a framer with 4 years field experience to build/design my house over ANY Dr. Thermal mass, and the difference between ICF and double wall. Don't wast your time nit picking what could be less than 5% of your envelope. And remember, at 200$/sqft. ICF can cost you 20,000$ in just lost floor space. Realtors measure the outside perimeter, than take the thickness of the wall for SQFT. So when you pay for a 2050sqft home, ICF gets you a 1950sqft home, ICF won't save near the money that you lose in living space just by using ICF, not counting the rediculous costs of construction with ICF.

  20. 7u8zfUR5vB | | #20

    Tyson, shut up and stop talking home energy for the greater good
    You really think homes should be tighter? Do you think if every house in this country were super tight, and had 'controlled" ventilation that every house would be seeing equal or better than .35ACH? Or would you be killing millions of people slowly becuase of broken fan motors, improperly designed fresh air, tainted single source of fresh air, dirty filters not allowing enough fresh air. I am sure as a Energy Star Verifier you are pure gold. But for ASHREA standards on health, STFU! Supertight homes will likely harm people, but you saved 15$/year from less "air infiltratin", good job! If you don't PRIORTIZE health, burn any licensing you have it is worthless.

  21. 7u8zfUR5vB | | #21

    To all you energy nerds
    using spray foam around windows, ANY spray foam will void their warranty. This could cost tens of thousands of dollars. Stop recommending it please.

  22. GBA Editor
    Martin Holladay | | #22

    Response to Greg Hecker
    I'm sorry, but you are wrong. You wrote, "Using spray foam around windows, ANY spray foam will void their warranty." Marvin Windows, for example, like most window manufacturers, advises the use of low-expansion spray foam in their window installation instructions. These instructions are online here:

    The Marvin instructions state, "Step 6: Insulating and Sealing the Installation. 1. From the interior apply a 1″ - 2″ (25-151) thick bead of low expansion foam insulation on the back side of the exterior casing. See figure 15."

    The illustration is shown below.

  23. E47mJdAip6 | | #23

    Efficiency is one part of the ICF story
    As some have mentioned, there are a number of other reasons why ICFs are a wise investment. Energy Efficiency is just one slice of the pie.

    It's interesting to me that there is such one-sided conversation, assumptions about "the" purpose of thermal mass. Reradiating heat in a passive solar design is one. That's not a great fit for ICF, as has been pointed out above, but that does not mean that it makes poor use of its thermal mass. The concrete is connected all the way to the ground, which is a relatively constant temperature. Granted, heat doesn't travel quickly in concrete, but because of its ability to store a lot of it, its constant connection to the ground makes above grade ICF construction have some qualities similar to the thermal consistency of below grade construction. The thermal mass in an insulated sandwich provides two things, among others. One, it makes it a pretty simple process to build a finish ready wall with attachment points and insulation already in place. Once you train your crew well, or find a good installer that knows how to move fast, with vigilance (is that unique to ICF?), you get a more complete assembly faster, with a sam hill of a lot less pieces, junctions, seams, and punctures, all necessary components of a decent (or any) wood assembly, and all potential points of compromise for your assembly. Two, you get an insulated buffer which greatly slows heat gain and loss. Who cares if you're not getting the passive solar gain? I'm not designing a passive solar ICF home. I can tell you that load calculations (admittedly that include great windows, properly installed) that result in ratios of 2400-3000 sqft. per ton do not lie.

    But, again, energy efficiency is just one slice of the ICF pie. It's not a perfect solution (nothing really is, across the board), of course, and they are not idiot proof in installation, as was also mentioned. When it comes down to longevity, comfort, health, safety, peace and quiet, a no-cavity wall of insulated monolithic reinforced concrete is worth hanging your hat on. How much value does disaster resistance really have? Clearly, not everybody is thinking about all of those things. If the value doesn't overlap with the needs and desires of the customer (or climate or geography), they will not pay for it.

    As far as real estate, ICF clients tend to be a different breed. They tend to be building forever homes. Not many folks want to sell their ICF homes once they are there. It does upset the real estate equation a bit, but for some folks, they simply aren't going to let that dictate what value they build into their homes. The right envelope is the foundation of a home that performs as it lasts. There are plenty of builders that can build $135/sqft ICF homes. They may not want to, but they can.

  24. 8MHFDgVVsu | | #24

    concrete - high energy to manufacture & transport
    ICF walls use concrete, which has its place, but concrete takes a HUGE amount of energy to manufacture. For this reason it is not the greenest material choice for applications where its features are not needed or can be achieved with a greener material.

  25. Michel Rochette | | #25

    Thermal Mass
    Take tow portable ice chests,cut and remove the bottom of one of theme and place the chest on a concrete slab, now put the same amount of ice in each cooler and wait...In some places,Thermal Mass is only better than insulation when you have a 2 feet thick wall.

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