Looks like the 2012 IRC is requiring R-10 underground insulation for a slab-on-grade build in Zones 4+

2012 IRC Thermal Code

It also gives a different R-Value requirement for mass walls and wood framed walls. The mass wall minimum is R-8 through R-13 while the wood framed is minimum R-20 (Zones 3-4-5). Almost all modern day ICF's are R-23 so they will pass code all to way up to Zone 8, which requires a minimum of R-19 for a mass wall and R-25 for a wood framed wall.

What is their reasoning behind this? I deduced that it is the thermal mass that is being taken into consideration.

Curious. This code relaxes insulative requirements for mass walls in my part of the world, and probably most everywhere else. Mass equivalency calculations using the Washington DC climate generate R12 as the required nominal R value. This code says R8 for zones 3 and 4. Washington is northern Zone 3.

Be sure to read footnote "i" The second number for mass walls applies if "more than half" of the R value is located on the interior of the wall. One suspects that ICF qualifies for the first number, although it really doesn't matter. What should matter to ICF homeowners is the code's acknowledgement that foam placement is a major factor in mass effect; that ICF builders in climates like the Southwest that are particularly suited for thermal mass are probably better off with less nominal R value; and that the ICF industry gives them zero help in determining how much less.

Also, I read the slab section as perimeter requirements, as in zone 8 or higher requires R 10 under the outermost four feet of the slab.

I did some checking and I was told that the 2012 IRC recognizes & takes into account ICF/ mass walls in two ways: it's thermal mass & the lack of thermal bridging that comes with a mass wall. That is why the 2012 IRC reduces the R-Value amount for a mass wall while it increases it for a wood framed wall.

For Zones 1-4, the insulation requirement for wood framed walls is double that of a mass wall. For Zones 5-8, that variance gets smaller but it's still 7-10 points lower than wood framed.

99% of all ICF have equal insulation on both sides of the form. Even with the addendum note, "The second R-value applies when more than half the insulation is on the interior of the mass wall." This wouldn't apply to 99% of all ICF out there but even then the R-Value increases by 5 points and is below R-21 in all but Zones 7 & 8.

Posted By Lbear on 05 Jan 2013 03:15 AM

It also gives a different R-Value requirement for mass walls and wood framed walls. The mass wall minimum is R-8 through R-13 while the wood framed is minimum R-20 (Zones 3-4-5)...

What is their reasoning behind this? I deduced that it is the thermal mass that is being taken into consideration.

Read the footnotes.  Part of the reason behind this is that the wood framed wall R value given is for the cavity insulation value, not the whole wall R value.  A typical 2x6 framed wall with R19 cavity insulation actually has a typical whole wall R value of R14-R15 due to the lower thermal resistance of the wood framing across the cavity.  The other code allowed wood framed option is R13 cavity + R5 sheathing.  This is because a typical R13 cavity insulated 2x4 wall has a whole wall R value of ~R10.  Add the R5 exterior sheathing and you are at ~R15 whole wall.  With mass walls, the insulation is continuous and not short-circuited by the low R structural components.  Another reason is better air sealing typically provided by mass walls vs. wood framed walls, so the net effect is less heat transfer with a mass wall even with lower R value components.

Posted By Lbear on 05 Jan 2013 12:43 PM

99% of all ICF have equal insulation on both sides of the form. Even with the addendum note, "The second R-value applies when more than half the insulation is on the interior of the mass wall." This wouldn't apply to 99% of all ICF out there but even then the R-Value increases by 5 points and is below R-21 in all but Zones 7 & 8.

I don't think that footnote was intended for ICF walls.  It most likely was intended for poured concrete and CMU block walls where all or most of the insulation is on the inside of the wall, i.e. a 2x4 framed R13 insulated cavity wall built inside the concrete wall.

As I wrote above, the reasoning behind publishing two minimums for mass walls should matter to folks considering ICF in some climates even if the nominal R value of the typical ICF wall exceeds requirements everywhere. If IRC 2012 requires a higher nominal R value when interior insulation is 51 percent of the wall's nominal value, why would you assume that the degradation of mass effect will be significantly less at the typical ICF mix of 50/50?

More to the point, how do R16 forms compare to R23 forms in a high desert climate, given less degradation in the former of the considerable mass effect there? Apparently, the answer is an ICF industry secret. Call me silly but I'd want all of the facts before building an ICF home in, say, New Mexico.

For a Zone 4 or 5 climate, insulating a slab-on-grade with R-10 foam, what would the ballpark cost of that be (labor and foam) for let's say a 1,500 square foot slab?

The foam would have to be capable of holding heavy 2-story structures, including concrete structures. How much psi can EPS foam hold? What would happen if some rodent decided to burrow through the foam and then you have a slab that is unsupported?

How much of a noticeable difference would it make having an insulated R-10 slab in a Zone 4 or Zone 5 area?

Posted By toddm on 05 Jan 2013 02:45 PM
As I wrote above, the reasoning behind publishing two minimums for mass walls should matter to folks considering ICF in some climates even if the nominal R value of the typical ICF wall exceeds requirements everywhere. If IRC 2012 requires a higher nominal R value when interior insulation is 51 percent of the wall's nominal value, why would you assume that the degradation of mass effect will be significantly less at the typical ICF mix of 50/50?

More to the point, how do R16 forms compare to R23 forms in a high desert climate, given less degradation in the former of the considerable mass effect there? Apparently, the answer is an ICF industry secret. Call me silly but I'd want all of the facts before building an ICF home in, say, New Mexico.

Even with the second number 99% of all ICF forms would still pass the code all the way up to Zone 8, which is basically the Arctic climates of Northern Alaska and interior Northern Canada. The way it stands, 2012 IRC recognizes & approves 99% of all ICF's all the way up to the arctic Zone 8 climates.

Your argument should be taken up with the 2012 IRC Committee Board, Scientists and Engineers. They had scientific reasons behind their thermal codes. You can choose to reject their findings but the code is pretty clear in what it states. They separate mass walls from wood framed walls and allow mass walls to have lower insulation needs because they recognize the benefits of mass walls. If they used the ORNL Thermal Mass Dynamic R-Value Equivalent, then an ICF form with a 5.25" EPS panel x 4",6" or 8" of concrete would be equivalent to an R-32.

No secrets here, taken from the Fox Blocks Data Sheet:

Energy Efficiency Data & Performance:
* Thickness of the EPS.………………………………………………… 2.625" / wall panel (5.25" total EPS thickness)
* EPS Steady State R-Value (thermal resistance of the material)…. R - 23 (R - 4.55 / inch @ 40 degrees Fahrenheit)
* CTL Group Thermal Resistance R-Value Calculation Report……. R - 23+ calculated in accordance with ASHRAE 90.1
* EPS K-Factor (thermal conductivity of the material)………………. K - 0.22 / inch @ 40 degrees Fahrenheit
* Air Leakage (infiltration rate).….…….……………………………….. 0.05 to 0.10 ACH (average air changes / hour)
* ORNL Thermal Mass Calculator Dynamic R-Value Equivalent…... Greater than R - 32


If you disagree with the above Data Sheet then you should file a complaint with the FTC and report them for false R-Value claims. Argue with the FTC and ORNL.

You know exactly what I mean, Lbear because we have argued this point repeatedly: Mass effect works best when the mass is exposed to the home's interior rather than tucking it behind 2.5 inches of foam, and its corollary, if you insist on the latter, less foam is better than more foam.

I have no reason to quibble with IECC 2012 because its use of a second, higher nominal R value for homes that have largely interior insulation makes this exact point. What's more, you keep trotting out Fox Block's once-size-fits-all nod to mass effect as if it would be of actual help in determining the optimal ICF wall thickness in Arizona.

GBT readers are not as dumb as you think. Of all the possible responses by ICF consumers (Yeah but.... So what... ) obfuscation is curious, and once again begs the questions of who you are and why you're here.

You know exactly what I mean, Lbear because we have argued this point repeatedly: Mass effect works best when the mass is exposed to the home's interior rather than tucking it behind 2.5 inches of foam, and its corollary, if you insist on the latter, less foam is better than more foam.

I have no reason to quibble with IECC 2012 because its use of a second, higher nominal R value for homes that have largely interior insulation makes this exact point. What's more, you keep trotting out Fox Block's once-size-fits-all nod to mass effect as if it would be of actual help in determining the optimal ICF wall thickness in Arizona.

GBT readers are not as dumb as you think. Of all the possible responses by ICF consumers (Yeah but.... So what... ) obfuscation is curious, and once again begs the questions of who you are and why you're here.

You know exactly what I mean, Lbear because we have argued this point repeatedly: Mass effect works best when the mass is exposed to the home's interior rather than tucking it behind 2.5 inches of foam, and its corollary, if you insist on the latter, less foam is better than more foam.

I have no reason to quibble with IECC 2012 because its use of a second, higher nominal R value for homes that have largely interior insulation makes this exact point. What's more, you keep trotting out Fox Block's once-size-fits-all nod to mass effect as if it would be of actual help in determining the optimal ICF wall thickness in Arizona.

GBT readers are not as dumb as you think. Of all the possible responses by ICF consumers (Yeah but.... So what... ) obfuscation is curious, and once again begs the questions of who you are and why you're here.

For the sake of completeness, if you insist on a steady indoor temperature (ie, will use hvac to get it) or want to use thermostat setback on a regular basis, then thermal mass on the outside beats thermal mass on the inside (in both cases, not covered with foam).

Posted By toddm on 07 Jan 2013 08:46 AM
Mass effect works best when the mass is exposed to the home's interior rather than tucking it behind 2.5 inches of foam

Agreed but covering it with 2.5" of foam does not make the mass wall of zero effect. It doesn't go from being "best" to having no effect simply by having 2.5" of foam on it. It reduces the dynamic thermal mass effect but it does not eliminate it. According to 2012 IRC, it reduces it by around 1-5 points (depending on climate zone).

Posted By toddm on 07 Jan 2013 08:46 AM

I have no reason to quibble with IECC 2012 because its use of a second, higher nominal R value for homes that have largely interior insulation makes this exact point.

The 2012 IRC/IECC assigns the higher R-Value number by 1-5 points. That clearly shows that having more than half the insulation on the inside of the mass wall reduces the R-Value by 1-5 points. It doesn't make it ZERO like you believe it does.

Posted By toddm on 07 Jan 2013 08:46 AM
What's more, you keep trotting out Fox Block's once-size-fits-all nod to mass effect as if it would be of actual help in determining the optimal ICF wall thickness in Arizona.

The data given shows that 2.5" of foam with 6" of concrete and then 2.5" of foam, has a thermal mass dynamic R-Value equivalent of R-32 or higher.

Building a guarded hot box and testing the wall assemblies based on different climate zones is not an inexpensive venture. We area talking tens of millions of dollars here. For now, using the ORNL standard is both legal and FTC legitimate for assigning an R-Value equivalent for ICF thermal mass. You can accept it or reject it, but it is valid, even with 2.5" of foam covering the interior mass. ORNL recognizes it, as does the 2012 IRC.

Degradation(n) -- A decline to a lower condition, quality or level. See any mention of zero, Lbear?

Helpful(a)-- Providing assistance, useful. Again, Lbear, tell us how "greater than R32" helps you optimize mass effect in AZ. BTW, Fox had to run all eight climates in ORNL's calculator in order to summarize. So why 32 plus? (Two possible answers: A, to avoid undercutting the many contractors tossing around wholly specious R value equivalencies; B, to cover the fact that there are better, cheaper ways to use mass in the desert Southwest.)

Jonr, while allowing temperature variations increases the efficiency of mass effect, it is not requirement. Using a normal thermostat setpoint will still save energy. The Delta T will be smaller in a mass house because it averages out extremes in daily temp. And you'd still want mass exposed to the inside.

In practice, there is probably less overall variation in a mass house. You'd set your Tstat for heating or cooling in spring and fall in the Midwest, and summer or winter in the high desert, pick one. But for part of the day you'll get the opposite condition and some change from your setpoint, Of course, even people in poorly insulated houses will simply tough it out. So there you have it, jonr: High mass, the house for weenies.

Posted By Lbear on 05 Jan 2013 03:15 AM
Looks like the 2012 IRC is requiring R-10 underground insulation for a slab-on-grade build in Zones 4+

2012 IRC Thermal Code

It also gives a different R-Value requirement for mass walls and wood framed walls. The mass wall minimum is R-8 through R-13 while the wood framed is minimum R-20 (Zones 3-4-5). Almost all modern day ICF's are R-23 so they will pass code all to way up to Zone 8, which requires a minimum of R-19 for a mass wall and R-25 for a wood framed wall.

What is their reasoning behind this? I deduced that it is the thermal mass that is being taken into consideration.

An R20 wood frame is ~R13-R14 whole-wall with the thermal bridging of the framing factored in.  In locations where R20 center-cavity is code, R13 + 5 continuous insulation (c.i) is also code, and  also works out to ~R14. 

It's really not a very different minimum U-factor, since mass-walls are by definition c.i..   It's not a different "whole-wall" R-value at all- it's the same!

An R25 studwall (2x8 w/ R25 fiber) with a 20% framing fraction comes in at ~R18, whole-wall, which is slightly lower than the R19 c.i.specified for mass walls. (But are we going to argue that a U-factor of 0.056 is dramatically different from a U-factor of 0.053? It's within the margin of building and actual framing design differences.) But looking at the chart it's NOT an R25 specification for framed walls, it's R20 + 5c.i., which works out to R19 , which is exactly the same.

For zone 4 and south, the whole wall numbers are quite different. In the case of no interior insulation, it's R8 for the mass wall vs R13 for stud wall in 3 and 4 . There is logic here. Mass isn't overly effective in cold climates.

Posted By toddm on 08 Jan 2013 12:44 PM
For zone 4 and south, the whole wall numbers are quite different. In the case of no interior insulation, it's R8 for the mass wall vs R13 for stud wall in 3 and 4 . There is logic here. Mass isn't overly effective in cold climates.

In zones 1 - 3 the rationale for R13 cavity fill  (R9-10 whole-wall)  is well over the mass-wall min-spec only because that is that it is the lowest practical cavity fill value that delivers a U-factor performance that meets or exceeds the values for mass-walls.  It's not a cost or overall performance factor, it's a minimum performance factor.  R8 whole-wall isn't a dramatically lower than R10 whole-wall.  An R11 batt solution comes in just shade over R8 whole-wall when the framing factor is 25% if it has wooden siding, but a shade under R8 if stucco or vinyl. It's different, but not too different in zone 2.  

In zones 3 & 4 the studwall requirement is R20 (which is ~R13-14 whole-wall), and identical with the R13 c.i. requirement for mass walls when most of the insulation is on the interior side of the mass.  But they'll allow R8 c.i. if over half the insulation is on the exterior, which is a real difference.

In zone 5 the prescription for timber frame is still R20 (R13-14 whole-wall), but they require R17 on c.i. for mass walls with most of the insulation on the interior(?), R13 if most of it's on the exterior.  This means they're requiring  higher whole-wall R for the mass wall than the studwall if most of the insulation is on the interior, but the same whole-wall R when most of the insulation is on the exterior.  (This seems a bit odd to me, but that's the way it's written in the table.)

From an ICF point of view it hardly matters, since all but the most minimalist R16  ICF meets or exceeds code in zones 1-6, and it's not a big bump for zones 7 & 8.

Posted By Dana1 on 08 Jan 2013 01:23 PM

From an ICF point of view it hardly matters, since all but the most minimalist R16  ICF meets or exceeds code in zones 1-6, and it's not a big bump for zones 7 & 8.

Exactly. For the 99% of all ICF's that are used today, the 2.5" EPS x concrete x 2.5" EPS will pass the 2012 IRC all the way up to Zone 8. The 2012 IRC recognizes mass wall benefits.

Don't forget that code requirements are minimums, kind of like graduating with a 2.0 in school. Why anyone would build with code minimums today in a cold climate is beyond me.

YMMV

Eric