From practical experience, all i can say is that using an asymmetrical ICF of 2" interior and 4" exterior thickness, and then an ICF poured deck system on two levels gives a significant thermal mass gain. Then add in the air infiltration gain with using the ICF as against stick built, and there is no comparison - especially if special attention is paid to seal around windows using something like Tremco illmod trio tape.

I agree with Eric. But to take it further, and specific to ICF in the question, a few other things are more pertinent. The daily temp fluxuation is very important. If it's a wide flux, mass storage becomes more optimal. That is, if it's a constant temp of 58 degrees outside, day in and day out, That wall will be 58 degrees, and you'll need to insulate from it the same as if it wasn't there.

Next is where that fluxuation is. If it is 100 in the day, and 60 at night, this is where the thermal mass comes in to play. And the more mass, the better. In a simple example, you would have an average of 80 and would only have to insulate for that temp.

Then whether it gets hit with a lot of sun. If the average temp is 70, but the searing sun is on it, you'd want to insulate it on the outside to keep the heat off.

Then, what is the soil temp and its exposure at the footing. You can get a lot of heat transfered to/ from the soil through the concrete. If the avg temp outside is 30 in winter and 90 in summer, and the soil is 70 degrees, you'd want to be insulated from the outside and take advantage of the soil temps and leave the interior un-insulated.
But in general, it seems that the greater the temp fluxuation is, and the closer that average fluxuation is to the desired temp, the more benefit you'll get from thermal mass. Of course you'll have to consider sun exposure and soil temp.

Me? I'll stick with knowing that the energy bills will be better regardless, and energy is only one of the MANY benefits of an ICF home; and I'll leave the theoretical number-crunching to the professors who have time for that stuff.

Upon further review, I'm going to agree with Eric, Dana, and with Jerry's post above. The paper noted by me before was found online and was conducted by an Engineering professor, so I felt compelled to throw it up for review and consideration. It apparently got a great deal of both! I'm not a huge number cruncher either, I try to leave that those that are better at it than me.

Ren

Posted By wes on 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.

Hey, I refrained from the electronic analogy of a low pass T-filter for the daily/hourly load swings, with a DC resistor to the center capacitor node modeling the heat flux to/from the subsoil, complete with the frequency response curves!

But it models fairly accurately that way.  Models even better if the C is a capacitor with an internal resistive impedence (a relevant secondary effect, since it's only ONE order of magnitude lower than the impedance of the EPS layers, not two.)

OTOH, the shift in R-value of the EPS with temp also needs to be included since the temperature coefficient of EPS is significant.  The labeled R-rating of EPS is @75F (per FTC requirements) but it is ~20% higher at 25F, and ~10% lower at 110F, which would be a bigger secondary effect than the heat conducted to the subsoil in many/most installations.

If insulated on only the exterior it's essentially an RC filter as far as the daily temperature swings of gain/loss through the insulation,  but the R-value of the concrete is still high enough relevant for changes in load occurring on the interior (from gains/loss from glazing or changing levels of energy use inside the home.)  The peak & net heating & cooling loads on the interior will always be lower if the thermal mass is entirely within the thermal envelope.

We live in an ICF house with passive solar design. We have cold winters and hot summers, and the thermal mass works better than I expected. To have "enough" thermall mass we used ICF 6" core and concrete floors (on slab and on Hambro). Works great! Warm in the winter and cold in the summer. I heat in the winter just few hours at night, if there is sun the next day. In the evening, when I come from work, I have 74F in the house.

Posted By eq1 on 16 Nov 2010 09:49 PM
We live in an ICF house with passive solar design. We have cold winters and hot summers, and the thermal mass works better than I expected. To have "enough" thermall mass we used ICF 6" core and concrete floors (on slab and on Hambro). Works great! Warm in the winter and cold in the summer. I heat in the winter just few hours at night, if there is sun the next day. In the evening, when I come from work, I have 74F in the house.

A few questions:

What are your interior & exterior R values on the ICF?

How much concrete do you have in the floors?

Where are you located? (or heating degree-day & cooling degree days for your climate)

What are your sub-slab & attic/roof  R-values?

How does the place fare in summer?

Hi,
This could be nice and great idea..
Thanks for the links you shared with us here..it really help...

Posted By eq1 on 16 Nov 2010 09:49 PM
We live in an ICF house with passive solar design. We have cold winters and hot summers, and the thermal mass works better than I expected. To have "enough" thermall mass we used ICF 6" core and concrete floors (on slab and on Hambro). Works great! Warm in the winter and cold in the summer. I heat in the winter just few hours at night, if there is sun the next day. In the evening, when I come from work, I have 74F in the house.

This is Solar Gain not Thermal Mass!

In the Michigan climate, we have found the the concrete core of an ICF is best used to buffer temperature swings, but if interior thermal mass is needed, it is quite effective to use an interior concrete floor. The ICF's with the thermal mass shifted (Logix was not mentioned, but their XRV panels allow this) are usually quite a bit more expensive than a concrete floor, and less effective per square foot of surface area.

If a floor (or an active water based storage system) isn't enough and you want the thermal mass where it can do the most good, then you may want to look at tilt-up construction.

Posted By Viking House on 28 Nov 2010 05:24 AM

Posted By eq1 on 16 Nov 2010 09:49 PM
We live in an ICF house with passive solar design. We have cold winters and hot summers, and the thermal mass works better than I expected. To have "enough" thermall mass we used ICF 6" core and concrete floors (on slab and on Hambro). Works great! Warm in the winter and cold in the summer. I heat in the winter just few hours at night, if there is sun the next day. In the evening, when I come from work, I have 74F in the house.

This is Solar Gain not Thermal Mass!

It's actually both Viking House. Solar gain is usefull by itself in the right doses, so is thermal mass. But a good passive solar home strikes a combination of both, with a mind to plentiful insulation levels as well.

For example if eq1 did not have the thermal mass of the slab  or the ICF wall, but still had the same southern glazing exposure, the house might well have been around 80+ degrees that evening after work. Though that may feel nice coming in from the cold, it would soon feel too hot and you'd want to open a window. Then shortly after the sun went down the indoor temperature would drop and you would need mechanical heating sooner in the late evening-night. The thermal mass tempers the solar gain and allows the gained heat to be "stored" for use in the early night, only requireing heat for a few hours each night. It sound \s like EQ1's home could use a bit more mass or insulation in the right places to help carry through the night longer, but overall it's a pretty ideal senario.