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ICF R-value

I have a choice to make between ICF and conventional build with exterior foam.... My builder has a block available to him that is rated at an R-30. Or I could build 2 by 6 with BIBS and exterior foam to an R-38 value ...

I am in Peterborough, Ontario, which is considered a climate 6...

The builder is experienced in both methods of construction and is convinced the ICF R-30 will outperform the traditional method of construction at very little cost difference ...

His blower door tests are always less than 1 ACH50...

What is the better way? I plan to heat with 2 mini splits in a 2100 sq. ft. storey-and-a-half on a slab on grade...

Thank you,
Bob Holodinsky

Asked by bob holodinsky
Posted Tue, 01/07/2014 - 11:07
Edited Tue, 01/07/2014 - 12:10


6 Answers

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First of all, I recommend that you double-check the math on the ICF blocks. Expanded polystyrene has an R-value of about R-4 per inch, so that R-30 ICFs would need to have two layers of 4-inch thick foam, one on each side of the concrete. If the ICFs are that thick, that's great.

Some ICF manufacturers have been known to exaggerate, and report "equivalent R-values" -- a meaningless term. So it's worth double-checking the foam thickness.

I don't think that one method of construction is better or worse. Here are some comments:

1. If the R-values are accurately calculated, R-38 will perform better than R-30.

2. Electrical wiring is easier in wood-framed walls than ICF walls. Talk to your electrician.

3. Remodeling is easier in a wood-framed house than an ICF house. Cutting in a new door through an ICF wall is possible but expensive.

4. ICF homes are more resistant to wind storms and earthquakes than wood-framed homes.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 01/07/2014 - 12:04

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Thank you Martin ...I will get more specific with him and get more information...regards,Bob

Answered by bob holodinsky
Posted Tue, 01/07/2014 - 12:11

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The better ICFs will have options for putting 2.5" on one side and more on the exterior, to better utilize the thermal mass of the concrete by putting it further inside the building envelope.

If the R40 is some dynamic mass modeling number, it's pretty bogus unless it was modeled for your exact climate and site factors. In a climate zone 6 a typical 2.5" + 2.5" ~R22 ICF might have a dynamic performance of R40-ish from a peak-loading point of view, but not from an energy use or average load point of view, so make sure to understand exactly what they mean when throwing out such numbers.

Also note that code-min center-cavity R20 2x6 walls are actually about R14-15 after factoring in the thermal bridging so to compare apples-to-apples you need to know exactly what the "whole-wall" R of the wall you're comparing it to is. An R20 ICF WILL outperform an R20 nominal center-cavity 2x6 wall by good margin, on both peak and average performance. A dynamic-mass-modeled R40 ICF will not outperform a true R40 whole-wall framed assembly by any thermal measure, but it will on structural strength. From a thermal performance point of view ICFs almost never beat high-R framed assemblies on a bang-for-loony basis, but the structural capacity and other values can tip the balance toward choosing an ICF approach.

Answered by Dana Dorsett
Posted Tue, 01/07/2014 - 13:34

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Thanks Dana ...the 2by 6 insulation was quoted to me asR22 and the 4 inches of foam at R4 per inch as R 16 for a total of R38 ..Does thermal bridging enter into the equation with the exterior foam present?

Answered by bob holodinsky
Posted Tue, 01/07/2014 - 14:22

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The thermal bridging ALWAYS counts. Assume the R22 batts or whatever comes in at R15, the 4" foam gets added pretty much at it's rated R, unless you go hog-wild with 1001 timber screws instead of 24" o.c.

If the4" exterior foam was EPS (R4 x 4= R16) and the center cavity fiber was R22 rock wool (R14 after thermal bridging), the center cavity value might be R38, but the actual average performance of the wall is about R30 due to the thermal bridging. That's a pretty good wall, and it's unlikely that even with optimal site factors an R22 ICF would outperform it on heating energy use, though the ICF might come to near-parity. An R22 ICF may or may not come close to R40 performance from a peak-load point of view, but that peak-load condition is pretty much a "who cares?" situation- since windows will likely dominate the peak load numbers anyway- the effect of wall-R on the whole-house load isn't nearly as dramatic as in a code-min wall, or an older 2x4 wall (R10-ish whole-wall, if insulated.)

For VERY large houses the mass wall construction might make a difference in the HVAC equipment sizing, but most R30-whole-wall low mass houses would have heat loads below the output of the smallest gas furnaces out there.

Answered by Dana Dorsett
Posted Tue, 01/07/2014 - 15:34

Helpful? 0

As always ,thanks for very good and precise explanations from both of you...

Answered by bob holodinsky
Posted Tue, 01/07/2014 - 17:35

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