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What is the best cold climate basement strategy?

We are building a new home in the Black Hills of South Dakota, about 20 miles south of Rapid City. The floor plan is nearly finished. The home is a ranch style with a partially finished walk out basement. There is a small loft area for an office above the mudroom/ laundry/ pantry. There will be about 4,150 sq. ft. finished. The home will face due south.

We have pretty well decided on SIP construction for the above ground portion - 61/2 inch walls and 81/4 inch roof, polyurethane. My big concern is the below grade portion of the house. Although I don't want to throw money away, I do want to make sure it is done right and well sealed. Therefore, I'm leaning towards vertical ICF's. Is that wise, or should I go with a standard poured wall with four inches of hard insulation on the outside and two inches on the inside that would join up with the insulation under the basement slab?



Asked by Tom Hanke
Posted Sat, 02/15/2014 - 15:30


9 Answers

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IMO, either will work if the R values are similar. We use standard 8" poured walls with 4" of interior polyiso which is less expensive than the same R value using ICFs.

Answered by Bob Irving
Posted Sat, 02/15/2014 - 17:08

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I agree with Bob. Either approach will work, so compare costs.

Remember that any approach that includes exterior foam (including ICFs) will require the installation of a material to protect the above-grade portion of the exterior foam. That's why many builders choose to install all of the basement wall insulation on the interior.

For more information, see How to Insulate a Basement Wall.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 02/16/2014 - 07:01
Edited Sun, 02/16/2014 - 07:02.

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Thanks for the help Bob and Martin. If all of the insulation is placed on the interior wall, does that create any problems with moisture migration, etc. Also, by putting most of the insulation on the interior, I assume it eliminates the possibility of capturing any of the thermal mass of the wall. BTW, we do plan on using some phase change thermal mass in the house so maybe the loss of the wall thermal mass is a mute point. What do you think of phase change thermal mass?

If you were going to build this home, where would you put the insulation on the basement assuming cost was not an issue?



Answered by Tom Hanke
Posted Sun, 02/16/2014 - 09:43

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There are pros/cons to either approach. The link Martin provided covers them well.

Generally speaking, in my view, and if cost is not an issue, I prefer exterior. Mainly because it allows for a continuous thermal break at the rim joist, and any area of exposed concrete above grade. That said, lets look specifically at your house, and options.


Here, you will have to address the the issue of your above grade wall sheathing (the outer OSB layer of your SIP) and exterior rigid insulation not being co-planar.

You mentioned 6" for your basement walls. Assuming your SIPs are flush with your rim joist/foundation wall, a 3/4" rain-screen gap and siding, this would result in exterior rigid foam being close to 5" proud of your siding. Probably not what you had in mind. Aesthetics aside, you would need to flash this ledge, and any exposed foam above grade. You will also have to protect below grade foam during backfill from damage.

Going with some insulation inside and some outside will reduce this "bump out" of course.


Here you lose 6" of floor space all the way around your basement. Maybe this is a big deal, maybe not. Only you can decide that.

More important than square footage is how to detail your control layers so they are as continuous as possible at the rim joist. How will you insulate this location, and how will your air and water barriers connect?


As for using phase change materials, they are exciting - but - I would say they are not the lowest hanging fruit at this point in your design phase.

I suggest you arrive at an envelope design you are happy with first. A design with solid, buildable details. Do your heat loss calculations. Then fine tune it with PCMs if you like. I say this because, in my opinion, the gains from PCMs will not allow you to radically alter your HVAC design, or slash insulation levels.

Moreover, PCMs require careful energy modelling in my opinion. You have not told us if you are doing this, or have engaged someone to do it for you. While it sounds simple on the surface, there is a lot to consider. My understanding is that PCMs are designed to store/release energy (heat) at set temperatures. This temperature must be selected with care, or you will potentially get little to no benefit. This also ties into your envelope's decremental delay. Solar exposure and occupant behaviour also play significant roles.

I am not saying don't use them, I am simply saying "do your homework" before you commit. They are not as cheap as a bit more insulation.

If you are really interested, there was a case study of a passive house duplex - two identical units - except one had phase change material. It's a good read.


Answered by Jason Hyde, Peterborough 6A
Posted Mon, 02/17/2014 - 10:37
Edited Mon, 02/17/2014 - 10:40.

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Jason has provided you with some good advice. I'll try to answer some of your specific questions.

Q. "If all of the insulation is placed on the interior [of the basement] wall, does that create any problems with moisture migration, etc.?"

A. No.

Q. "By putting most of the insulation on the interior, I assume it eliminates the possibility of capturing any of the thermal mass of the wall."

A. Yes. This issue is addressed in the article I linked to, How to Insulate a Basement Wall. For more information on thermal mass, and why it probably doesn't matter as much as you think, see All About Thermal Mass.

Q. "If you were going to build this home, where would you put the insulation on the basement assuming cost was not an issue?"

A. There is no single answer to this question, but my article lists the pros and cons of each approach.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 02/17/2014 - 10:57
Edited Mon, 02/17/2014 - 11:01.

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Regarding the SIPs...

Almost all closed cell polyurethane in the US is blown HFC245fa, has a very high global warming potential (GWP),.at about 1000x CO2. In high-R assemblies where the predominant R-value is from the polyurethane, it's pretty clear than on a lifecycle basis the environmental impact is strongly net-negative.

This is not the case for high-R attained with open cell polyurethane, most of which uses water at the blowing agent, or EPS & polyisocyanurate, both of which are blown with pentane (at ~7x CO2 GWP).

Unless the manufacturer will stipulate that the polyurethane is blown with one of the very few low-impact blowing agents such as HFO1234yf at (~4x CO2 GWP), it's going to be nicer to the planet to use something else. It's been marketed for both automotive refrigerant (to replace HFC134a, which runs ~1400x CO2, which is also used for blowing XPS), as well as a blowing agent for closed cell polyurethane. Honeywell's trade name for it is "Solstice", but there are now multiple vendors of HFO1234yf, since Europe has banned HFC134a in automotive air conditioners, and it's fairly close to a drop-in replacement for those systems.

Answered by Dana Dorsett
Posted Mon, 02/17/2014 - 12:59

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Jason, Martin and Dana: Thanks so much for all the help! I'll get busy and do some more studying as soon as I return from the business trip I'm on right now. I'm sure I'll have more questions after that.

Thanks again!


Answered by Tom Hanke
Posted Mon, 02/17/2014 - 17:59

Helpful? 0

Have You considered a "PWF"? A mineral wool insulated (Roxul comfort bat) wood basement can save considerable money. and offer far better insulation compared to the plastic foam insulated concrete designs. In many ways a PWF is much "greener" as well. The PWF will, however not tolerate typical shody construction and site prep. The original question, "best" has many aspects and may be controversial.

Answered by Jerry Liebler
Posted Mon, 02/24/2014 - 16:01

Helpful? 0

if you properly insulate and isolate your basement floor, that mass will act as heat storage.

Answered by Bob Irving
Posted Mon, 02/24/2014 - 19:28

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