SIP Walls vs. ICF Walls


  I’ve never been convinced with the “thermal mass” argument claimed by some ICF manufacturers (and log home builders), so I contacted a representative of a manufacturer who sells both ICF’s and SIP’s and asked him about the thermal mass issue.  He recommended an ICF foundation (“R” value 10.3) because of the strength of concrete and the added insulation of the foam (solid concrete has an “R” value of about .1 per inch, so an 8” solid poured concrete foundation wall has an “R” value of less than 1) and SIP walls and roof because of the higher insulation value.  Here’s why.


  I understand that you might be more concerned with cooling than heating your house, but the issues are the same, whether you’re trying to keep heat in or out.


  First of all, the very best insulating material is nothing.  Well, let me rephrase that.  The best insulating medium is a vacuum.  For example, a vacuum Thermos bottle will keep a drink hot or cold for hours.  The next best is closed cell polyurethane foam, like an ice chest.  A thermal mass works best when completely contained within the insulating barrier, like a chunk of ice in an ice chest.


  If you can heat your thermal mass with solar energy, like sunlight coming through the windows to heat the floor, furniture, ceramic tile, rock or whatever you have in your house during the day, and let that radiate heat at night, then you have a functioning, efficient thermal mass.  The same is true with cooling your house.  Cool the thermal mass at night, and it helps cool the house all day.


  ICF’s do not make a good thermal mass because the sun can’t get to the concrete, i.e. the 2” foam on both sides isolates the concrete. Even if concrete was exposed to direct sun light for 4 hours, it will loose the heat in less than 1 hour, plus it would transfer the heat on the inside of the house to the outside so you would have a cold house.  And why would you want to put insulation between your thermal mass and the area that you want to heat?  With ICF walls you cannot get an Energy Star Rating (which requires a 35% energy savings) or the Federal Tax Credit which requires 50%+ energy savings.  According to Federal testing only SIP’s can provide 50%+ energy savings over conventional framing.


  “R” value is the measurement of the amount of Resistance to the transfer of heat, whether it’s from inside to outside or visa-versa.  According to thermodynamics, heat will transfer to cold and to eventually reach a thermal equilibrium.  Heat transfer always occurs from a hot body to a cold one, a result of the second law of thermodynamics. Heat transfer can never be stopped; it can only be slowed down.  The only way to slow this is to create a barrier between the heat and cold.  The better the barrier (the higher the “R” value), the slower the heat transfer.  The big point here is that heat always travels from hot to cold.  The concrete in the ICF wall will transfer the heat in your home to the colder air outside.  In the summertime, it will transfer heat from outside to the cooler air inside.  Granted, the foam insulation will slow the process, but not as much as solid foam insulation.


  To understand the thermal energy efficiency of different building products, see the U.S. Federal Government’s Oak Ridge Report.  In the table of test results under #3, the second update is CIF’s, which is the same as ICF’s and has a Whole Wall R-Value of 10.3.  By the way, the Engineering Department at the University of Colorado has confirmed these numbers. 


  In the report, look down to #11 for Stressed Skin Panel, which are SIP’s and it has a Whole Wall R-Value of 21.6.  This was later updated to 21.9.


  If we divide SIP’s 21.9 / by CIF’s 10.3 = 2.1 or SIP’s are more than twice as energy efficient as ICF’s (or CIF’s).  In fact, if you look at #9, which is a standard 2X6 stud wall with R-19 batt insulation, it’s rated at a Whole Wall R-Value of 13.7, which is even better than a CIF wall. 


 The U.S. Federal Government’s Oak Ridge Report is a completely unbiased government report that tested the efficiency of the current building and insulating methods available today.  As with any government report, there is a lot of information to wade through, but it contains many useful facts and information you can consider.  You can find it at


  I think the lesson to be learned here is, it’s not so much ICF’s vs. SIP’s, but using the right product in the right place for the best overall performance.  SIP’s don’t make good foundations and ICF’s don’t make the most efficient walls.


  One more thing I wanted to mention.  Some people are tempted to put in very large windows to allow the sun to come in during the day to heat their house.  The drawback is that windows have a very low “R” value and contribute to a large amount of heat loss during the night.  The same, but opposite problem, would be true when cooling your house. The sun coming through the windows during the day would tend to heat up your house.  If you were to extend your eves such that they would shade your windows during the summer (sun high in the sky) and allow sun to enter in the winter (sun low in the sky), it would assist your energy efficiency.  Careful choice of windows and insulating drapes would be beneficial.


Thanks for your time, Steve