Discussions

MAS™ along with others find equivalent R-value to be a useful way to summarize the energy efficiency of a MAS™ structure. Since there is wide spread confusion between a “conventional R-value” and an “equivalent R-value” it is important to understand the difference.  There are a few important cautions. First, equivalent R-value is not the same as the conventional R-value and you shouldn’t pretend it is.

A MAS™ structure can be built with 3rd party insulation products which typically use OPEN cell panel insulation in the MAS™ system corners and OPEN cell spray insulation on the arch sections.  The MAS™ structure can obtain a REAL conventional R-value of 60 but will offer an equivalent R-value of 60 for significantly less insulating material. Following are the calculations which show this.

Using 3.0″ open cell foam insulation on the corner wall and hip roof gives an approximate convention R-value of 15 in this area.  Using 3″ of  closed cell foam on the arches gives an approximate conventional R-value of 60.  See the table at the end of this discussion from the US Department of Energy, National Laboratory System, Science and Technology Information Center.

With NO 2×4’s, you get approximately 10% more insulation per square foot based upon 16″ stud spacing (1/16).  You also eliminate voids which accounts for about 1/2 inch of extra insulation.  Thus to get the conventional R-value you take 13.72 times 1.094 = 15.  This is based on 3.5 inches.  However we can subtract 1/2″ as there are no voids (missing insulation) due to imperfect fitting to 2×4 studs.

Using 1.0″ closed cell foam insulation on the arches give an approximate conventional R-value of 22 in this area.  Thus the to get the conventional R-value you take 20.09 times 1.094 = 22.

The MAS™ walls are not R-60 in the above example (but they can be). They still allow heat to conduct through like R-15 and R-22 walls because that’s what they are. It’s just that they have other, different ways, to save some energy, Equivalent R-value is a way to boil all the energy savings down and summarize them in one number, But it’s a different type of number.  MAS™ Considers this configuration to be an equivalent R-value of 60.

IS IS POSSIBLE TO ACHIEVE A CONVENTIONAL R-VALUE OF 60 by using more insulation AS FOLLOWS:

It is possible to use 1″ of open cell foam insulation on the corner walls and hip roof and cover it with  2.5″ of closed cell foam to achieve a conventional R-value of 60.  This is calculated as follows: Using 1″ instead of 3″ of open cell foam yields a conventional R-value of 15/3 = 5.  We desire an R-60 and thus need to make up and R-value of 55 (60-5) with the closed cell foam.  As shown in the calculations above, we get a conventional R-22 per 1″ of closed cell foam.  Thus 55/22 = 2.5.  Thus with 2.5″ of closed cell foam sprayed on top of the 1″ of open cell foam we get a conventional R-value of 60.   Applying 2 3/4″ of closed cell foam on the arches would achieve a conventional R-value of 60 in this area.  Thus the entire MAS™ structure can be built with a conventional R-value of 60.

In summary, the MAS™ structure can achieve a conventional R-value of 60.  However, an equivalent R-value of 60 is achieved with significantly less insulation due to the lack of voids, thermal mas and other factors that have a profound effect on the use of energy for heating and cooling.