https://energycode.pnl.gov/EnergyCodeReqs/?state=Indiana

I'm unable to post the table in a readable fashion so please click the link and select a random county.

Mass wall value i
Why is a higher R-value needed when >50% of the insulation is on the interior of the wall? Ignoring (possible) practical problems I would say insulation on the interior wall is the best/most efficient place, because indoors it's dry, less air leak/circulation, less/no rodents nesting in it, etc

Wood Frame Wall R-value
Why the R2 difference? To compensate for the thermal bridging of the frame?

Why is a higher R-value needed when >50% of the insulation is on the interior of the wall?

The theory is that people often turn off the heating/cooling and let the interior thermal mass absorb and release heat if the daily temperature rises above/below the desired interior temp. But this implies some significant swings in interior temperature and in reality, many people will get tired of the temp variations, opening and closing windows and the humidity issues and set the thermostat to 70 something and leave it there. In this case, the interior thermal mass is doing pretty much nothing and exterior mass would outperfom it.

Thermal bridging in stick built homes is another factor in WHY they require the higher R-Value if most of the insulation is on the interior side. Typical wood frame homes have a 35% thermal bridging factor, sometimes more if you have a lot of headers and 2x's stacked for bracing. Every time there is a 2x4 or 2x6 stud that R-Value drops to R-3 / R-5 and it creates a thermal bridge from the exterior to the interior. Adding insulation (like rigid foam) to the exterior helps to reduce that R-Value loss.

Posted By jonr on 27 Nov 2014 09:41 AM

 many people will get tired of the temp variations, opening and closing windows and the humidity issues and set the thermostat to 70 something and leave it there.

It all comes down if they get tired of paying the $400 a month electric/gas bill or dealing with a 68F-78F temperature swing.

So adding EPS to the exterior seems to the be green answer. Does that almost require using EIFS instead of brick/vinyl if you want to add more than 2 inches? If solution is so easy, why don't more homes use a few inches of EPS on exterior?

You can use up to 4" of EPS with just about any siding you want.

Posted By James02 on 28 Nov 2014 10:57 PM
So adding EPS to the exterior seems to the be green answer. Does that almost require using EIFS instead of brick/vinyl if you want to add more than 2 inches? If solution is so easy, why don't more homes use a few inches of EPS on exterior?

It is no problem with brick because brick is supported by the footing or brick ledge with a 1" air gap between the brick and wall.  Metal brick ties installed approximately every 2-3 sq ft are used to tie the brick back to the wall while maintaining the 1" air gap.  If you had more exterior foam, you would just need longer screws/nails for the metal ties to span the thickness of the foam.

Posted By jonr on 29 Nov 2014 09:03 AM
You can use up to 4" of EPS with just about any siding you want.

Many siding companies strictly indicate furring strips are needed if exterior foam is greater than 1". 4" of foam also makes siding guys unhappy because it is very hard for them to find a stud to nail into. Exterior foam without furring strips complicates matters. Exterior foam with furring strips complicates matters. The added complexity and cost is what prevents people from doing this. Since most homes today are built by builders, not homeowners, its a simple matter of profitability and headaches.

Posted By Surfsup on 29 Nov 2014 08:42 PM

Posted By jonr on 29 Nov 2014 09:03 AM
You can use up to 4" of EPS with just about any siding you want.

Many siding companies strictly indicate furring strips are needed if exterior foam is greater than 1". 4" of foam also makes siding guys unhappy because it is very hard for them to find a stud to nail into. Exterior foam without furring strips complicates matters. Exterior foam with furring strips complicates matters. The added complexity and cost is what prevents people from doing this. Since most homes today are built by builders, not homeowners, its a simple matter of profitability and headaches.

Makes sense. I was wondering why more didn't add that extra few inches of EPS. Does this cause cost spiraling upwards or just a bump in the price? I'm curious, for when I build, if I should avoid this idea or stick with it.

So I did some research on how to attach your siding onto furring strips? This is assuming you did exterior insulation with furring strips. It appears there is not any International Residential Code for attaching siding to furring strips. One recommendation was to use ring nails?

http://www.greenbuildingadvisor.com/blogs/dept/musings/fastening-furring-strips-foam-sheathed-wall

Since most furring strips are 1x4, then how could brick siding attach to furring strips with confidence and quality it will hold and last?