Exterior Wall Foam Insulation

[MountainDon]

I researched this as a part of my renovation and re-insulate project. Thought I'd pass it along.

This information is directed at walls that use rigid foam insulation on the exterior of walls and also have insulation in the wall cavities. To make this information relevant to your situation you need to know the climate zone for your location. Climate Zone information is located here.

In some climate zones you can have as little foam on the exterior as you please. In other zones adding too low an R-value of foam to the exterior can result in problems with moisture in the walls. Climate zones 1, 2, 3, 4A and 4B have no minimums. Climate zone 4 Marine and everything else all have minimums.

Walls with foam sheathing on the exterior and no cavity insulation are exempt from these requirements Their only restriction is that the foam insulation should be whatever minimum R-value is required to meet the locations energy code. Walls that are filled with spray foam insulation only, also only need to meet the energy codes R-value. The so called flash and batt installation where foam is sprayed on the inside of the sheathing and then the remainder filled with batts have more or less the same foam R-value minimums as walls with rigid sheets on the exterior and batts in the cavity. More on that down the page.

Problems can arise because insufficient foam on the exterior makes the sheathing material colder right next to the insulation in the wall cavities. The cold surface can cause condensation. Or rather, as I learned, the cold OSB or plywood absorbs moisture more readily. The moisture in the air does not necessarily condense into a liquid on the cold wood surface. The moisture can be directly absorbed bypassing the liquid on the surface stage.

Thicker insulated walls, like those framed with 2x6's also require more exterior foam than insulated 2x4 walls. This is because the thicker cavity insulation keeps the sheathing layer colder. To counter that more foam is needed.

Walls with a flash layer of foam, then batts can have actual condensation of moisture on the foam if the foam layer is not thick enough. The practice of a very thin layer of spray foam to air seal the wall can be a bad idea because of this. It is recommended that spray in place foam be about as thick as the minimum given for exterior rigid panel foam. The word 'about" is in there because the sprayed in the cavity foam takes up some space and reduces the amount of batt infill insulation. The reduced amount of batt type insulation causes the foam layer to be warmer. But I never found any definitive numbers for spray in place foam thickness recommendations for any climate zone.

Once you know the climate zone and the thickness of the wall and cavity insulation the required thickness of exterior rigid foam is easy to determine. In the IRC, use Table R601.3.1.  The information is also to be found in the 2007 supplement to the IRC, Table N1102.5.1.  This is all based on changes that allow wall construction in cold climate zones without the use of the usual interior vapor barrier, like 6 mil plastic. Basically with sufficient exterior foam, the wall is permitted to dry to the interior just like the walls in the warmer climate zones.

So, many older homes that were built without truly effective vapor barriers on the interior are excellent candidates for increasing insulation by adding rigid foam on the exterior. Just add enough R-value for the climate zone. Remember as with everything in the IRC, these are minimum requirements. Thicker foam is fine, thicker foam is better.

Here is the essential information for Minimum R-Value of exterior foam for the pertinent climate zones...

Climate Zone      Required Exterior Wall Foam
Marine 4      R-2.5 for 2x4 walls; R-3.75 for 2x6 walls
Zone 5      R-5 for 2x4 walls; R-7.5 for 2x6 walls
Zone 6      R-7.5 for 2x4 walls; R-11.25 for 2x6 walls
Zone 7 & 8      R-10 for 2x4 walls; R-15 for 2x6 walls

Remember, the above foam R-values are for walls that also have non foam cavity insulation.


There is similar information in the IRC regarding foam over cathedral ceilings that are unvented and have cavities filled with some other insulation. See IRC Table 806.4.



Technically our home is in Zone 5, realistically we are in Zone 4B. Zone 5 and 2x4 calls for R-5 foam added to the exterior of the walls. Zone 4B has no restrictions listed.  I'm using a total of R-10.7. That's a virtual doubling of the R-11 fiberglass batt insulation. Removing the stud thermal breaks will give an even greater overall increase in R-value. That batt insulation has a Kraft paper face, there is no other vapor barrier/retarder. We're safe.

[UK4X4]

http://countryplans.com/smf/index.php?topic=11903.0

has guides and manuals for the same- for my area I needed 2" of foam to keep the moisture outside the framing

[waggin]

If one were to consider something like this, how is the issue of increased thickness addressed at the top where the bird blocking is, and around the windows/doors.  This will put the finished siding quite a ways out beyond those; what is used to compensate for that?  Is it ok to have those features recessed?  Are there some specific methods for trimming/flashing when something like this is done? 

[UK4X4]

Take a look at the link I posted - it has links to design guides and installation guides explaining the choices and water planes.

there are two main thoughts/ methods -
windows set back installed as per normal and extended sills
or
using the foam as the water plane and installing the windows just beneath the siding

[Alan Gage]

Innie vs. Outie windows:

http://www.greenbuildingadvisor.com/blogs/dept/musings/innie-windows-or-outie-windows

Alan

[waggin]

Thanks!  That and pages 16-20 of  http://cchrc.org/docs/best_practices/REMOTE_Manual.pdf  (from your other links) were really interesting and informative.  Sorry for missing that on the first go round.