Exterior Insulation Reasoning

OK lets talk about insulation. This is a touchy subject. I know. It usually gets people fired up so that being said....put your helmet on. I'm going there.

We are choosing to use rigid exterior insulation. Why? There are so many options and ways to skin this cat. (I'm a cat lover I swear...) We could blow in cellulose, fiberglass, pump low expanding foam into the wall cavities. Why in the world are we using rigid insulation?

The reasons are pretty simple:

1. We do not want to alter the interior finishes. It is fine the way is it and we intend on leaving it alone. "if it ant broke, don't fix it"
2. We want to keep the wall cavities open for future routing of utilities and communication lines.
3. Continuous insulation eliminates thermal bridging associated with framing. Thermal bridging derates an assembly R-Value.

*put on nerd had*
Example:
In a 16" OC wall using standard framing members there is ballpark 20% framing & 80% cavity. Just for fun lets say we had a 2x6 wall @ 16" OC and filled the cavity with R-6/inch spray foam (R-33). Wood is approx R-1/inch so the framing would be ballpark R-5.5.

(% x U-Value)+( % x U-Value) = Weighted U-Value
(.80 x (1/R-33))+(.20(1/R-5.5)) = .060 U-Value

1/.060 = R-16.5 Weight Assembly R-Value

FYI... R-Value = 1/U-Value
U-Value = 1/R-Value

Just because there is R-33 spray foam in the wall, it does not mean that the wall is R-33. The thermal bridging knocks it down to R-16.5. )
*nerd hat off*

We don't want thermal bridging to rob our assembly R-value.

4. Thermal bridging creates variety of temperatures within the wall assembly and on interior surfaces (look at the THERM images below). This variety of temperatures increases the potential for condensation inside the wall assembly. Condensation leads to mold growth & material degradation. We don't want that.

5. The building's siding and windows are at the end of their life cycle. They need to be replaced regardless of what insulation system we choose. Adding a layer of continuous rigid insulation while the siding and window replacement is going on is relatively easy to do.

See the therm images below:




Part 2 of this blog:

Q: How are you going to attach all of that insulation to the building?
A: Big Screws!

Q: Wont the screws bend from the weight of the siding?
A: No! Look at this back of napkin moment calculation:

*nerd hat on*



*nerd hat off*

Basically the compressive strength of the foam prevents the screw from being allowed to rotate and bend down. Ballpark...each screw in this system can support 700lbs of wall before the foam will begin to compress and let the screw bend. Unless we are going to be hanging cars from the wall...screws bending is NOT AN ISSUE!

These are the actual screws that Bruss will be using.







The screws will be fastened into the studs as seen above.


Questions / Comments?
ryan@petersenengineering.com