Question of the Day - #109 Easy question!

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Question of the Day - #109 Easy question!

Postby RDavidson » Wed Dec 17, 2014 12:16 pm

I have a simple question that should have a simple answer. What is the minimum R-value required for basement walls in a home built under the 2015 Minnesota Residential Energy Code? Table R402.1.1 says I need an R-15 but contains footnotes c and, what I think is, i. Now footnote c directs me to R402.2.8.

R402.2.8 says “Basement walls. Walls associated with conditioned basements shall be insulated from the top of the basement wall down to 10 feet below grade or to the top of the footing, whichever is less. Foundation insulation shall be installed in accordance with the manufacturer’s installation instructions.”

That sounds pretty straightforward. If I have your typical conditioned basement then the full height of the wall must be insulated and the insulation installed per the manufacturer. Piece of cake!

Then I read on: “Walls associated with unconditioned basements shall meet the requirements of this section unless the floor overhead is insulated in accordance with Section R402.1.1 and R402.1.7 and the following requirements:”

Ok, so maybe I have an unconditioned basement. And it says that the basement walls must be insulated unless the floor overhead is insulated via the two code sections. So I go to R402.1.1 to find some floor insulation requirements. But R402.1.1, and R402.1.7, address foundation insulation requirements, not floors! Ok, so I am a little bit confused. And what if the space that I have is by definition a crawl space. I know, I know, it isn’t what you said, it was what you meant.

I read on. There are two other requirements that I must follow for this unconditioned basement. Because the second requirement applies to wood foundations and I have a poured foundation I can ignore the second requirement. Cool! The first requirement reads: “R-15 insulation for concrete and masonry foundations shall be installed according to R402.1.1.1 to R402.1.1.8 and a minimum of a R-10 shall be installed on the exterior of the wall. Interior insulation, other than closed cell spray foam, shall not exceed R-11. Foundations shall be waterproofed in accordance with the applicable provisions of the International Residential Code.”

I think what this is saying is that I can split the R-15 insulation requirement so that I have a minimum of an R-10 on the exterior and make up the rest on the interior. But, I cannot exceed an R-11 on the interior unless I use a closed cell foam and then I can go hog wild! I understand this so now I feel better except I’m wondering why the code gods specified a different waterproofing section here than for all other applications. Makes no sense….

Then I read the exception to this requirement. It says: “R-10 continuous insulation on the exterior of each foundation wall shall be permitted to comply with this code if the tested air leakage rate required in Section R402.4.1.2 does not exceed 2.6 air changes per hour and the total square feet between the finished grade and the top of the foundation wall does not exceed 1.5 multiplied by the total lineal feet of each foundation wall that encloses conditioned space. Interior insulation, other than closed cell spray foam, shall not exceed R-11. See footnote c to Table R402.2.1.”

Now I am confused again. This exception sounds like the rule that it is excepting. Both result in R-10 on the exterior. Both are limited to R-11 on the interior unless I use closed cell foam and then I can fill the whole unconditioned space. BUT, the exception includes an air leakage rate requirement and limits the amount of foundation wall exposed above grade on the exterior! Why would I use the exception that is more restrictive than the rule? I have to test for air leakage and I am limited on amount of exposed foundation! Some would call that bass ackwards. And, what is the business about limiting the amount of exposed foundation THAT ENCLOSES CONDITIONED SPACE! We are talking about an unconditioned space! C’mon! Now I am really confused. I will never build anything with an unconditioned underfloor space. I will build it on stilts! But I am also wondering, was it intended that some of this section that references basement walls enclosing unconditioned space might apply to foundation walls enclosing conditioned space? No one would ever write something this confusing for such a common and simple application, would they?

So I finish my reading where the last sentence directs me to footnote c to Table R402.2.1. Maybe there is some logic in that footnote that I overlooked. I read footnote c and it states “See Section R402.2.8”. THAT IS THE SECTION THAT JUST CONFUSED THE HELL OUT OF ME!!! As my good friend Larry would say, “WHAT #$%%^&&**(&^^(@$!” There is a bald guy in Iowa who is rolling around on the floor hysterically laughing at what is going on here.
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