Midcoast Green Collaborative Forum

July 2, 2008

Sustainabilty Post #23 — How Much Insulation?

Filed under: Information — Tags: , , — Topher @ 10:50 am

Last week after our  talk on insulation, I sat down to write a post here about what the optimal amount of insulation in a house should be.  Given the recent prices of heating fuels, I thought the speaker undervalued the benefits of insulation.

After a week, and some forays into the world of economics that I would have been happy to skip, I finally have this web page which tries to tell you how much insulation is most economical.  It takes values for fuel type, and price, insulation type, and price (price is divided into fixed costs and variable costs based on thickness of application), mortgage interest (or opportunity cost if no mortgage), and several other things, and computes the optimal amount of insulation, free of any building constraints.

So, how is this useful, you ask, given that there are always building constraints?  Well here are some examples:

1) Your attic,  it is the one place in many house which is free from building constraints.  You can put as much insulation up there as you want.  So, how much is that?  Say, your attic is 26′ by 40′ (1040 square feet), you live in midcoast Maine with around 7500 heating degree days, burn #2 fuel oil at $4.40 / gallon, have a high efficiency boiler at 87%, you think that oil price increases will continue on the current line from 1946 – May 2008 of 2.81% per year,  you are on a fixed income, you can get a 30 year home equity loan for 6.00% to do home improvements, plan to put fiberglass batts perpendicular to the rafters (and existing insulation), which should improve the whole wall factor to 90%, the current insulation is R-20, and the contractor say it will cost $900 (total) for labor, and 10 cents (per square foot) for each inch of thickness.  How much insulation should you add?   19 more inches.  Given that fiberglass only comes in specific thicknesses, this probably means a 6 inch batt and a 12 inch batt.

2) You are building a new house, and are wondering about whether to do 2 x 4 construction, or 2 x 6 on 24″ centers for the walls with dense pack cellulose (both including 1″ foam insulation on the outside).  Using the same economic characteristics as the previous example, and 2000 square feet of wall surface, The whole wall calculator (follow the link on that factor) says that 2 x 4, 16″ on center with 1″ foam outside, has a whole wall factor of 87%, and 2 x 6, 24″ on center with 1″ foam outside has a whole factor of 77%.  And you could discuss that with your builder to see which is cheaper in the long run.  However, there is a problem.  The calculator is telling you that you want 20-22 inches of cellulose in those 2×4 or 2×6 walls.  Oops.  Maybe you should throw out the idea of using old fashioned stick framing altogether.  Perhaps look into double walls or Larsen trusses.

3) You are looking at oil prices and think they are going to track the last five years growth rather than the last 50 years.  That is 36% increases every year.  You are prepared to live under a mound of insulation if you have to, and are planning on buying the newest 97% efficient boiler.    The calculator returns 0, which means you exceeded the limit of 200 inches of insulation.   The solution to your problem is not insulation, but rather ceasing to use oil altogether.

* * *

There are websites (such as this one) which give recommendations for how much insulation you should have.  I have found a couple of problems with them. First, the assume status quo in terms of building construction techniques.  They recommend R-19 for wall because that is what fits in a standard 2×6 wall.  Second, they have embedded assumptions about the price of fuel, and the rate of increase of said, as well mortgage rates and other things.  These assumptions are all hidden and there is no way to change them without going through governmental committees.  Hopefully this site will provide insight into those issues.

It shold be noted that this software is in beta test, and there could be bugs, and it could change without warning.


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