R value vs Heat Loss Reduction
papalohnes
| Posted in Energy Efficiency and Durability on
It seems in many cases that more insulation in walls or ceiling is the best way to go. However, I understand that increasing R value is good to a point then the percentage of heat loss reduction reduces with a increased R value. Is there a table to illustrate this? The table would have R value increasing from left to right and the % of heat loss reduction increasing from bottom to top. Thanks
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Replies
Don,
The type of table you envision has been created, but many such tables are misleading, and have been deliberately created to show that thin insulation is adequate.
The deception comes in deciding what to choose for a starting point. Many such charts start with an R-1 wall (probably impossible to build -- more like a tent).
It makes more sense to start with a code-minimum wall or a code minimum ceiling, since it is illegal to build a wall or ceiling with less insulation than the minimum amount required by code.
Here's the general rule: every time you double the thickness of your insulation, you always cut your heat loss in half. The rule never changes, no matter how thick your walls or ceiling.
Obviously, doubling your insulation thickness gets more expensive every time you double it. At the same time, the amount of escaping heat diminishes with each doubling. This is often referred to as "diminishing returns."
Don,
Here is a link to a bar graph (Figure 1.1) that shows what you want. The document is posted on the web site of a spray foam contractor.
The text surrounding the bar chart fails to explain that most of the suggested options shown on the chart are illegal.
Here is the link:
http://www.insulright.com/PDF%20FORMS/economical%20thickness.PDF
Thanks, both answers were helpful.
Just imagine any generic logarithmic curve.
As R-value increases, heat loss keeps getting smaller, but at a slower and slower rate. There is no particular point you can pick out anywhere where it is obvious to stop insulating. It's just a gentle sloping curve. I'm still a little confused about what happens in the range of R-0 to R-1, but putting that aside, say you start off at 100% heat loss, and at 1 inch, you get to 50% heat loss. Then you double the insulation to 2", and get to 25% heat loss. At 4" you're at 12.5%. 8" is 6.25%. 16" is 3.125%. And so on.
I've found it useful to compare a heat loss calculation against the cost of installing more insulation, and see how long the payback is for every additional inch. That's how I get a ballpark idea of what is reasonable. In my case it turned out to be about R-60 in the attic, which coincidentally is a very common recommendation.