Energy simulations comparing wall R-values.
Over the last couple of years, I’ve become a bit of Green Building Advisor junkie… learning as much as possible on the building science details, application, etc., but the results of a recent house simulation suggest that the choice in mechanical systems is more than twice as effective at lowering energy costs than wall R-value.
My project is with a small house (1275 ft2 + 775 ft2 below grade finished basement) that I’ve been asked to provide energy simulations for various building envelope configurations. Taking a nod from BSC’s research on wall systems, I decided to use their “whole wall R-value” for each wall type (i.e. 2×6 + fiberglass + VB compared with double stud wall + 9.5″ cellulose – R13.7 and R30.1, respectively). The simulation software is Carrier’s HAP, which I have almost 10 years of experience with, so I consider myself relatively fluent, however, the results are somewhat disappointing.
Given the house is in Atlantic Canada (high delta T in winter) and the infiltration is set to minimal amounts, the result is a reduction in energy costs of only 17%, which equates to only $369/yr in savings. Even though I know a well sealed, well built house is much more comfortable, it’s hard to justify the extra cost of the insulation. I’m hoping that maybe the software is somehow not accounting for something significant. Is there any real world data out there that indicates otherwise, or even better, can justify the added insulation?
Hope someone can straighten me out.
Darcy Grant
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Replies
Darcy,
I have no reason to doubt your figures. The fact is, energy is now cheap. Energy is so cheap, in fact, that it's hard to justify superinsulation methods in most climates based on the current cost of energy.
To justify superinsulation methods, you need to have a long view. Models that include the assumption that energy costs are likely to rise steeply in the future show that investments in insulation make sense. Models that assume that energy prices will remain stable show very long payback periods for insulation improvements.
Finally, there are other reasons to invest in superinsulation, including increased comfort and a lowered carbon footprint.
If a given surface only accounts for say, 25 percent of energy loss, what is the most amount of money you can save by insulating it?
In a 'regular' houses with 'good' windows and 'regular' insulation, half your heat goes out the windows. Half of what is left goes out the roof. Everything else is the rest.
This is what I don't like about software packages, they give you great answers, but they take away the thought process.
square feet * deltaT / R for walls roof and windows with a pocket calculator and a sheet of paper. THEN use the software to optimize.
To me, it is always about the windows. If I was budgeting, I would choose an R20 wall and R6 windows instead of the R40 and R3 that many seem to choose.
Not that there is anything wrong with an R30[or 40] wall..............
Darcy,
Ignoring mortgage interest rates, other possible investments and energy cost inflation for a moment...over just 30 years your model suggests that the savings for the double-stud wall is about $11,000. Based on your description, you probably have in the neighborhood of 1,000ft2 of net above grade wall area. I suggest you then ask yourself: "Can I build the double-stud wall for less than $11/ft2 increased cost over the 2x6 wall?"
Hint - the cost difference for the insulation should be less than $2/ft2.