Wall and Ceiling Superinsulation suggestions
MarkDMacLeod
| Posted in Construction Techniques on
This question really bridges both construction techniques and energy/insulation. It’s a question about the FHB article on R40 walls from Dec/Jan and the preferences of the builders on this site. Of the options presented – what are people’s preferences? What do you find the most cost effective and what wall system would you choose if you were building for yourself. Just a general question – more out of ignorance than anything so please don’t be insulted – but when is too much insulating value too much? At what point does ceiling or wall insulation pass the point of gains in efficiency balanced with cost? Thanks Mark
Replies
Look back a couple of issues for the Foam Insulation article. It shows the graph that describes how payback is reduced as R value goes up. Basicly, it says that at a certain depth you might save $500 a year on heating/cooling, and if you double the depth you might only save $50 more... or even just $5 more.
Cost of the materialis a linier line, more is more. Effect of the materials is a line that rises then flattens out.
Great comment - thanks very much. I had forgotten about that article - went back and looked at it last night. It was very helpful.
Mark
Be careful about interpreting such heat loss vs R curves. I've posted comments on this before. Foam insulation contractors seem to like to use curves showing percent of heat saved to show that a wall that barely meets code is all you need. Adding more R value using foam is expensive, and proposing more without being asked by the client risks pricing the job out of the competition.
The right amount of insulation for new construction clearly depends on the climate. Those curves that use "percent" saved usually don't show the reference point (percent of what?). A heat loss of the last 5% (of some reference point) may be insignificant in a mild climate or still serious and worth chasing in a cold climate. Further, chasing the residual heat loss may not be worth it if the incremental insulation is expensive, or well worth it for an inexpensive insulation, such as cellulose.
Compounding the situation is that to add insulation to a design you need space to put it. A closed cell foam between studs provides the most R value you can fit into a given space, but is most expensive per R. Also, if you don't do something about the thermal bridging through all the wood framing (upwards of 20% is typical), then the percent degradation of the whole wall R value due to thermal bridging is much more pronounced with a high R cavity fill than with a low R insulation.
So many things to think about! The best thing to do is to model all parts of the whole proposed building envelope, using whole wall R values (not center of cavity), and adding up the total U*A for the shell. The free RESCheck program is one tool that can be used, but you could do your own Excel spreadsheet to do the same calcs. Then you can evaluate various tweaks to a proposed wall assembly in terms of heat loss and thus cost over a heating season.