Cost analysis of thermal bridging in zone 2
Bbrown3381
| Posted in Energy Efficiency and Durability on
Adding an addition to the house. 2×6 at 24″. About 60 feet of wall. Cedar shingle siding. Concerned about the cost of installing ridged foam to prevent thermal bridging. Will the energy cost savings ever provide a return on the additional foam, horizontal battens at 5″, labor, etc.
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Replies
Robert,
There are several factors to consider -- many of which we don't know. These factors include your current energy costs, your assumptions about future energy cost inflation, the cost of construction work in your area, the specific details of your home's construction and orientation, and your expectations regarding a reasonable payback period.
To find the answer to your question, you need to (a) determine the construction cost of the energy-efficiency measure that you are considering, (b) perform two iterations of an energy modeling program, one with the measure under consideration, and one without the measure, and (c) perform a cost-effectiveness calculation to determine whether the annual energy savings meet your personal criteria.
For more information on this type of calculation, see Payback Calculations for Energy-Efficiency Improvements.
If I had to guess -- and this is only a guess -- I would say that the payback period for your suggested energy-efficiency measure is probably very long in your climate zone, and that there may be better ways to spend your money if you want to reduce your energy bills.
If you take a peek at table 2, p10, the starting point for discussion on a long-term financial rationale in US climate zone 2 is a whole-wall R of about R15:
http://www.buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones (Read the whole first chapter to know what they are talking about with those recommended starting points.)
A 2x6 wall single top plates and AF framing corners, with R21 fiberglass or R23 rock wool with cedar shingles the whole-wall R comes in at about R15, so you're kind of in the sweet zone anyway. (Open cell foam or cellulose would put you just shy of that R15, but close enough that it doesn't really matter.) If you used 1/4-3/8" fanfold XPS for siding underlayment you'd be at about R16.5-R17, still in the zone. Since fan-fold can be detailed as the weather resistant barrier that may be the most exterior foam that really makes sense.
If you have unusually high electricity costs and net metering of rooftop solar isn't allowed there may be a rationale for more exterior foam than that, but that's probably not the case here. If net-metering is allowed, you may be better off spending the next few $1000 on more rooftop solar at some point rather than on exterior insulation.
The single most cost effective thing you can do with your 2x6 wall is to get religion about air sealing as you build, putting a bead of caulk or construction adhesive under the bottom plate of the studs, and caulking every stud to the sheathing between stud bays, taping the seams of the sheathing, taking care to air seal the window installations perfectly all prior to insulating.