Biggest bang for your buck?
justinmurphy
| Posted in Energy Efficiency and Durability on
We are about to begin construction of a new home near coastal Connecticut. We are trying to incorporate green approaches where feasible. Three main components are: Insulation; energy source (ie Geothermal); and energy distribution (ie in-floor radiant heat or forced air). Of these three green approaches, how would you rank them in terms of biggest bang for yurt buck, assuming you can’t afford all three. Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Everything is climate-specific, but by and large, insulation is by far the best bang for buck.
Forget about the ground-source heat pump and the radiant floors. Both are expensive boutique products that often become disappointments (especially the radiant floor).
For "energy source," use PV instead. To really save on energy distribution, use extremely cheap electric resistance radiant heaters powered by your PV array instead of a forced air system. If cooling is a requirement, you pretty much have no conventional choice except for forced air A/C, which makes a heat pump more attractive if you have heating needs too (since the one single system can do both).
I sense that you may be a relatively new visitor to this website. If you want to get some solid ideas regarding designing a new house, you could profit by using the Search box and looking for an article by Martin Holladay - use "How to Do Everything" as your keywords. It is a great reference source for...How to Do Everything. Near the top of the list are a few articles related to overall design of green residential houses. Read those first, including the Pretty Good House article. This may help you start your wanderings in a useful direction.
Energy use is a key part of being green. Almost every article at this website or any other website that talks about building science says that air-sealing and insulation are critically important and also have large effects on energy use and comfort at the lowest cost. And, you only do them once, without any maintenance thereafter. Proper orientation of the house with respect to the sun can also have a big effect.
Aside: if you have a builder who is proposing ground source heat pumps and radiant floors and calling that design green, you may wish to look for second opinions.
In CT geothermal heat pump installations have been running about $9000/ton, and in coastal CT their seasonal efficiency is barely better than modulating air-source heat pumps (and sometimes not better at all), that have an up-front cost of less than $3500/ton.
On a typical 2000-2500' house spending the money on the building envelope (insulation, air tightness, better than code windows) is a far better way to spend the difference in up-front cost of heating systems.
Even spending the difference on rooftop solar photovoltaic systems will likely pencil out favorably.
On the building envelope, see the recommendations on the zone 5 row of Table 2 on page 10 of this document:
http://www.buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones
Note, those are "whole assembly" performance values with all the thermal bridging factored in, not center-of-framing-cavity values. eg: They recommend R30 as a reasonable starting point for wall-R. That isn't an R30 batt stuffed into a 2x10 studwall- the thermal bridging of the framing would undercut that performance to something in the low-20s. A buildable R30 wall would be a 2x6 wall with R20-23 of cavity fill, with 4" of continuous EPS or 3" of polyiso foam on the exterior of the sheathing, between the sheathing & siding. IRC 2012 Code min for climate zone 5 CT would be just the 2x6 wall with R20 mininum of cavity fill (not R19s), no exterior sheathing.
Justin,
Your arbitrary list includes three categories: insulation, energy source, and energy distribution.
#1 on the list should probably be "making your house as small as possible."
#2 should probably be "air sealing."
Insulation should probably be #3; choosing the right type of insulation, the right details, and the correct insulation method matter just as much as the R-value.
Good window specs might be #4 -- but I'm not sure.
Here is a link to an article that may be useful: Green Building for Beginners.
But the question was: "...how would you rank them in terms of biggest bang for yurt buck, assuming you can't afford all three."
#1 is hands-down it's insulation, assuming "insulation" is shorthand for "building envelope efficiency".
#3 is energy distribution- the energy bang for buck of low-temp hydronic floors vs. ducted or ductless hot air is "pays never". Though there is a marginal comfort upgrade with the low-temp hydronic solutions, the comfort different shrinks dramatically when you have the higher efficiency building envelope.
That leaves ground-source heat pumps as #2 , but only by default, and in CT it's harder to rationalize due to the very high local costs for GSHP, and the fairly good performance of cold-climate air source heat pumps in that not-super-cold climate.
If you can only afford one, go for the high-R building. There are good / better /best and expensive / moderate / cheap ways of going about that too.
Thank you all (1-5 as of 3/2) for your very helpful responses. I did find Martin Holladay's posts (thank you Andrew C). And thank you Martin Holladay for your reply. I am a beginner but my list was not entirely arbitrary. These were just the 3 things that we have not fully decided on. Your posts are most helpful. I wish my house was smaller, but I lossed that argument :), but by insulation I did mean "building envelope efficiency" (thank you Dana Dorsett) and "air tightness" and we are focused on those aspects. Thanks again!
Part of the reason that the envelope is #1 is that a better envelope allows the heat source to be smaller, and thus cheaper, and it also helps you get the temperature inside uniform, even without a fancy distribution system, thus saving you even more on that.
In principle, ground source heat pumps make a lot of sense in your climate. Unfortunately, we have a market situation right now where high-performance air-source heat pumps ("modulating" ones) are amazingly inexpensive (at least the mini-split variety), whereas ground-source heat pumps using much less advanced technology are much more expensive, before you even get into the cost of the well drilling. I'm in a minority here in hoping for a ground-source heat pump revival, but right now it is not a good way to spend limited funds.
BTW: I neglected to mention, The "whole-wall-R" of a code min 2x6 / R20 of between R13 (16" o.c. typical framing) and R15 (24" o.c. with single top plates and most advanced framing techniques being deployed.)
That means the R30-ish wall recommended in the Building Science Corp paper as a starting point for what has long-term cost effectiveness is literally half the heat loss/twice the performance of code min. Of course how you get there and local energy costs have dramatic effects on cost-effectiveness, as well as lifecycle climate footprint. CT is a fairly high-energy cost market relative to most of the US, and if you're doing it with lower cost materials & methods R40 whole-wall performance isn't necessarily crazy. There are several vendors of reclaimed/recycled rigid foam board operating in southern New England that can make an R20 cavity + R20 continuous foam wall cost-comparable with R20 cavity + R6 foam if using virgin-stock goods.
Even though this wasn't in your question, I second nathaniel g's suggestion to look at PV. Connecticut's "CEFIA" incentive is fantastic, and electricity is pricey. We recently installed 6.5 kw, and i calculate approx 6 year pay off at current electricity rates. If you have a good site, you can think of it like a tax free bond yielding about 6%!
+1 on the rooftop PV! Retail electricity in CT is among the highest in the 48 contiguous states- expensive enough to make PV financially rational even without the current generous subsidies, and WITH subsidy it's a no-brainer, especially if financed at standard mortgage rates.
when designing and siting a new house paying attention to the photon-farming capacity of the roof pitches is well worth it even in places with cheap electricity. The 40 year "learning curve" of PV has about a 22% reduction in cost every time the world's installed base doubles, which is now every two years. (The past 6 years it's been closer to a 35% learning curve.) Within the lifespan of the shingles rooftop PV will become a no-brainer investment everywhere in the US, with or without incentives.