BTUs to Dollars?
Dear GBA Community;
I am looking to build a pretty good house in New Hampshire (zone 6, 1500 feet elevation.) I’ve read Matin’s heat loss calculation blog (which was extremely helpful, as always!- THANK YOU MARTIN!). I also stumbled across this blog entry which was very helpful: http://www.prettygoodlakehouse.com/2013/11/heat-loss-calculation.html
Although I punted a bit on calculating heat loss from the windows and ventilation system; the approximate result is around 14,500 btus for outdoor temps of -4 F (97% of days for my area) and 6,000 btus when it is 47 F degrees. I have already settled on two small mini-splits- one for upstairs and one down stairs. Will likely add some resistance heaters for basement /bathrooms.
I have two questions:
1.) We are still exploring different design options and are debating between double stud (R-42 walls) vs standard 2×6 with dense pack (R-20 walls). The house will feature large, south facing windows and we are debating between lower performing (low u-factor/ low SHGC) American windows vs superior European windows. We assume attic will be R-60 to R-70 given the ease/low cost of insulating it. Slab will be R-25ish. Question: We are trying to determine what makes the most sense from a cost perspective. Adding double stud walls while keeping our floor plan dimensions will cost at least $12,000 more than a standard 2×6 wall. Opting for American windows will also decrease costs some (and minimize complexity) as our builder will not install European windows. If our heat loss goes from 14,500 btus to 19,000 btus, how much extra will this cost us each year? Are the efficiency upgrades worth it?
2.) How much heat can we expect to receive from our south facing windows? Although I imagine it won’t change our heat loss calculations, will it affect our equipment sizing?
Thank you all in advance!
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Replies
Rick,
A software program like REM/Rate will provide your annual energy costs (and will allow you to see how different envelope choices affect that annual energy cost).
If you are using a pencil and paper to determine annual heating energy cost, you need to start with your heating degree days. (Look it up.)
Here is the formula:
BTU/year = (HDD*24*Area)/R-value
As an example, let's assume that your location in New Hampshire has 7,000 heating degree days.
Let's assume that your house has 260 square feet of windows.
Let's assume that you are comparing windows with a U-factor of U-0.18 (that is, R-5.5) to windows with a U-factor of U-0.30 (that is, R-3.3).
Ordinary windows:
BTU/year = (7000 * 24 * 260) / 3.3 = 13,236,000 BTU
High-performance windows:
BTU/year = (7000 * 24 * 260) / 5.5 = 7,942,000 BTU
So the high-performance windows save you 5,294,000 BTU/year.
How much does the difference in BTU cost? It depends on your fuel and fuel cost.
If you are burning oil in a boiler with an efficiency of 80%, and your oil costs $3.00 per gallon, here is how you figure it:
Fuel oil has 139,000 BTU/gallon
To create 5,295,000 BTU/year, you will need:
5,295,000/(139,000*0.80) = 47.6 gallons of oil
If you're paying $3.00 per gallon, you'll be spending $143 more dollars per year in a house with ordinary windows compared to a house with high-performance windows.
This is an example. Plug in your own HDD, window areas, window U-factors, fuel type, appliance efficiency, and fuel cost to make your own calculation.
-- Martin Holladay
Thank you Martin. That formula is exactly what I needed. I ran the numbers comparing walls (r-40 to r-20) and windows (r-7 to r-5), converting btus to kwh and assuming a COP of 2 for the mini splits.
At 16 cents a kwh in NH my break-even point will be about 23 years for the walls and 28 for the windows.
Lot of options to bring those numbers down though. Thanks again!
When running long term payback calculations it's important to use a net-present-value approach, using reasonable interest rates as the discount rate, test it for sensitivity to both energy price inflation and energy price deflation.
The cost of the equipment would also be factored in to a more sophisticated financial model approach. The lifecycle of a mini-split is something like 15-20 years, whereas electric baseboards are good for 30-40. At the point of mini-split replacement it's likely that the replacement will be both more efficient and cheaper, but that too has to be tested against inflation/deflration.
The current unsubsidized levelized cost of rooftop PV is roughly on par with your 16 cent figure, but with tax credits and other rebates it's often substantially cheaper on a 20 or 25 year basis. That too would have to be measured against the cost of the higher-R assembly, along with maintenance and replacement costs of PV, which should be MUCH cheaper than today, but may not be eligible for net-metering at retail in 20 years. eg: At $2.50/watt (net. after subsidies) with a 4% discount rate 15% capacity factor and a 20 year lifecycle analysis the levelized cost of PV is about 14.5 cents/kwh. If you can get that down to $2/watt it's about 11.5 cents.
http://www.nrel.gov/analysis/tech_lcoe.html
Rick: When running the numbers, remember that a 2x6 stud wall filled with R-20 insulation will have a whole wall R value of considerably less than R-20, due to thermal bridging through the studs.
As for windows, you may want to consider the comfort factor. Good windows aren't drafty. You can sit close to them and not feel a chill.
Finally, your builder refuses to install European style windows? If he or she can't figure out how to do that, I'd be concerned. My builder had never done it either, but he certainly was willing to do it. It isn't rocket science.
Dana, Thanks for sharing your thoughts. Very helpful, as always. The higher performing home will require a 16000 BTU mini split and the conventional one will need a 21000 BTU unit(s). Prices comparisons are a bit hard to come by but it appears that the larger unit will cost at least an extra $1,000 plus added installation costs as I presume it would require more than one head. I will run some of the other scenarios you mentioned. I didn't think about PV implications!
Stephen, thanks for the insight. You are correct! The 2x6 wall would probably be more like R-17ish. We are pricing our design with a modular company. They've installed Euro windows before but have since stopped because the shipments often delay their tight production schedules. I saw my first triple pane window in Portland, ME when I visited Performance Building supply. It was about 30 degrees out but the interior glass on the north facing Intus window was shockingly warm. I've wanted some ever since.
Rick:We got our windows from Performance Building Supply and are more than happy with them. I have a thermometer on a window sill. The outside temperature is 30 and the wind is about 40 mph, gusting to 50 and the thermometer is reading 71.
Since we got our windows two years ago, it seems like there are more North American manufacturers making triple pane, highly efficient windows.
Stephen- that is amazing!
We live in a recently built house in Jackson, NH that we wrote about on GBA....first post is: https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/building-small-house-white-mountains
Although our budget was small (or we tried to keep it small), we were focused on high insulation levels, good windows, and solar...so we didn't really consider standard walls or cheaper windows. I can't help with your detailed numbers, but can offer some advice/perspective.
Walls
You can do whatever wall you want and either upsize or downsize your heating system, but I think there are some benefits to high R walls. I like how a super insulated home can have such a simple heating system, we heat with just one fairly small mini split (Mitsu FH12NA) on our first floor and some rarely used wall mount electric resistance in bathrooms/bedrooms. Power outages in the past couple years have been minimal, but it's nice to know our house could weather a sustained winter power outage better than standard construction.
Windows
I'd suggest taking a look at Alpen windows from their dealer in Maine: Pinnacle Window Solutions. The turn around time is reasonable and we've been happy with the windows. We went with Alpen (their 525 series, what they call Zenith now) over Intus and Marvin for various reasons including time, performance, and cost. Another thing to consider with windows is condensation resistance. This depends on your indoor relative humidity and other factors, but better performing windows will be less likely to develop condensation on those super cold nights. Solar heat gain from south windows can be significant, good in the winter and bad in the summer. Even though NH is a heating dominated climate, don't go overboard with the south windows and try to incorporate some roof or dedicated window shading for summer sun angles. On sunny cold winter days (even with highs not much above zero), the solar heat gain will bring our house up to 70+ and the mini split (set at 68) can shut down for the day.
I'm realistic and I know it doesn't make sense to dump money into walls and windows just to say they are high R, but I'd give the double stud wall serious consideration and I'd definitely get the best window you can afford. By sticking to our energy efficiency goals, our house has been net positive (we generate more electricity than we use over the course of a year) with a modest 6.72 kWh solar PV system.
Brian- thanks so much for adding your thoughts/experiences/recommendations. I've read your blog and love your house. (Building it seems like it was quite the adventure! Hats off to you for making it happen!)
I know that you learned a lot from other homebuilder's blogs when building your own home. You did a great service to all of us by creating your own blog. I am new to NH and had never heard of "Ledge" until I read your blog about you having some issues with it.
My wife and I looked at hundreds of lots to build upon and, in light of your blog, I would dig a small hole with a shovel on each of them to see if there was ledge. It took us a year, but we finally found one with no ledge. (They don't call it the Granite State for nothing!)
You bring up so many good points in your brief post. I hadn't thought about solar until Dana mentioned it above. The R20 wall (more like R-17 wall) would require an 8.5 kwh array on its own (I think). That can't be cheap.
I hadn't heard of Alpen windows but I looked into them and they may be, alas, the perfect fit. I love how you don't need to run the heat even on a cold, winter day so long as it is sunny out. We definitely went overboard with the solar glazing and I hadn't thought about exterior shading much. This could be an opportunity to decrease window costs and increase the comfort level.
Thanks again for the post and your blog, Brian. Despite being a horrible writer, perhaps I should pay it forward and start my own blog! :-)
Brian,
I appreciate hearing the update too. Glad things are going well in your very nice house.