Best 2 or 3 energy efficient strategies for new build?
Brian_Pettee
| Posted in Energy Efficiency and Durability on
Hello,
I am in Northern Utah (Zone 6) and I am getting ready to select a builder to construct a 1800 – 2200 sq. ft. Rambler with 9 ft ceilings and full walk-out basement. Lot size is 0.5 acres and home will be oriented East and West along it’s long axis and have southern exposure for the living areas and back yard.
What I am unsure of starting out is how to get the best value into the home from an energy efficiency perspective. I am not sure I can afford a custom build but several of the local homebuilders appear willing to do a “semi-custom” home if I choose one of their existing plans. Is it realistic to ask them to do some “non-standard” strategies for energy efficiency? I know most will not deviate far from their typical process, so I plan on asking for 2 or 3 efficiency upgrades to the design of the home. Could anyone provide suggestions on which strategies provide the best ROI and are most likely to be considered by a production builder. A few things I was thinking of are:
1. 2” Continuous Rockwool on exterior of the building w/ strapping for siding
2. Air sealing at the sheathing level (zip vs taped plywood/Tyvek)
3. Raised Heal trusses
4. Basement slab insulation (they don’t do standard here in UT)
5. PV on roof
6. Others?
Thanks!
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Replies
Brian,
I think your list is excellent. Although... They may balk at the exterior Rockwool.
Some other efficiency upgrades from an ROI perspective:
1. Heat pump water heater
2. Right sized ductless mini split heat pump(s)
3. Exterior shades/overhangs to prevent over-heating in summer.
4. A house design with only 4 corners and a flat ceiling
5. Gasket for mud sill
6. Avoid can lights and try to make ceiling drywall airtight.
7. Avoid expensive spray foam. Cellulose or dense pack fg all the way
Brian,
The only thing I'd add to the good advice you've already received from Rick, would be to perhaps take the list and prioritize it so that you end up with things that are essentials, things you would like to include, and those that you can plan to add in the future because you have made provision for them.
Brian,
Unless you are in a fire zone, exterior mineral wool is expensive material and very labor intensive. Exterior insulation is great, going with foam rigid insulation is much simpler. If you are not in a heavy rain area, and you can go down to 1.5", lot of siding options can be installed directly over it which would further reduce install costs.
Another wall option would be to go with 2x8 walls on 24 OC without exterior insulation. This would be an R23 overall assembly vs R25 for 2x6 with 2" mineral wool but way cheaper to build.
Also consider getting a drain water heat recovery installed. Save a fair bit of energy, quick payback.
+1 on the 2x8 framing, something that isn't a stretch for 2x6 framers to figure out.
Not so much on the drainwater heat recovery, unless it's a larger than average and mostly-showering family. Both natural gas and electricity are substantially cheaper in Utah compared to the rest of the US:
https://www.eia.gov/state/?sid=UT#tabs-5
If heating water with electricity, the better bang/buck is to put the drainwater heat recovery money into a heat pump water heater.
Another, more subtle energy performance improvement is to simulate the energy use (BeOpt or other) under different glazing options and different amounts of roof overhang. In most of zone 6 UT the best bang/buck on windows would be a double low-E double pane with low E coatings on surfaces #2 & #4. The #4 surface coating is limited to hard coats, since it's in contact with the room air, but the coatings on #2 can be either high solar gain or heat rejecting. For the south and north facing windows a high gain coating is better, as long as there is sufficient overhang on the south side to shade the windows during the summer. East and west facing windows should not be high gain, since the low sun angles make it hard to reject unwanted seasonal gains.
It generally looks better to have the same type of glass in any given room (or rather, it looks wierd if they're different) so it may be worth shrinking the east/west windows in rooms that have mostly north or south facing glass.
When it's -10F outside it's possible to get condensation on the windows (defeating the low-E characteristic of surface #4) if you're actively humidifying the space in winter, but as long as you don't overdo it that isn't going to lead to copious condensation pooling on the sills or adding much to the seasonal energy use for those limited hours of extra-cool weather.
Among the bigger window vendors, I believe both Pella and Anderson use Cardinal Glass for their double-low-E double panes. With argon fill the center-glass U-factor is typically ~U0.20 (which is getting into triple-pane type performance) and the SHGC of their highest-gain glass (LoE-180 + i89) is 0.6, nearly twice the solar gain of typical code-min single low-E double panes. It's definitely an up-charge for the #4 coating, but nowhere near the upcharge it is for comparable performance triple panes.
Tuning up the glazing specs isn't going to change your peak loads by very much, but in sunny Utah it can make a HUGE difference on your heating energy use.
Wow, thanks for the excellent detailed explanation Dana. I forgot to consider windows and will look more into this as you and Scott suggest. We definitely get a lot of sun here in UT!
Thanks for all the suggestions!
Rick, I agree with you on the mineral wool. I really like the idea of the continuous Rockwool insulation but figure the builder will probably consider it too much of a PIA to install. Thanks for the other suggestions, I will add them to the list.
Malcom, I will definitely prioritize the list. I figure I may only get 1 to 3 of the "big" requests.
Akos, the reason I selected mineral wool was because 2 inches of continuous exterior foam is not adequate to keep the sheathing below the dew point during the winter in my climate zone. Mineral wool will allow sheathing to dry to the exterior as well as the interior with the warmer temps in the spring as the house dries out.
Thanks again for all the suggestions!
Brian, two inches of Polyiso should be just fine, especially if you use cellulose between the studs. It's greener than the Rockwool anyway.
Not sure the cost is worth it though.
Brian,
Up here in Ontario (zone 5 and 6), there are many houses built without issues with R5 continuous exterior insulation over 2x6 walls. They work because there is a reasonably well sealed interior vapor barrier.
I believe US code also allows for reduced exterior insulation ratio provided there is an interior barrier (poly, craft faced insulation or paint on vapor barrier).
You can also get vapor permeable rigid insulation such as EPS or non-foil faced polyiso (ie IKO energAir), which will still allow for drying to the outside. These are cheaper to buy and significantly easier to install than rigid mineral wool.
This is a good read about this:
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-026-they-all-laughed
Why not consider something like Phoenix Haus, out of Grand Junction? Should be close enough to keep the shipping cost down and would give you what you appear to be looking for, performance wise. I haven't used them, as I don't have an empty lot, but think it would be a serious choice.
Torreyho...interesting idea. I might use them for a cabin I am thinking of building soon. Thanks for the suggestion.
https://www.greenbuildingadvisor.com/article/getting-the-biggest-bang-for-your-air-sealing-buck
https://www.greenbuildingadvisor.com/article/green-building-for-beginners
https://www.greenbuildingadvisor.com/article/martins-pretty-good-house-manifesto
https://www.greenbuildingadvisor.com/article/the-pretty-good-house
Thanks for the links Jaceen. I will read through them.
Consider running BEopt software to really answer the question.
Hi Brian,
First of all, I agree with everyone that you have a solid list to get you started, but I also agree with John. Why speculate and guess at what improvements you should ask for when an energy rater could do some modeling and tell you for sure? That might be the best money you spend.
That said, air sealing is probably the most important and cost-effective thing you can do to improve an average house. It's important to think not just about the large swaths of air barrier, like taped wall sheathing and an airtight drywall ceiling, but all of the transitions. As an example, how are you going to keep the air barrier continuous from the sheathing to the ceiling? And make sure that air sealing is tested with a blower door before it's too late to make easy fixes to the air barrier.
If the insulation is going to be on the attic floor, I also believe the raised heel trusses will have a big impact and don't change the builder's process, they just have to be ordered properly.
Also, consider that now may be the time to put your money into the envelope, which will lower demand on heating and cooling allowing you to spend less on equipment. Of course, that all needs to be modeled.
And you can make the house PV-ready, but wait to purchase the panels which will allow you to spend your money on the built aspects of an efficient house now and easily add renewables later.
Thanks Brian. I agree with the importance of continuity of the air barrier and testing for it early. I will have to educate myself on energy modeling and see if I can find someone local to help me there.
Thanks everyone for the helpful responses!
Air sealing, raised heel trusses and pre-wire for PV will be my starting point.
I think I will still try to get some exterior insulation on the home if one of the builders is confident they can do it right.
If the standard build puts ducts and air handler in the attic, a PLENUM truss would be worthwhile:
https://www.greenbuildingadvisor.com/article/use-plenum-trusses-to-keep-ducts-out-of-your-attic
Most tract home builders pick a furnace & AC sized for the biggest/lossies house in the development, upsize from there by 50% or more for guaranteed margin, then use THE SAME equipment for every home. A typical 1800-2200' 2x6/R20 type home with code-min double panes would come in with a heat load around 25,000 BTU/hr @ 0F (Vernal UT's 99% outside design temp, Logan's is +2F) and at the ASHRAE recommended 1.4x oversize factor would call for a 40,000 BTU/hr condensing gas furnace, most 2000' homes seem to get 60-100,000 BTU/hr equipment. and 3-4 tons of cooling (for a 1.5-2 ton load). While that isn't a huge efficiency hit or equipment up-charge, it IS a comfort issue.
Assuming with all of your design tweaks you knocked even 20% off the "typical" load numbers you'd be in range of ducted modulating mini-split solutions, which may be slightly more expensive to run than a 30K BTU/hr condensing gas furnace, but far more comfortable, and would work with pretty good hourly synchrony with the output profile of your future PV array's summertime output, ramping it's modulated output up & down with the sun.
Dana...I believe the HVAC duct and air handler will be in the basement. I will confirm with the builder.
I will also look into the mini-split solution you suggest.
Thanks again!
1. Continuous outsulation.
2. Soapstone masonry heater.
3. 4 head mini split heat pump.
4. ERV ventilation.
Air sealing. I know it's already top on your list, but make it the top three. And, have it tested several times during construction. The first is generally pre-insulation or pre-drywall, depending on where your primary air barrier is. In my region, I have yet to see new construction that even comes close to code on air leakage. Everyone is taking the "by inspection" approach, but nobody is inspecting. With air leaks the size of a Tesla, nothing else matters.
Second for me would be to have an independent party perform the Manual J, S, and D (if appropriate) calculations, and have the HVAC system designed properly within those standards. And have the HVAC installation commissioned and tested by an energy rater. Pay for these outside services yourself if you have to, to provide some oversight on the builder's subs.
These are all Code, but routinely ignored. Even code minimum here will improve most house's energy performance by quite a bit.
Thanks Peter for the HVAC suggestion. I will look into this.
One thing to consider would be the number, size and type of windows you choose. The very best window makes a lousy wall so if you can swap out an R5 window for an R25 wall you're way ahead at lessening energy losses. Also, people tend to put in too many operable windows in each room. You really only need one in a small room, and probably 2 in a large room for cross ventilation. Having more fixed windows can help lower energy losses.
You can calculate your overhangs using this free online calculator.
https://susdesign.com/overhang/
You should also consider installing outswing doors for access to patios, etc. They seal better than sliding or inswing doors since the pressure of the wind blowing on the door pushes the door against the frame and seals it tighter when closed.
The problem with out-swing doors is the same as we used to experience with storm doors. A heavy snowfall leaves you unable to open them.
Good call on windows and overhangs. I totally overlooked this for some reason.
Akos: The non-foil faced polyiso is IKO EnerAir, not energair.
https://www.iko.com/comm/product/iko-ener-air/