Hoped to get near net zero, not even close
I’ve been planning a small 980sq ft house near Vancouver, for which I was hoping to get near net zero. It’s a rectangle with a recess on one wall and a double pitch roof. The plan was to build to level 5 of a new energy code that’s being phased in in BC. Level 5 isn’t due to be a mandatory step until ~2030. It’s basically net zero, <1.0 Ach50, <15kwh/m2. Our energy modeller has run his simulations, played with the envelope and insulation design and it’s not much more efficient than an average house. The main issue seems to be the poor volume to surface area of the enclosure ratio, where the small size of the building is actually a negative. The modeller comments that they increased insulation values but it didn’t change the outcome by much. I’m a little surprised it hasn’t come out better, but it also raises the question to me if small home builders in future will be penalised for building smaller homes The only way for my build to reach net zero is a 10kw solar system at a cost of CAD $25-30k, which is a cost I’ll struggle to ever get back in energy savings.
Was I naive to think a small house could be highly efficient without costing top dollar? If this energy code is to be standard in future how else can small homes meet these targets?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Even if your kwh/m2 is higher than a larger home, you will use less total kwh. So don't get too hung up on the kwh/m2.
I'm curious where all the energy is going. Did the modeler break it down, and show you what assumptions were made.
980sqft is about 91m2,
15kwhx91 = 1365
In your area you can expect about 1000kwh anually from each kw of installed solar. So you should be able to cover it easily with a 2-3kwh solar array.
Something is way off if you need 10kw of solar to zero out on a 980sqft house in a mild climate like Vancouver.
Small houses are efficient. Forget flawed metrics like "net zero energy" and kwh/m2 and use kwh/year/person. Even better, forget about energy use and estimate lifetime environmental impact per person, including externalities.
Could you elaborate on these points Jon? Thanks.
Brendan did a good job below on house size. Regarding energy efficiency being a flawed metric, when energy is used also matters. For example, 1 kWh at 8pm might cause more environmental damage than 2 kWh at noon. "Net zero energy" gets this backwards.
For the same price, about twice as much solar can be installed by the utility as can be installed on a residential roof. Yet "net energy" usually favors the latter.
Then there are construction practices. An energy efficient house built with XPS might be worse for the environment than a less energy efficient house built with cellulose. A house close to town might eliminate a lot of commuting fuel use. "Net Zero" accounts for neither.
Metrics matter. And while environmental damage is a less precise metric, it's lack of perfection shouldn't deter using it to come up with better designs.
Can you post any of the details of the modeling that was done? Most of the modeling programs generate a summary sheet.
Thanks for the replies. I thought it seemed odd. The modeller is sending me a report soon. Seems like the large majority of usage is going to heating. I will post more when I have it.
>" The modeller comments that they increased insulation values but it didn’t change the outcome by much."
That's probably because in a higher R house most of the losses/gains are from windows, not walls & ceiling. Going to an R100 wall or roof instead of an R20 wall & R50 ceiling isn't going to change the U-factor or solar gains of the windows. Being very selective about window sizing and even optimizing the low-E coatings used on each side of the house can change energy use by quite a bit.
Even in foggy-dew Vancouver passive solar gains from windows matter, and moving more of the glazed area to the south face using a higher gain glass can move the needle on heating energy use. Cardinal Glass loE-180 on surface #2 combined with i-89 on surface #4 with argon fill in a double-pane glass unit results in a solar gain almost as high as clear glass double panes (no low-E), but a U-factor as low as some triple-panes. That's great for the south side, but could induce a significant cooling load if used on a significant area of west facing glass, which gets direct sun well into the evening this time of year in your area. A lower gain glass (or even no glass area) would be better on the west side.
Even north facing windows have some solar gain, but should be used primarily for daylighting, without excessive area. Most of the year daylighting from north facing windows would be shadow-free, with no direct sun except the weeks around the summer solstice.
Does the energy modeling include the efficiency of a heat pump? In your area a right sized cold climate modulating heat pump would use about 1/4 the amount of heating energy of electric baseboard heating.
Hi Dana
Thanks for the reply. I'll need some time to digest all that! The house is being modelled with both Minotair Pentacare unit and a Fujitsu 9RLS3. 75% is the glazing is on the south east facing wall.
Have you done a comparison between that system and a conventional ERV with a larger Fujitsu? A 15RLS3 will only cost a few hundred dollars more than the 9RLS3, while the ERV will likely be thousands of dollars cheaper than the Minotair. The Minotair is an interesting product, but unless you can actually forgo another heat source, it's hard to imagine a case where it makes financial sense.
Hi Trevor
The idea was that the Minotair would be sufficient for our heating needs without another heat source. If it's not, I wont be using it.
I see, I misunderstood. I thought it was the Minotair and the Fujitsu, not either/or.
>"'A 15RLS3 will only cost a few hundred dollars more than the 9RLS3..."
That's right, but it's almost a "so what?".
A 9RLS3 has enough capacity for the whole house, making the 15RLS3 would be overkill on capacity for a high-R house in that location. I suspect the reason for the Minotair Pentacare is distribution behind closed doors. The 9RLS3 has enough capacity to heat a 1000' higher-R house, capable of 15,000 BTU/hr @ -20C/-4F. That's probably enough for a code-min house at the much tamer -3C-ish outside design temp :
https://ashp.neep.org/#!/product/25332
The 15RLS3 would have something like twice the capacity needed for the load of a high-R house that size at -3C/+24F:
https://ashp.neep.org/#!/product/25336
A compact-ducted Fujitsu 9RLFCD and a separate HRV would make more sense than a 15RLS3 or the 9RLS3 + Minotair Pentacare
https://ashp.neep.org/#!/product/25310
15,000 BTU/hr @ -15C/+5F and 13,500 BTU/hr+ @ -20C should be enough for a high R house- it's already overkill for capacity for the load at -3C, assuming the place isn't crazy-overglazed with U0.35-ish windows.
My comment was based on the premise that he was modelling a 9RLS3 plus the Minotair, which is good for about 6kBTU. I didn't do any evaluation of what his load actually was. Taking that premise away, yes it makes no sense.
Deleted
Dana, what do you think about the Minotair unit alone for this build? A scenario where bedroom temperatures can be altered independently would be ideal. Can the compact ducted Fujitsu provide that?
Just to phrase what other's have said in a slightly different way:
A small building can make it more difficult to hit kwh/m2 EUI (energy use intensity) targets as compared to a larger building.
But your overall energy use should still be less than a larger, but otherwise similar, home. I wouldn't expect you to have significantly more trouble than average hitting net zero in your 980 sf design.
Also, to follow up on the point Jon R was making, in terms of environmental impact, it's usually more relevant to consider how much energy each person uses, as opposed to how much each sq ft of house uses.
For example, take a 1000 sq ft house. Which of these is a more environmentally friendly way to house two people?
- Two 1000 sq ft houses each with one occupant
- One 1000 sq ft house with two occupants
(assuming the houses are identical in each scenario).
Obviously the house with two occupants is more efficient, yet the EUI (kwh/m2) in the two examples will be very similar (slightly different due to the loads of the 2nd occupant of course).
Since you are not required to hit the level 5 code, and you are finding the EUI metrics may not be appropriate for your build, it might be worth looking at other performance targets. One target could be hitting net-zero with an amount of solar panels that fit comfortable on your roof.
While we are on the topic of energy use targets, here's a really interesting one that might inspire some ideas: https://en.wikipedia.org/wiki/2000-watt_society
Thanks Brendan, this was really helpful and makes a lot of sense.
Something is way off. The owner of a 2500 sf house I built several years ago on Vancouver island - by no means a high-performance one - is offsetting his entire Hydro bills with a smaller solar array than your modelling suggests.
I totally agree. Either the modelling has to be off the rails here, or the house is in a dark canyon that gets no sun.
I have relatives on Whidbey Island, WA heating a ~1400' 1990s code-min house with 1.5 tons of ductless that could hit Net Zero with less than 10 KW of PV.
I also know someone living in a 1960s vintage double-wide trailer (about 800') heating the place with a single 1.5 ton Mitsubishi and a bit of electric baseboard near Port Townsend WA who could hit Net Zero with 10KW of PV.
Can you post a floor plan so we can see if there are any problems with the layout or window placement/orientation?
Deleted
Wow, lots of replies, this is such a cool community. Thanks for everyone for chiming in. I've attached external views and floor plan. The wall with the majority of the windows faces south east and has a sea view. To the north east is sheltered by tall trees nearby. West is partially sheltered by trees.
I've attached a floor plan and external views. There's ~150sq ft total of glazing and 2/3rds of that is in the south east facing wall.
Walls are 2x6 with r24 Roxul plus 2 inches of Roxul on the exterior. Roof is approx r58 blown cellulose. Windows have a U-value around 0.15 (triple pane).
Specifying low flow water fixtures, energy star appliances and typical usage temps has reduced the projection to a 8kw PV system.
I'm trying to build as green as we can afford, the budget is modest and I will be doing a lot of the work.
Alex,
One part you can save a fair bit of money and labor is by swapping out the rigid mineral wool. Unless you are in a fire zone, it is a waste of money and a pain to strap out flat.
Essentially the 2" board is an 8" batt squished down. You are paying for R30 of material but getting R8. You are much better off with going with something like 2" felt faced polyisio which is vapor permeable, cheaper and gives you more R value.
Depending on what your modler say, you might be able to save a bit and get away with double panes on some of your facing windows. The triples are worth the upcharge for comfort if you will be sitting near them though.
In terms of the overall house design, I think your overhangs a bit to low to make efficient use of your south facing windows. If you raise them up and extend them slightly, you can get the right amount of shading plus allowing more winter sun in.
Outdoor ares with roof coverage are more useful than open decks. This lets you use the space when it is rainy or very sunny. I would aim for enough coverage over the deck for a decent seating area.
Thanks Akos.
The choice of Roxul is in large part due to fire concerns. We live in a wooded area that hasn't seen rain from June to September the last few years and the fire risk is high in summer. We also prefer to avoid using foam.
I like your idea about a roof over the deck, I'll give that some thought as well as the overhangs.
Alex,
In that case you are probably better off with cross trapping your wall with 2x3 on edge and going with 2.5" AFB. The strapping would have to be at 17.5" OC for the insulation to fit. If you are using vertical siding, you can hang it straight off the 2x3 strapping, if not install the rain screen over these. You would need some spacers (larger washer or thin plywood) under the strapping to allow for drainage.
This would give you slightly higher R value than the comfort board and be way cheaper and easier to install.
It seems the higher r value would be offset by the lack of a continuous thermal break? In your area, what cost savings would you envision from your suggested method?
Alex,
By cross strapping I mean the 2x3 at 90deg to existing studs. This mostly limits the thermal bridging. With R24 cavity insulation and R10 AFB you end up with something like R30 overall wall VS R25 with 2" rigid.
Rigid insulation is not cheap and it takes a lot of work to get strapping straight enough not to have wavy siding.
Thanks Akos, I follow you now. One of the potential window suppliers also makes a product that would do the same thing as the 2x3. I'm going to look in to it and see how it compares.
https://www.cascadiawindows.com/products/cascadia-clip
The generous use of acronyms on this site is driving me up the wall. Please tell me what AFB is. I plan to build in a wooded area that is not now at great risk of fire but who knows what it will be like in ten or twenty years.
Nils, it's not this site in this instance, it's the product name.
https://www.rockwool.com/products/afb/?selectedCat=rockwool%20afb%C2%AE%20downloads
It stands for Acoustical Fire Batt.
Nils,
Rockwool AFB (acoustic fire block). It is an insulation commonly used in commercial construction that is readily available through drywall suppliers in a number of thicknesses unlike Comfort batts.
The only catch with it is that is sized for steel studs, so its fine with TJI but you have to either go with non standard spacing (ie 17.5" on center) or trim them with dimensional lumber.
AFB is also great when you want a thin noise isolating wall. 1 5/8 steel double stud wall with 1.5" AFB is something like STC 60 in only 5.5".
Akos,
Interesting note on using felt faced polyiso in lieu of rockwool.
Do you happen to know if there is a difference in perm rating between them? And does it matter ?
On my current build I used comfortboard 80 (for fire rating) but also because I wouldn't get away from not having interior vapour barrier so I needed to dry the wall assembly to the outside. You're correct in that installing the furring level over comfortboard is a PITA. Next time I'd try comfortboard 110 as it's a little more dense, but certainly polyiso would be the easiest to get furring level.
Jamie,
You'll have to check what is available locally. You might have to do a bit of digging into the data sheets for the actual spec. There should be a couple of options with greater than 1 perm. Not as permeable as mineral wool but still allows for drying towards the outside.
IKO has made a business out of higher-permeance polyiso board with a pre-applied WRB:
https://www.iko.com/comm/permeable-polyiso-wall-insulation/
In looking at your plans: the front hall is too narrow. Do you really need a tub in the main bathroom? If you went with a glass shower you could move that hallway wall back in line with the bedroom wall below it (the room would also "feel" more like a powder room).
Your master bedroom (although large) has almost no storage space (dresser/chest of drawers). I would move the two closets to the opposite wall and have a lower dresser between them under the window. The bed and night tables would go on the north wall and slide right towards the master bathroom(as will the bedroom entry door).
With the bedroom door now further to the right you should flip your kitchen layout top to bottom. The fridge and stove will be along the bedroom wall and the kitchen sink looking out to the left under a window. Now your dining table will be near the windows on the south.
The small inswing exterior door and the small window next to it should be changed to a double outswing door.
Where are your linen and broom closets?
We are set on the interior layout. It reflects our usage of space and we do not own a lot of "stuff". The closets are large for our needs. There wont be a dining table and extra storage for non-essential items will be built in under beds and mechanical room instead of giving up limited square footage to dedicate space to linen/broom closets that will barely have anything in them.
At this point fine tuning the envelope performance is the goal.
I like the modest design and also encourage a roofed over deck. Not sure why the roof notches back on the entry side...kinda fussy to my eye. A small house requires some sacrifices but it looks like there's an attic and crawl space to store a few things...
Yes agreed on the roof, the reason for that was that we have some tight spaces to work with and an existing auxiliary building that would have been too close without the notch. I also like the idea of a roof over a portion of the deck, but I haven't come up with a way to do it that looks good. Would have preferred the deck to extend to the south but a neighbour has an easement that prevents this being possible. Thanks for the comment.
For a balcony I extended part of the main roof for cover. Beside the extra cost for materials, it was relatively straight forward.
You might be able to do something similar on your west side by extending the roof lookouts. Looks wise, it might work if the deck was longer on that side.
Good idea, thanks
I would like a clear explanation of why this is supposedly happening:
“The main issue seems to be the poor volume to surface area of the enclosure ratio, where the small size of the building is actually a negative. The modeler comments that they increased insulation values but it didn’t change the outcome by much.”
If you put a lot more insulation into the house compared to an ordinary house, make it more air tight, and do every other thing possible to increase the energy efficiency; how does it end up performing not much better than an average house?
Use irrelevant metrics for "better" and "poor" and that is the result.
Compare Alex's 980 sq ft design to a typical 980 sq ft house and the former will outperform.
What I want is a definition of a “poor volume to surface area of the enclosure ratio,” and an explanation of why it would poor, how it can be optimized.
I also want to know what it means for a “small size building to be a negative.”
I think its getting at how a two story building with 980sq ft on each floor would have the same size roof, thus only the wall surface area increases. However I still don't understand why higher insulation didn't show improvements.
So it turns out the loads are coming from hot water (~9.91GJ) and base usage (~21GJ). Heating load is low hence why insulation makes little difference. How do those numbers sound? We use maybe 6-8 min of hot water a day, run the dishwasher 2-3x per week and washing machine 1x per week. Thats the extent of our hot water usage. Appliance wish the whole house will be energy star appliances and led lighting, normally dimmed down. I had looked at heat pump dryers but in Canada the options are limited and the only reasonably priced model is Whirlpool with terrible reviews.
For a couple of rentals with electric hot water and natural gas heat, I'm seeing between 280 to 350 kWh per month outside of cooling season. These are recent units with high efficiency appliances, led lights and washer and dryer.
That is around 3.5MWh/year for all electiciy, so the 2.7 from the simulation seems high. I find about 1/3 is for hot water 1/3 for dryer and 1/3 for the rest.
A heat pump water heater will cut water heating energy use by about 2/3, while adding a small amount of heating energy use during the heating season.
If the 6-8 minutes of hot water use per day is a shower, a drainwater heat recovery heat exchanger can cut the hot water energy requirements for that shower by half or more. A heat exchanger that size requires at least ~4-5' of vertical drain downstream of the shower drain, with the tepid potable water output feeding both the cold side of the shower mixer (or the whole house cold supply) and the input side of the water heater. The EcoDrain VT-1000 series is a 3rd generation version with very good efficiency numbers that outstrip the best in class 2nd generation Renewability PowerPipe versions on most specifications, but either would be a huge benefit if showering is your primary hot water load.
https://ecodrain.com/en/products/VT1000/
https://canada.renewability.com/
Natural Resources Canada used to maintain searchable listings (and a downloadable spreadsheet) of third-party performance testing of drainwater heat recovery models to a standard protocol to help compare apples to apples, but that web page seems to have gone away or moved.
So my takeaway would be in the PNW climate, a small house built to the recent energy code requirements is already efficient enough that plug and appliance loads dominate. Further improvements in the building envelope aren't that useful.
That's probably true! (And not just in a temperate PNW climate.)
Many Net Zero Energy and PassiveHouse efforts flounder on the rocks of hot water use & plug loads due to a hyper-focus on heating/cooling. This house built (by some folks I race against in the local skiing beer-leagues) in 2010 fell short of Net Zero primarily due to hot water use, and probably would have made it with current 2019 technology heat pump water heater &/or a drainwater heat recovery heat exchanger:
https://www.energysage.com/project/5302/erik-and-jessica-haugsjaa-stow-ma-solar-pv/
When the standard you're trying to meet is based on kwh/m^2 small houses suffer. An 8 minute shower in a small house takes the same kwh as an 8 minute shower in a gia-normous house, but the kwh/m^2 for that hot water use is smaller in the big house.
So with that in mind, do you think we might see the end of threads about how to eliminate the thermal bridging on front door locks?
>"...do you think we might see the end of threads about how to eliminate the thermal bridging on front door locks?"
Not a chance!
There will always be people focused on the minutae while missing the big picture.
Malcolm I think you're right too and Dana sumarises exactly what we have run in to. Our house was modelled at 1.5 Ach50, r28ish (maybe slightly lower) walls, r57 roof, r17 slab, triple glazed windows. Our space heating demands have come out at <1 GJ a year in all scenarios with a mini split or Minotair unit as primary heat source. Interestingly, the Hot2000 modelling software doesn't allow for a heat pump to be selected as the primary heat source, only as a back up source, even though newer units will operate at temperatures well below anything we have seen locally.
Our modeller said we could remove the exterior insulation and still have no significant changes in heat load but obviously wanted to keep it for other reasons.
Is DHW recovery possible in a slab on grade?! In winter we might have 1 bath a week after a hard days skiing and we'll be adding a kid to the mix in the near future.
Still, the model seems to overestimate our base electricity usage comparing recent electrical bills from months where we have no heating or cooling loads. The values are close to our current situation in a 50 year old house with 3 people in it (we will only have 2 in the new build), runs of exposed copper pipe from the hot water heater, 50% led lighting at best, 1 or 2 energy star appliances (tested once at 14ach but hopefully lower now...).
It makes the r50 or 60 walls on a passive house not far from us seem like little more than an expensive way of making marginal improvements in heating/cooling loads perhaps to offset some of the factors Dana mentions. Either way it sheds some questions on the financial sense of such approaches and the standards being used to assess efficiency as others have highlighted in this thread.
It seems like the "pretty good house" standard or even a "marginally better than current code" standard is plenty good enough in terms of envelope design.
Which begs the question: What is the use of the Step Code?
That is the challenge with designing efficient buildings. Once you get above around R15, adding extra only reduces your heat loss by a small amount. This is a good illustration of the issue:
https://www.buildinggreen.com/sites/default/files/articles/SPFA%2520picture%25201.gif
It doesn't really make sense to go much above R25 overall for a wall except in very cold climates.
In terms of heat recovery, some units can be mounted horizontally (Ecodrian A1000). They are less efficient than the vertical units.
Malcolm I'm left wondering the same thing, now that code calls for airtightness testing and not just prescription the step code appears to make very little difference to the outcomes. Granted, the aim is for net zero to be adopted in another 13 years but at this point in time there's little financial incentive to pursue net zero on a house like mine. The price point of appliances that may make a difference like heat pump dryers or heat pump water heaters is high, options remain very limited and the cost of solar is still too high. The pay-off is a very long way down the road.
Maybe the step code makes more of difference further north or for commercial buildings...
Alex,
I don't know if it is the right answer or not, but I think we are seeing it getting adopted by cities as being an attractive way to show legitimate concern about needing to reduce carbon emissions - and not based on any useful scientific metrics.
I'm inclined to agree with you on that Malcolm
My house is more than twice the size of yours in a much colder climate, and my house is certified net Zero under the CHBA.
Like your house, plug loads and hot water dominate.
You need to make sure you are assuming energy Star appliances, LED lighting, and reasonable estimates for ventilation energy usage. We are net Zero with a 10kw array.
I think the problem is with your model, not the idea.
>"Is DHW recovery possible in a slab on grade?!"
It is pretty straightforward if the shower is on a second floor.
If the shower is at the slab level the EcoDrain A1000 or B1000 can deliver at least some reasonable return (probably not half), and may require a drain pit to be designed and build into the slab to accommodate it:
https://ecodrain.com/en/products/A1000/
https://www.greenbuildingadvisor.com/article/drainwater-heat-recovery-comes-of-age
https://ecodrain.com/en/products/B1000/
>"In winter we might have 1 bath a week after a hard days skiing and we'll be adding a kid to the mix in the near future."
Drainwater heat recovery does nothing at all for tub-filling performance, since the drain needs to be flowing concurrently with the hot water. For a mostly-showering family they're great, and increases the "apparent capacity" of the water heater, since it takes less flow out of the fully-hot tank when mixing with pre-heated tepid water from the heat exchanger to achieve the necessary temperature at the shower head.
So the modeller reviewed some things with a colleague and the solar array can shrink to 7kw. HW demand can be reduced by just over half with a heat pump water heater. 7kw is still a fair bit higher than I had anticipated.
I did a lot of ‘what if’ for modeling my 2000 sq ft passive house using US Dept of energy software , BEopt. The main take away is that the whole envelope must have continuous consistent insulation. Adding roof insulation did very little if there was no underslab insulation. Ground tempered surfaces like the slab will double the R value of the insulation so R-30 underslab will equal R -60 in ceiling. Get rid of thermal breaks and air leakage have moderate insulation that is continuous in the whole building and you will have an economical and efficient house. Get a heat pump water heater that uses outside air and heat the house and your hot water . I’m betting 6 kw solar will make you net zero.
Kamilche Passive House