Effects of House Size on kWh/m2a

[ericstorm]

Has anyone noticed, dealt with, or read about the effects of house size on the results of the PHPP calculations?

I am working on a small house (22'x28'; 616 sf exterior dimensions) with 10" walls (531 sf interior), and while the U-values of the envelope elements are all lower than the Darmstadt example (and the windows are a net gain and the PE and ventilation are fine), it is coming in at about 24 kWh/m2a.

In playing with the variables, it seems that smaller homes have a harder time qualitfying and mostly to be a factor of using energy use per area as the metric. I can get it to 15 kWh/m2a if I INCREASE the size of the house without changing the construction. A larger home, of course, uses more energy than a smaller home, but uses LESS PER SQAURE METER. I don't think it's about volume to surface ratios, because I can increase it in a long skinny way (52'x28' or 22'x91') or a square-ish way (37'x37') to make it work. And the numbers get "better" the larger the house gets, and "worse" the smaller the house gets.

Any thougths or experience on this? Is there a multiplier or some other correction for smaller homes? I'd hate to think that one would have to build a larger home just to qualitfy for an energy standard.

Eric Storm
Portland, Oregon

[Mark Siddall]

Eric,
The issue of Very Small Passive Houses was raised in the PassivHaus Conference Proceedings 2008 (pp 411.) As the surface area to volume ratio is not ideal the performance requirements of the envelope increase significantly thus achieving the 15kWh/m2/a becomes increasingly difficult the smaller the building. The U-values in the example are:

wall 0.124 W/m2K (alt option 0.087 W/m2K),
roof 0.092W/m2K,
windows 0.68 W/m2K,
floor 0.018 W/m2K,
MVHR: 92% efficient HR, elect eff. 0.36Wh/m3

The energy use was 15.9 kWh/m2/a and could using the alt option be brought down to 13.5 kWh/m2/a.
Sounds like you need to go around full cricle one more time and toggle the building elements (U-values and MVHR) to finally achieve the desired performance target.

Hope this helps.
Mark

[DMcEvers]

Marc and Eric,

This raises an interesting point, is the ultra green McMansion any better for mother earth than a tidy little bungelow that has been weatherized to the full extent and uses a fraction of the energy of the McMansion? I really think one of the declarations that needs to be made for any new home is total yearly energy consumption and A CO2 statement. You could take it further with a total embodied energy for the new build, we then might take a different look at dwelling size. This is probably the main reason why as a builder I gravitated towards refurbishment of existing homes as opposed to building new, it seemed logical 15 years ago and still does. My quest these days is to see how efficient I can make a home that was built years ago.

I am all for Passive House and the building standards imposed, this is where we need to be for new construction, I do however feel that total energy consumption on an annual basis should be identified.


Doug

[ericstorm]

Thanks, Mark, for the quick reply.

Here is some feedback based on your example and my thoughts on surface to volume ratios.

Working from your example I tried:
wall 0.091 W/m2K,
roof 0.093W/m2K,
floor 0.096 W/m2K,
windows 0.61 W/m2K,
MVHR: 92% efficient HR, elect eff. 0.36Wh/m3
= 14.45 kWh/m2/a

So it works, but that's R-62 in the walls (for those of us who think that way), 16" of cellulose, for Portland, Oregon (HDD=4,400F or 2,444C not unlike East Anglia...with the Latitude of Venice). And I haven't found windows (or an HRV) in the US that can meet those values. Compared to the Darmstadt (HDD=2,616C) example:
wall 0.138 W/m2K,
roof 0.108W/m2K,
floor 0.131 W/m2K,
windows 0.70 W/m2K,
MVHR: 83% efficient HR, elect eff. 0.40 Wh/m3
= 13.79 kWh/m2/a

So, Mark, you are correct that higher performing envelope elements can get me there. But that only begs the question of why small buildings need to (should have to?) perform better than larger buildings to qualify.

In looking at the surface area to volume ratios, I am not sure they are that important. The larger the numbers used, the lower the surface to volume ratio (interestingly this makes it look worse in square feet than square meters!). I think the treated floor area (bigger is better) is the main factor, and here's why.

I start with a surface area (229) to volume (128.8) ratio of 1.778. I can double the height of my building (but not add another floor), making it closer to a cube with less surface (323.3) to volume (553.7) and a ratio of .5838, and the kWh/m2/a value gets worse. But it gets much better if the extra height also gives another floor. So the same surface to volume ratio (tall one-story vs two-story) can result in dramatically different kWh/m2/a values (25 and 8 respectively!). It's the floor area not the ratio.

And, I can double the long side (doubling the treated floor area), making it have only a slightly better surface (406.7) to volume (269.7) ratio of 1.507, and get a better kWh/m2/a value (10.41). Here it's because of the increase floor area, not the lower surface to volume ratio. If the added size does not increase treated floor area on only envelope, the kWh/m2/a value gets worse not better (32.8).

I see that the party wall of the Darmstadt example helps its results a lot. (FYI: I think there may be an error on the height of that wall, Areas K45 shoud be 8.8925, but it's not used in calculating the heat loss calcs.) If the party wall is changed to an exterior wall (Areas D45 changes from 18 to 8, and ignoring the above error), the kWh/m2/a goes from 13.7 to 29.59.
With 14 fewer square meters of floor area but the same envelop, this building would not qualify. Similarly if you shorten (east west direction) the building by 1.33 meters, it no longer qualifies.

So I am still open to being shown how this works another way, but as far as I can make sense of it, it seems that the amount of treated floor area is a much bigger factor than surface to volume ratio. And so small buildings are penalized in some way by the way things are calculated or by the energy per area metric.

In determining how comfortable an individual will be in a building all these factors come into play: outdoor temperature, how well insulated and sealed the building is, the amount of solar radiation reaching the interior, the internal heat gains, the amount of wind outside, and individuals' opinions about what constitutes a comfortable indoor temperature, relative humidity indoors. And we are trying to achieve comfort with the least possible energy.

Perhaps the "Balance Point", the temperature at which a building begins to need space heating, may be a good number to watch. Energy per person? Or as Doug mentions total annual energy. Well, I am still letting this settle in. So if someone can see something I'm missing here, I'm all ears.


Eric Storm
Portland, Oregon

[ericstorm]

Stepping back a little to see where this all needs to go, I read a document on the Passiv Haus Institut website about translating the PH standard to other regions. While I think Portland, Oregon is similar to some European climates, working from principles and goals may also apply to different house sizes. Here are the relevant passages, with my bolding for emphasis.

First Steps: What Can be a Passive House in Your Region with Your Climate?
http://www.passiv.de/07_eng/FirstSteps/First_F.htm

(pages 4-5)
"the method for investigating a Passive House solution will be the following:
1. Attempt to use passive technologies to reduce the peak load demand of the service in question. Possible approaches include insulation, shading, use of subsoil heat exchangers and reduction of internal heat loads by using high efficiency appliances.
2. If comfortable indoor climate conditions differ greatly from outdoor conditions, it is always recommendable to use a ventilation system with heat recovery (or vice versa with cold recovery) to maintain a high indoor air quality without the need of huge heating or cooling demands. See Ole Fanger’s work, “Thermal Comfort” or ISO 7730 for a definition of “comfortable indoor climate”.
3. There will be a certain point in the cooling/dehumidification demand so that with lower demands, there will be an appreciable simplification of the active technology needed. This defines the Passive House Solution in your climate!"

(pages 1-2)
"...it would be folly to directly copy details, especially those for insulation, windows and ventilation from the Central European example to other parts of the world. Instead, the details should be found to suit the climate and geographic conditions to develop a Passive House solution of each location. ...
By dramatically increasing the energy efficiency of a building, the HVAC systems can be radically simplified upon reaching a certain level of efficiency. ...
But the most significant threshold appears when the peak heating load reaches 10 W/m². When the peak heating load is less than 10 W/m², independent of climate, the ventilation system can easily be used for space heating, and a separate heating system is no longer required."

(page 4)
"If the maximum load is lower than 10 W/m², the ventilation system can distribute all heat needed throughout the building as well. The definition of a Passive House is therefore that the peak heating load should be projected to a lower level than 10 W/m². In warmer climates, this value may be easy to achieve, however in colder climates, careful planning is required.
There is almost no extra benefit gained by increasing efficiencies beyond this point the 10 W/m² threshold. The construction costs could rise dramatically if the goal is to construct a “Zero Energy House” instead of simply a Passive House. As well, there is almost no additional environmental benefit. A Passive House has a very low energy demand for maintaining interior comfort in the heating season. The heating demand is so low that the environmental impact is negligible even if fossil fuels such as oil, gas, or coal are the heating sources."

From this I conclude that, regardless of size and climate, if a design uses passive technologies and principles to allow for heating via the ventilation system, or more generally allow for the use of a radically simplified and non-separate heating system, that it could be considered a Passive House. Would anyone disagree with that? (Of course actual certification would be subject to review by the relevant parties.) And if that is the goal, which metric would you suggest to measure that?

Eric Storm
Portland, Oregon

[Mark Siddall]

Eric,
The 10W/m2 peak load is determined by the max temp that you heat air to before burning all the dust and degrading indoor air quality. You can make any building form into a PassivHaus, this issue is that by keeping a close eye on the S/V ratio and seeking to minimise it you avoid can unnecissary capital expenditure. A 1 storey building will be less cost effective than a 2 or 3 storey building of equivalent habitable area.

I agree with Doug that an upper limit should be set. Within a PH this is done but it is not highlighted very much. PassivHaus calcs allow 30-35m2/person (I think that this relates in part to ensuring that relative humidity is adequately managed) so therefore by default the PH standard limitation is 450-525kWh per person. To extrapolate on this basis a 3 bed 4 person house would have maximum area of 140sqm (~1500sqft) and the energy use would be a maximum of 2100 kWh/annum..... Not sure how this accords with the McMansion standard.

At the PH Conf a chap from an American energy agency did a presentation. He suggested that Energy Star will eventually develop space standards thereby, within the confines of ES, preventing the "pseudo-Eco" McMansion (my term not his).

Mark

[MarcR]

My two cents (three with the devaluation of the dollar):
- I think relevant metric in achievability of PH is Surface area/TFA, not volume - small houses usually worse
- Energy and resource budget per human seems ultimately the most equitable (shoot, I'll have to get rid of some of my bicycles)
- Achieving 10W/sm in cold climates is really tough. There's some point where a simple point source heating system (think minisplit heat pump) in a superinsulated house plus the photovoltaics to run it costs less than the additional envelope upgrades, and using the heat pump uses less energy due to the COP than ther PH with electric resistance heat.
Marc

[Mark Siddall]

Marc,
- Surface Area to Floor Area is useful though double height spaces would be an issue... this would be a distortion in the calc as a PH based upon 2.5m floor to ceiling (should really have checked this statement by refering to PHPP).... as an architect I like double height spaces but this can cause havoc in a PH if not managed appropriately)
- Agree
- The 10W/m2 is the European determinate where the design temp is between -14C to -10C. I agree that more extreme climates, where the design day is more challenging, other more cost effective solutions may be sought in order to address the peak load. The cost effective range of insulation needs to be carefully considered an IEA study suggests that in a PH a U-value of 0.15W/m2K is ideal. My own study suggests that, within the UK at least, even U-values sub 0.1W/m2 may be quite economic over 25 years.

Mark

[ericstorm]

Thank you, Mark and Marc, for your input and thinking on this.

I agree with the Surface Area to Floor Area comment, which makes sense given the Heat Load/ Floor Area methodology and that Heat Load is related to Surface Area using a similar envelope U-value range.

Likewise, on the Energy/Resource per person idea, not unlike the Ecologial Footprint.

As for 10W/m2, I'd like your help in disecting this so that I can see how well this will apply to my climate and house size.

My building (which as designed comes in at 23.95 kWh/m2a) has a Heating Load (Ph) of 924W, and 17.7W/m2 (using the city with the mose similar climate--Bolzano, Italy--for heat load data). Using the envelope upgrades Mark suggested earlier I can get to 14.45 kWh/m2a, 677W, and 14.2W/m2, respectively. As you can see, while the building would then qualify for PH and the Heat Load is low, the W/m2 is still over 10 and not dropping as fast as the others. I think this is again a factor of the small size of my building. Since I was using minimal ventilation, if I increase the ventilation operation so that the heating system can supply the heat (Heating Load Q95), I get 15.59 kWh/m2a, 691W, 14.5W/m2. But still not 10W/m2, and I've got to tighten things up even more to meet the 15 kWh/m2a.

Perhaps I'm beating a dead horse here. But any thought or observation you have would be much appreciated.

If I ignore the 10W/m2 issue for a moment and I look at my options for supplying this amount of heat, a simple electric baseboard or wall heater could handle this quite economically. It would seem that I could also put something in the the HRV system (electric or hydronic) ... or is that where I bump into the 10W/m2? Either way I am planning to use solar PV and solar thermal. Any thoughts?

Thanks again.

Eric

[Mark Siddall]

Eric,
A post heater in the MVHR can supply up to 10W/m2 before your get into pyrolysis or the air (incidentally hydronic coil linked to gas or perhaps ground source heat pump is preferred to direct elec resistance as it has a lower carbon footprint).

Often people add a rad to the bathroom to help with drying towels and to provide a little more comfort (say 24C). A medium size towel rail can supply 450W. Over 55sqm that's an extra 8W/m2. (A large towel rail can supply 800W which over 55sqm is about 14.5W/m2.) You could try to heat the whole house from the rad but I'd encourage using post-heater as principle heating and then using the rad as the back up. The reason being that this will result in smaller temperature gradients within the house thus greater comfort. (Also if the bathroom is located on an external wall, which in a small house is likely, then the direct heat losses to the outside will be than much greater than if centrally located within the home.)

Masonry or timber frame? I ask as this could impact upon the options. If masonry construction is used the heat can diffuse through the house easily (low thermal resistance of masonry partition walls). In timber frame it is unlikely that diffusion would be as good due to acoustic insulation within the partitions which then causes greater thermal zoning. This zoning can be exploited by the way, I spoke to a German architect that uses small radiant plaster board panels (hydronic coils set within) so each room can be managed with a degree of independence (he reckons it only cost and extra 1000 Euros).

As I understand it when you have a peak load of less than 1kW (you have 924W) then a simple water heater could heat the space and the DHW....

I'm no services engineer but I hope that this illustrates the kind of space heating options that I think are available, and are fairly realistic, when you can't satify the the 10W peak load criterion.

Cheers,
Mark