in pursuit of energy efficient minimalism
10 tricks for optimizing a Passivhaus
With a recent run of projects featuring over-insulated assemblies in mild climates, we’ve been pondering a way to get out the message that R-90+ assemblies just don’t make sense for most of the country, and really (if properly designed) can be avoided. Let’s say we’re designing a stunningly affordable, modern Passivhaus for you – but due to site constraints, we’re running just north of meeting the specific space heating demand (SSHD) of 4.75kBTU/ft2a. There is no need to fret – there are methods of optimizing PHPP to cross that threshold in a compliant manner. These aren’t necessarily cost effective, but as we prefer to keep things in the ‘normal’ range of things, most are. What follows is our list of potential tricks to help bump you across the PHPP space heating threshold.
1. Increase living space within the thermal envelope
Can you squeeze a loft or partial height story in the attic? Spaces w/ sloped roofs between 1-2 meters still get calculated at 50% of TFA. Do you really need a 5’ wide stair? Shrink the width of that puppy for a significant increase in TFA (and corresponding decrease in SSHD) and intimate vertical encounters. Granted, it looks awesome – but do you really need that double height space? Does the utility closet (calculated at 60%) really need to be as large as a master bathroom? Let’s design it right so things are still accessible and serviceable, while eliminating the wasted space. Finding ways to increase footage WITHIN the existing envelope can lower your SSHD.
2. Keep interior column area below 0,1 m² (15.5 in²)
Keep column cross sectional areas below this threshold to prevent deduction from TFA (which will increase SSHD). Granted this may necessitate the use of steel, but that’s not always a bad thing (see: Tom Kundig). The PHPP manual states that columns with a base area below this aren’t subtracted – so if a larger wood column is needed, maybe you could get away with some sweet minimal steel cap and base action that doesn’t exceed this threshold.
3. Extend windows to finished floor
By extending windows to the floor and insetting them more than 0,13m (5.12”) from the interior finish – you can add this area to the TFA, which will reduce the specific space heating demand. It also has the added benefits of phenomenal aesthetics and allowing casements and tilt/turns to do double duty as exterior doors – usually at a lower price than an actual door. This can also increase solar gain in winter – which would further reduce specific space heating demand. Sextuple-duty bonus!
foto: djd/Deutsche Energie-Agentur
4. Better insulation for thinner exterior walls
Maybe 16” cellulose-stuffed double stud walls sounded enticing in DD, but now in CD/pre-certification polyiso or vacupor are in play. Reducing the wall thickness on a 50’x30’ house by 6” could potentially add another 75 ft² (assuming the outer extents of thermal envelope stays same). Not too shabby.
5. Similar to above, only for interior walls
Do we really need to use pocket doors and 2×6 wood walls everywhere? Not only does this add unneeded costs, but reduces the livings space within the thermal envelope (and therefore the TFA). Maybe 2-5/8” steel (or 2×4 wood) stud walls would work just as well. If you’ve got 150 linear feet of interior partitions, you could conceivably add about 36 ft² of TFA.
6. Window Psi-install
Big thanks to Bronwyn Barry, whose affinity for window porn (pdf) exceeds our own, for this tip. The ‘built-in’ psi-install for PHPP for most assemblies is 0,05 W/mK (0,029 BTU/hr.ft.F). Over-insulating window frames and positioning optimally within the wall assembly can push the psi-install closer to 0,01 W/m2K (or less given right detailing). This can have a dramatic affect on specific space heating demand. The PHPP manual states, “The standard value for Psi-installation is relatively pessimistic. Please pay attention to a thermal bridge optimized window installation and the corresponding value… Through this the heat demand can be lowered significantly.” By significant, we’re talking specific space heating demand reductions exceeding 0,4kBTU/ft²a.
7. Ventilation Efficiency
Utilization of Certified Passivhaus components ensures the values entered into PHPP are verified and accurate. This can be crucial and have significant implications when it comes to HRV/ERV efficiency. If the product is not certified, 12% must be deducted from the heat recovery efficiency. For a (currently) uncertified product like the UltimateAir Recouperator, which claims 95% efficiency, we can only utilize 83% in PHPP. On a prefab project we’re working on, that deduction causes a 10.5% increase in SSHD, from 4.19 kBTU/ft²a to 4.63 kBTU/ft²a. This deduction may also require looking at a competing supplier, such as Zehnder, Paul, Drexel + Weiss, Luefta, etc.
8. Change locations!
Ok, while it may be easy to move your project in PHPP from Seattle to Denver and freak out when the specific space heating demand reduces by nearly half – it’s not so easy to move your project in real life. However, it would be prudent to still verify your climate info. In Seattle, the weather station altitude is 0.00’. Houses at the top of Queen Anne Hill can be upwards of 400’. Elevation differences from the Meteornorm weather station can result in drastically differing specific space heating demands, so verify if the elevation of your house is significantly different from the weather station – if you’re lower, it may result in an improvement in your SSHD. Higher elevations could be the opposite, though.
9. Verify Shading
PHPP assumes a 25% shading factor for all windows. Entering the actual reveal depths, overhangs, etc. overrides the shading assumptions. This can improve the performance of the windows, which can potentially improve the SSHD due to increased solar gains. Be cautious, though – as the increased solar gains can also increase the percentage of overheating.
10. Thermal mass
Sure, 5/8” GWB in a Passivhaus is ‘thermal mass’ – but some of us coming from a passive solar background like a little more shake to go with our fries – and long-lasting interior finishes. Adding a little more thermal mass (such as a topping slab, or perhaps a stunning Ando-esque concrete wall with exterior insulation) can also change your numbers – however this depends significantly on design, orientation and climate. This adjustment can be significant, so it’s worth looking into. On that prefab we’re modeling, switching to a topping slab would drop the SSHD from 4.30 to 4.19kBTU/ft2a. There is no resulting increase in the space cooling demand or the frequency of overheating for this project (again, subject to design/location). This is the trick I showed some starchitects to ‘save’ a sweet Passivhaus that’s being planned for Calgary. For those that don’t remember this in training, the specific heat capacity input is at the top of the ‘Summer’ tab in PHPP.
Add all these up and you could see some drastic shifts that allow you to reduce any absurd assembly values. Alternatively, it could mean getting away with less-expensive windows. Or perhaps just lowering your heating bill by another 25% per year (on top of the 90% by achieving Passivhaus) – all decent goals in our book. However, the greatest trick we can think of is to build appropriately, like multi-family housing to reduce infrastructure/embodied energy/habitat depletion.
That’s our rough and dirty guide to squeaking out a Passivhaus that just isn’t quite there. If you’ve employed these tactics – or know of any others – feel free to leave a comment. We’re big proponents of open-source knowledge and gladly share our discoveries, so please share yours.
14 Comments
Leave a comment
Powered by WordPress
Great article !
Regarding trick #8 “Changing location”, it’s also better to build in a well protected area instead of an open space in order to minimise the effect the “Wind Protection Coefficients”.
And about trick # 10, this seems to only have an influence on the “monthly method” calculation.
Cheers,
.vince
Comment by .vince — August 27, 2011
Vince,
Very true on the wind protection coefficients. It can get a bit tricky, because to take advantage of the “moderate screening” you may be in a development where adjacent houses are blocking solar gain in winter.
You are right that thermal mass only works on the monthly method – PHPP manual states that this is only adopted into the monthly method – and I’m guessing it is because of the monthly energy balance being slightly more accurate. Our projects tend to utilize a higher percentage of glass, which usually means we’re forced to use the monthly method (due to Qf/Ql being larger than 0.7).
Comment by meliason — August 27, 2011
Great post guys. Having just completed the training this is very timely. It’s been fun to play around with these tips to see how they can impact the heating demand.
Regarding #1, a bottom stair landing less than 3 risers from the floor is another way to squeeze out (in) a few more SF. Curious how you determine the actual window PSI-install?
Comment by jeff — August 28, 2011
jeff,
these can have quite the cumulative effect to the space heating demand.
for window psi-install, you should run a THERM calc. PHIUS said in training if you had a similar detail to something done in EU, you could utilize that psi-value.
Comment by meliason — August 28, 2011
Great stuff. Not only shake with those fries but mustard on that burger.
Can you please provide a larger version of the window detail in #6. Thanks.
Best,
Geoff
Comment by Geoff Briggs — August 29, 2011
Geoff,
Thanks. Slightly larger image can be found here:
http://www.building.co.uk/Pictures/web/r/w/u/passivhaus_jamb_detail.jpg
Comment by meliason — August 29, 2011
[...] trick we have looked at to boost passive solar gains in winter (and briefly discussed in our 10 tricks to optimize a PH post). Finally, the compactness of this house is what we would expect – and not just to keeps [...]
Pingback by brute force collaborative » PHBdW: Passivhaus Bau der Woche 16 — September 5, 2011
Love those floor-to-ceiling windows.
Some more tricks:
Make your closets larger than 0.5 m2 so they count as TFA.
Make at least one room in the basement habitable and the entire basement counts as TFA.
Keep ceiling heights above 2 m, again so the areas count as 100% TFA.
Comment by Greg Duncan — September 5, 2011
Greg,
Soooo difficult for me to get away from the floor to ceiling glazing… Soooo difficult.
Getting closets larger than 0,5m² (5.38 ft²) isn’t too difficult, either.
Good call on the basements. And again on ceiling heights larger than 2m, we luckily haven’t run into head height issues with PHPP.
Comment by meliason — September 5, 2011
One more “trick”. This isn’t specific to Passive House. When optimizing the amount of insulation, first look at areas with the poorest performance. Even a small area with a low R-value can result in a lot of heat loss.
Comment by Greg Duncan — September 8, 2011
This is a great list, thanks for all the great suggestions i’m especially interested in tip #10 as my project that i’m currently designing has a lot of finished concrete slab exposed. as well as some of the concrete walls will just be finished with all the insulation on the outside. giving us what i imagine is a large body of thermal mass, just wondering if you could explain how you go about calculating this exact value. as the PHPP just gives 3 sample values. it it makes a much more radical difference than i expected… so if you were able to point me in the right direction where i can read up more on this that would be fantastic.
Thanks
Tim Naugler
Comment by Tim Naugler — September 8, 2011
@greg – definitely. there are ways of analyzing which assemblies are losing heat in PHPP. though to be honest, i generally adjust assemblies in first pass until the delta doesn’t ‘feel’ right.
@Tim,
There is a calculation you could apply. In the PHPP manual:
C (spec. capacity) = 60+n(heavy) x 24[Wh/m2K]
n(heavy) in the I-P PHPP is listed as “signifies number of massive enclosing wall surfaces (anywhere from 1-6 surfaces which enclose a box approximating a single-zone building).”
I think this means you would probably qualify for the ‘mixed’ 23BTU/ft2.F.
Comment by meliason — September 8, 2011
Mike,
Good stuff. Having been working on narrow lots, the tip I would put at position 1 is Maximize Compactness! It kills when a 20′ buildable footprint means long and narrow in Chicago. Especially when there is a neighbor 8′ to the south. Also, if the volume of the interior is much larger than it needs to be, that adds to the ventilation losses, so it isn’t just surface-volume, but total volume.
Tom B-D
Comment by Tom Bassett-Dilley — October 8, 2011
Tom,
We’re presently working on a project in a fairly extreme climate, and the compactness is just as critical there. A huge issue with urban conditions is ideal solar access may be hard to come by. On a tour of a project recently, we were brainstorming methods to increase solar gain without overinsulating – well-shaded skylights or pop-up monitors seemed to be interesting alternatives.
Good point about the ventilation losses as well – that has become readily apparent in the extreme climate. Our projects tend to be fairly compact anyway – it’s kind of in our design DNA – but until we started looking in other locations, that wasn’t on our radar. With increased volume, it’s really a double hit – ventilation losses increase and so do surface area losses. It’s those projects were the 12% reduction for the RecoupAerator really hurts.
Comment by mike eliason — October 8, 2011