Oct '11 - Feb '12
Air infiltration is a fairly new concept to me. We never discussed the consequences of a building enclosure's air tightness in my undergraduate architecture studios. When I worked as a facade consultant, each facade came with a thick set of specifications that dictated the maximum allowable air infiltration per lineal foot of curtainwall joints and operable window joints. At the time, I thought these requirements were merely to cut down on breeze or were just an indicator of the building's water tightness. It wasn't until our studio collaboration last spring with Columbia as part of their CBIP program that I got a glimpse of the power of an air tight building.
At PAE our first task in the CBIP studio was to analyze one of the existing NYC building's selected for study in the studio. We modeled the building as-is in IES to get the current Energy Use Intensity (EUI), or energy use per unit of area, and then performed incremental modifications to the enclosure to gauge their effectiveness in reducing EUI. We tested horizontal shading, increased thermal mass, increased roof and wall insulation, and improved glazing specifications to name a few, but the results were minimal compared to the effects of increasing the air tightness of the building's enclosure. I found that for my building, a midrise brick residential building, the building's heating loads were cut in half with an increase in air tightness from a relatively leaky building (1.0 air changes per hour) to a fairly air tight buidling (0.5 ACH).
It's pretty simple. Hot air with high pressure wants to relieve itself of its high pressure by mixing with cold air with low pressure. So in the winter, when your bedroom is heated to 72°F, and its 20°F outside, regardless of how much insulation there is, all of that warm air is going to try its hardest (and succeed) to sneak out of any little crack in the wall or leaky window jamb it can find. The air will even force itself through somewhat porous materials such as plywood or bricks if the pressure differential is great enough. Thus, any heat that is pumped into a leaky home will need to be replaced at a quick enough pace to replace the heat that just escaped. Think of a leaky bucket; if you were to attempt to keep the bucket filled to the brim, it would require a continuous flow of water to keep up with the water leaking out of it; not very efficient.
Looking back, a lower heating load is a pretty obvious result of increasing air tightness, but there were two very surprising aspects to it.
• First off, I had no idea it would have such a great effect. I mean cutting your heating bill in half anywhere in the northeast, just by properly constructing an airtight home, is pretty incredible.
• Second; if you take a look at the chart above, increased air tightness actually had the inverse effect in the summer. The cooling loads for the building I was studying increased with the increase in air tightness, negating some of the savings in heating.
If an airtight building envelope could effectively keep conditioned warm air inside the building in the winter, wouldn't it also effectively keep the hot outside air from making its way into the cool conditioned interior, thus reducing the amount of heat that would need to be removed from the building in the summer? That is scientifically correct, hot air wants to move towards cool air, and an air tight barrier effectively rejects the hot air from getting its way. What I did not expect, was that the interior of the building I was analyzing was actually hotter than the exterior. Thus the heat flow was from the interior to the exterior (just like in winter) rather than exterior to interior, but the tighter envelope was not allowing it to.
This concept is called "free cooling". For poorly insulated, leaky buildings, like the masonry buildings I've been living in in the outer boroughs of NYC for the majority of my life, the combination of the conductive heat gain through exterior walls, solar heat gain through windows, plus the internal heat gains from occupants, mechanical equipment, electronics, and lighting actually create a hotter interior climate indoors relative to the exterior climate during the summer. In these cases, while it is not incredibly effective, there is some cooling that occurs by the exhaustion of heat from the hot interior to the not as hot exterior. So increasing the air tightness of one of these buildings would increase the required cooling to maintain a comfortable interior climate.
So why do energy efficient building standards such as Passivhaus specify airtight envelope constructions as a requirement if it could potentially result in higher cooling loads during the summer. Airtight construction is clearly beneficial in winter, especially when Heat Recovery Ventilators (http://en.wikipedia.org/wiki/Heat_recovery_ventilation) are utilized to warm the cool exterior intake air with the already heated internal exhaust air. But in the summer it seems that a breathable envelope is better than a suffocating airtight one. And that is true, but rather than air leaking out of hundreds of cracks across the building envelope, cooling would be much more effective by sealing the cracks and intentionally concentrating the breathability of an envelope at well placed openings (operable windows) to induce much more effective natural ventilation and cooling. Natural ventilation depends upon pressure differentials and an airtight building with well-placed windows, will focus those pressure differentials to induce proper cooling breezes. Any unintentional leaks in the building envelope will diffuse the intended pressure differentials.
Let me know how much of this makes sense, is unclear, or just completely wrong. Thanks for reading.
The Product-Architecture Lab at Stevens Institute of Technology is a pioneering graduate program integrating the study of Architecture, Engineering, Product Design, and Interaction. The program focuses on a fusion of design culture and technology through the disciplines of computation, analysis, and advanced production methodologies.
6 Comments
Cool project!
Perhaps the source of your confusion is not looking to reduce the internal loads of the building and the effect of higher insulation. Multifamily residential buildings have lots of internal heat sources (fridges, stoves/microwaves/coffee pots, lights, computers/tvs/stereos) that aren't under the jurisdiction of the architect/building management.
Passivhaus reduces all internal loads to their minimum (mostly through daylighting and high performance appliences), and reduces heat gain by having right sized glazing with high performance coatings/multiple layers. Controlling these sources of heat (which are good in the winter, not the summer) reduce the need to cool in the summer - plus you can also use various forms of passive ventilation.
Another path to take is looking into humidity control - which makes the energy recovery ventilator a benefit in the summer too - lower humidity reduces the need to cool.
Multi-story buildings significantly suffer from stack effect (in a way that typical single family houses don't). Passivhaus isn't aimed at multi-story/multi-family structures. On the super-tall projects I've worked on, we have utilized consultants for both wind-tunnel testing/reducing wind loading, and for reducing the stack effect. Tightly sealing the envelop is the key method to prevent major breezes being induced by the height.
just a minor correction of barry's post - passivhaus is suitable for non-single family, and is more affordable/easier to attain on multi-family. the austrians have been pumping out some stellar multi-family passivhaus projects, key players are walter unterrainer, dietrich|untertrifaller, the kaufmanns (hermann and johannes), baumschlager & eberle.
despite the reduction of heat loss through multi-family projects (less surface area/volume) - they actually tend to be more energy intensive (on a per square foot basis) than single family (see 2030 EUI chart). this also means that achieving passivhaus on non-single family projects should be a priority, as it has a more dramatic effect in terms of energy usage and CO2 emissions.
passivhaus claims that the extreme airtightness is needed to prevent moisture migration through the envelope - leading to building science issues. it should also be noted that most EU countries require an airtightness of 1.5ACH50 or better when using MVHR.
in terms of envelope leakiness and cooling loads, i don't buy the model. or rather, i'm not sure that the building you are modelling is an exemplary project. especially at passivhaus levels of airtightness, you don't get increased cooling loads - you get significantly reduced cooling loads. part of this is because in passivhaus commercial projects, by definition, a passivhaus can't overheat more than 10% of the year anyway - which means the designer has to take issues of thermal mass, night cooling, avoiding solar gain into consideration.
http://www.ig-architektur.at/cms/index.php?idcatside=296
Holz, many thanks for the correction. Hadn't seen the passivtowers.
one other observation, brick/masonry construction is responsible for many folks dying from heat related illnesses in humid summer climates. They heat up from the sun (the thermal mass is all in the wrong place) and don't cool down.
barry, no worries. it's my biggest beef w/ the 'passive house' translation - it's applicable to almost every typology (even bathing halls). here's a quick presentation on a PH commercial building that recently wrapped up in UK - key things to look at are the significantly reduced loads, and the cost savings (not first cost, but operational - though it's definitely possible to achieve PH in the EU w/ no add'l cost now)
http://ukpassivhausconference.org.uk/sites/default/files/MWrate%20-%20Interserve%20Office.pdf
Thanks for all the info. Its important to note that the building in study was built in the 60's before any of this stuff was very high on the majority of architects and engineers agendas. Holz.box, as you pointed out, it should not be extrapolated to all PassiveHouses, I'm not trying to say PassiveHouses do not work in the summer. It was more in respect to the idea of energy efficient (potentially PH) retrofits to multifamily masonry buildings (all of the buildings studied in the studio were 50,000 sf+).
One of the shortcomings of the CBIP studio, in which we studied retrofits for existing NYC buildings, was that we completely ignored mechanical systems and any other sort of equipment or appliances. It was an architecture studio at heart, so the retrofit ideas focused on architectural modifications to alter daylighting and envelope performance. So I think the real lesson for me in regards to retrofits is that you cant just tighten an envelope or add insulation without making an effort to improve the mechanical systems and also reduce internal heat gains. In addition, I think improved mechanical ventilation (or the introduction of mechanical ventilation for many NYC buildings) would be necessary on an air-tight retrofit to meet minimum ventilation standards for healthy indoor air quality.
I've read about many successful single brownstone passivhouses in New York where the interiors are pretty much completely gutted and rebuilt the PH way. I think it becomes much more challenging (but with much higher reward) in the double digit tenant apartments across NYC. Have either of you seen any apartment size PH retrofits in the states or abroad?
ben,
there have been several. a recent and very interesting one is this apt. tower (conc.) in freiburg that was retrofitted to PH. another is this dorm in wuppertal, which utilized a really innovative process - pushing existing envelope out ~1,5m and then finishing it in prefabricated superinsulated panels. there have been a number of buildings over 50,000 sf that meet or are really close to PH. if you google 'passivhaus umbau' you'll find a lot of stuff, but there's a lot to wade through.
retrofitting is not only easier - but more cost effective (and produces greater cost-savings) for non-single family projects (for the most part). and yes, PH isn't just about envelope and infiltration - it's whole systems thinking. with commercial retrofits - you're talking about significantly downsizing MVHR equipment, which tends to make the ROI a lot easier. breakeven numbers for several PH retrofits that i've seen lately have been falling between 0-7. for institutions that shell out $150k in operational expenses, getting that reduced to $10-20k frees up a lot of cash in the long term.
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