I have lived and practiced green design in Germany, and I witnessed the introduction of the WSVO (Waermeshutzverordnung) firsthand in 1994. These were mandated changes to the levels of insulation required to be placed in the walls, roofs and floors of new buildings. At the time of my first R2000 build near Stuttgart (1996), I felt that the Germans were only getting half of the picture. Insulation is useless unless you can control the airtightness of the envelope. To do this, you need to know how to build ‘tight buildings’. In Germany, land of ‘Massivbau’ or masonry buildings, homes have been naturally ‘leaky’ since well before the middle ages. The idea with Massivbau is that you have a source of heat inside (fire), that warms the mass, and the mass in turn keeps you warm. But anyone that has visited or lived in castles, churches or older homes in Europe knows that these spaces will suck the life-force out of you in the Winter unless you are under a duvet quilt or in the direct line of sight of a fireplace or radiator.
Now imagine if that duvet or quilt were hovering 6″ above your body? The insulating effect of the quilt without a somewhat airtight seal at the edges is almost zero and that is the problem with highly insulated, but non-airtight buildings. So I was happy to see the emergence of the null-energie and Passivhaus standards that considered air-tightness ratios as an integral part of the standard, as it closed the loop and made the total building envelope energy efficient. However, no home is ever completely airtight, but only sort-of. This is measured with a doorfan, or a powerful fan that installs in an exterior doorway that basically produces a 50 Pascal pressure difference between the interior and exterior of the home. This allows you to measure the amount of air flowing through the fan across a range of pressures, and thus determine the air leakage ration or ‘Air Changes Per Hour’ of a home at both the artificially induced 50Pa value or under normal conditions. The ACH@50Pa has become the international standard of building air-tightness, and basically represents how many times the entire volume of air in a given building is cycled out due to leaks in the envelope.
I’ve performed over 500 of these tests with my R2000 mentor in Canada, John Godden, and installed about as many mechanical ventilation systems as well. What I learned, was that very careful detailing around every service penetration, electrical outlet, door, window and any other opening needs to be carefully detailed and air-sealed or the building will fail the Blower-Door or Doorfan test. Just by way of comparison, here is a range of typical, and exceptional ACH values:
- 5.0ACH@50Pa – Older or shoddy construction
- 4.0 ACH@50Pa – Typical New Construction in the USA
- 2.0ACH@50Pa – Energy Efficient New Construction with consideration of airtightness detailing
- 1.5ACH@50 Pa – R2000 Housing – A Canadian Federal Standard
- 1.0ACH@50Pa – Extremely Tight Construction
- 0.6ACH@50Pa – Passivhaus Requirement*
- 0.0ACH@50Pa – Spacecraft
While by no means an official or definitive list, the above gives a good anecdotal range of what is achievable in building design, and what kind of ACH ratio to expect in a building of a given age and performance type. Now I am all for adopting aggressive building performance standards for every kind of new construction, but why does Passivhaus require such a high air-tightness ratio, and is this 0.6ACH@50Pa even readily achievable? From my experience, having measured and evaluated thousands of R2000 homes, even the 1.5ACH number is not a slam dunk. Many of my own projects have failed to meet 0.6 the first time around, and that was not for want of trying. I always use spray-in-place polyurethane insulation, a thermal break layer of rigid (extruded) Styrofoam that is taped at all seams, and all penetrations and openings are sealed with spray foam and tape – but sometimes that’s still not good enough. That said, I have also achieved up to 0.4 and 0.25 on a couple of jobs – but those were very unique builds.
On a recent project in Napa, we started to evaluate the proposed model homes in terms of their energy efficiency, and to determine whether we should be pursuing the Passivhaus standard as well as LEED Gold. It turns out, the PH standard’s extreme airtightness ratio offers very little real benefit in terms of energy savings, as is explained below. In the computer simulation analysis we undertook with the Hot2000 program, the only variable parameter in our model was an ACH of 1.5@50Pa vs. 0.6@50Pa.
The difference in dollar values re. energy savings was predicted to be $9/yr. The difference in Carbon footprint is 3.282MT (Passivhaus ACH) vs. 3.310MT (R2K ACH). With numbers like this and the considerable additional expense required for PHPP compliance (airseal labour and multiple testings), it is difficult to argue for the 0.6ACH as a sensible number for even the most conscientious North American builder. I don’t take issue with ANY of the other PHPP requirements – but I think the 0.6 is a manic, hyperbole of performance and in this case, it’s pure dogma. 0.6ACH is just awfully hard to hit – and the payback for 0.6 vs. 1.0 or even 1.5 is slim to none. Well, the 15kWh/m2*yr for heating is pretty extreme also when the project total TEUI is a whopping 120kWh/m2*yr – but John Straube speaks to that below.
US Builders have a hard time with air-sealing because it has not been part of the mainstream construction practices, whereas in Canada, at the very least, the approach is understood by most competent contractors. To expect inexperienced US builders to hit 0.6ACH@50Pa on their first airtight home is both cruel and unreasonable.
I would strongly argue for a reduced/more relaxed ACH value if this standard if it is to take root at all in NA. John Straube and others have said the same here. Is America ready for the Passivhaus Standard is just the wrong question – let’s build green with proven approaches for this continent, and look to R2000 as a more relevant standard framework for evaluating all American and Canadian energy efficient construction. Now those that argue R2000 construction encourages mold because of the use of interior, 6mil Polyethelyne Vapour Barriers that can never be properly sealed, it is not necessary to use PE barriers, one can in fact build an R2000 house using Baubiologie principles, and there are product like Certainteed’s ‘memBrain’ that allow bi-directional drying. Please contact me if you’d like to learn more.
*From John Straube’s article:
…the requirement to limit the heating energy demand to only 15 kWh/m2/yr is perplexing: depleting energy resources and environmental damage are already limited by the 120(kWh/m2/yr) number, why constrain the design further with no reduction in energy? And what is special about 0.6 ACH@50Pa? If a builder can deliver a house that uses less than 120 kWh/m2/yr, with 1.5 ACH@50, why does this matter? Wolfgang himself offers some clues, as he states in an interview in the UK (at http://www.aecb.net/feist_
videos.php) that the exceptionally low number is intended is to avoid interstitial condensation that can damage the structure. Far too many superinsulated homes of the past suffered this fate. Of course, we now have the practical and technical knowledge to completely avoid interstitial condensation in a house with 2 ACH@50 Pa and also know that dangerous rot could still occur at 0.6 ACH@50 if a double stud wall design is used. Again, there appears to be other lower-cost paths to reaching a low energy house target that are blocked by these prescriptive restrictions.