Seattle Passive House – Measure twice, cut once. Measure once, blower door twice?

I left you with quite a cliffhanger regarding Dan’s project and the blower door test.  I thought I would explain a bit about the Passive House concepts that are affecting the results.  Passive House projects are measured in a variety of ways. 

For determining the Annual Heat Demand, the Gross Enclosed Volume of the building is used – that is the area enclosed by the  extreme outside of the thermal envelope.  In this cross-section of one part of Dan’s project the Gross Enclosed Volume is represented by a dashed blue line.  The yellow represents the insulation in the walls and under the slab.  The red line is the OSB and top of the slab that create the air tight layer.

For Ventilation purposes, the Net enclosed Volume of the building is used.  This is where the question of the project passing the blower door test got a little sideways.  In America, pressurization tests use different volume measurements normally and a number equivalent to the volume enclosed by the air tight layer (including interior walls and floors) was calculated.

For Passive House purposes a more conservative number is to be used for the Net Enclosed Volume.  Basically  it is the empty area that is ventilated within the thermal envelope.  For this project that is all the blank space within the red air tight layer EXCEPT for the bright green area that makes up the floor and interior walls.  The argument can be made that these constructions are not built air tight and will become pressurized and ventilated, but Passive House measurements are conservative and these volumes are not included in the Net Enclosed Volume.

Dan will be crunching the numbers, air sealing some more with the wonderful tape that Siga sent for the project, and trying again to keep under the .6ACH @ 50 pascal requirement that certified Passive House projects must reach.  He is very determined to do so.  He allowed for this scenario when he planned the blower door test.  His air tight layer is still accessible and he does have the opportunity to go back and make changes easily.  When planning your Passive House project, make sure you plan ahead so that you too can tighten the air tight layer if necessary without a lot of fuss.

Dan has really done a remarkable job with his project and I for one am learning so much from following it and presenting it to you on this blog.  Thanks for following along!


[updated to clarify interior wall and floor volume are not included in the Net Enclosed Volume.]


5 responses to “Seattle Passive House – Measure twice, cut once. Measure once, blower door twice?

  1. Thanks for keeping us posted.

    Any ideas/info on the long term durability of the construction (in general) or the tape (specifically?) It’s probably a given that a 30-year-old passive house will have significantly better performance characteristics than a 30-year old conventionally built house, but do you expect any specific ongoing maintenance/repair/replacement activities to keep the house up to passive standards? (e.g. is there a recertification process or does there need to be?)

    I realize I’m getting way ahead on a house that’s still being built (!) but low /zero maintenance construction will definitely assist with long-term adoption of and adherence to greener standards. Today’s houses have pretty simple maintenance criteria: mechanical systems working, maintain structural integrity and keep moisture where it belongs (aka “fix stuff that breaks, watch for termites, and apply new paint/shingles as needed.”) Adding energy use criteria is (relatively!) simple at the outset, but over the longer term…?

    Not a negative comment, just trying to think over the horizon. Keep the updates coming!

    • Great comment Josh! As you have surmised, the long term success of Passive Houses lie not only with the care taken with the original design and construction, but also in the maintenance and after use of those structures. Homeowner education will be a key concept in these projects succeeding in the long term. By understanding the concept of what is done and why, a homeowner will be able to act in a manner that will aid in this success. While that may sound “all good on paper”, it is actually being indicated by follow up surveys of projects in Europe. Here are the results of a study done by CEPHEUS (Cost Efficient Passive Houses as European Standards.) In the report details of multiple projects are spelled out and the results of building operation as well as the impact on/by residents in a variety of circumstances is reported.

      Currently there is not a recertification process and obviously future retrofits would need to be done in such a way as to preserve the ability of the Passive House to operate efficiently. Perhaps as we get further along in the process issues such as these will be addressed. It is good to ask the questions now and to be thinking ahead for the future of Passive House construction.

  2. Nice post Linda! You really lay out the issue well. No matter the volume, air sealing will be one of the key methods in developing high performance envelops. Without air sealing, the other measures can’t be effective and the heat will follow the air.

    I see developing and sharing simple methods as an important development. It’s no doubt to me that code minimums will move to an air sealing requirement of some level in the next few years as the issue and it’s relation to energy efficiency is understood.

    To Josh’s point, well have to start paying attention to longterm durability reports. For that we’ll have to look to Europe to supply the info on how their High Performance projects are aging.

  3. Back to Josh’s original question about tapes and longevity. I should have commented on this already but missed the point from all of the interest in watching if Dan’s project passed or not.

    As the first SIGA distributor in the US, I’ve been learning about what’s on the market while evaluating other products and manufacturers. I feel that SIGA became the right choice for me due to what I think is the best longevity potential. Some of the competitors to SIGA are Pro Clima and Wurth. Wurth is an extremely large company and offers many many products worldwide. Pro Clima is also a good competitor and was showing the Passiv Haus international Conference in Dersden this year.

    I like SIGA for two reasons: Size, and solvent free adhesives. SIGA is still operated entirely by the the 2nd generation of the founding family. Air sealing products is all they do. Hence they have the time and attention to do it well. The adhesives are all acrylics and solvent free. This means that there is no shelf life like the competitors due to solvent off-gassing. It always got me to wondering how… if there was a shelf life to sell the product, wouldn’t that indicate an overall longevity issue with the adhesive used in the tape?

    The SIGA products test well in application and the best data available is by “rapid aging” studies. The company assures me that the product is made and expected to last for a 50 year application. By that time most of the structure will be on schedule to be re-built with new windows, doors and general update remodel.

    All air sealing strategies are subject to settling, poor maintenance and puncture. The methods range from glueing on exterior sheathing to using the interior drywall as the air barrier and everything in between. Vapor diffuse membranes and sealing tapes from SIGA or other manufactures offer a cost and time efficient strategy with as much reliability as any other method that I can see. It’s all new to us here in the US. In recent years we’ve been yearning for the safety of a poorly constructed house that “breathes” because we went through the “too tight without vapor diffusion” stage and trapped ourselves in enclosures with all our off-gassing building materials and water vapor. That wasn’t fun!

    The good news is that we are on the way to learning how to build good tight, vapor diffusive homes while our customers have more access and education to look for the least off-gassing materials in flooring and furnishings. I think we are headed to good projects and good results!

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