Mass Timber and Passive House, Together at Last

Would you buy an electric vehicle (or better – an e-bike) without a battery? No, but this is effectively what happens when building with mass timber, but failing to meet the Passive House standard. Passive House is the secret ingredient that makes mass timber buildings truly sustainable (bonus: nearly every other building, as well).

I was first exposed to mass timber while working with "brettstapel" (dowel laminated timber, or DLT) in Freiburg, Germany nearly 20 years ago. I’ve been advocating for mass timber for over a decade, but it wasn’t until two years ago I finally got to use it – on the second-ever DLT project in the U.S. I have been in the weeds of Passive House for over a decade, also. I moved to Bayern, Germany, for greater exposure to these subjects; it was both educational and depressing. I found a number of public projects that incorporate both – but it cemented the reality of how far behind we are in the United States. However, this opportunity also showed me how there is near-perfect synergy with these – especially in the public realm.

A large reason is cost. We are still in the early stages of both movements in North America. There is only a single source of brettstapel panels, and high-performance mass timber panelization companies are almost nonexistent. I’ve previously written about how the U.S. construction industry is vastly underwhelming versus the E.U. on this issue. Most developers, builders, or agencies are still risk-averse to doing new things – which means there is at present a premium to building with mass timber. Unlike the E.U., there are no U.S. jurisdictions requiring anything close to Passive House, or utilization of mass timber, for new buildings. It is precisely this premium that requires planners to incorporate Passive House to mitigate and/or reduce overall costs.

Here is How Achieving Passivhaus Can Reduce Costs

Mechanical rooms in public projects can be quite large. I know of two code-minimum mass timber projects that feature mechanical rooms that are double what would be required if the project was designed to Passive House. My Passive House colleague Nick Grant tweeted a picture of the heating system for a 2.500m2 (26,900 square feet) Passive House school in the UK designed by Architype. This isn’t an insignificant saving – the cost per square foot of new public construction in Seattle can be as high as $350 per square foot. A 500 square foot reduction (through meeting Passive House) could net a savings of $175,000.

Other potential mechanical system reductions with Passive House include reduced duct lengths versus a traditional HVAC system, with more options for utilizing decentralized systems as well. Since Passive House ventilation is fresh, filtered air (versus air supplying heating and/or cooling) the ducts can have smaller diameters. Granted, with a Passive House, care must be taken to ensure the ventilation system is not loud, and similar strategies are also needed for the acoustical treatment of mass timber.

100% Fresh Air

As jurisdictions move to require 100% fresh air ventilation, this is already a requirement for Passive House. Fresh, filtered ventilation is rapidly becoming a necessity for public buildings with the onset of Covid-19, as well as increasing fire seasons on the west coast. As global warming unfolds – this requirement only becomes more imperative.

Speaking of heating, one of the largest advantages pairing with Passive House and mass timber is that the drastically reduced heating system (reference the boiler linked above) requires significantly fewer penetrations through walls and floor decks. There also aren’t mechanical facilities along the exterior wall, opening that up for storage or egress. If a design has convectors or radiators along the exterior of the building, with multiple penetrations – this will require significantly more coordination, as well as increase "table time" in the shop for panel production. Table time should be minimized to keep mass timber fabrication costs down. Passive House gets ignored for being "dumb" technology – but it is precisely this low-tech, climate-friendly solution that leads to a less expensive mass timber buildings – that actually cost much less to run as well, through significantly reduced operational costs. Dumb boxes are truly #BoxyButBeautiful!

Mass timber also dovetails with airtightness for the Passive House standard. Cross-laminated timber is relatively airtight, owing to the glues and layups of the wood. This means the weak link will be the seams. There are several effective solutions for these junctions, including high-performance air sealing tapes, and gaskets – for addressing panel junctures, penetrations, and openings. With Brettstapel/DLT – the safest bet for airtightness is to keep the structure inside of the thermal envelope. If a cantilever is ultimately needed, there are a number of ways of addressing the airtightness of this as well, including fabricating DLT panels with integral gaskets.

The Perfect Wall

Perhaps my favorite Passive House win with mass timber – it is the embodiment of Joe Lstiburek’s "perfect wall." Lstiburek is the founder of the Building Science Corporation, and its Building Science Insight (BSI-001) is on the perfect wall. Lstiburek describes the system: ”In concept, the perfect wall has the rainwater control layer, the air control layer, the vapor control layer, and the thermal control layer on the exterior of the structure. The claddings function is principally to act as an ultra-violet screen.”

This is how nearly every mass timber exterior wall is insulated. The control layer is the gasketed/taped seams of the mass timber panel structure. The majority, if not all, of the insulation, is outboard of the structure. The façade sits outboard of all of this, protecting from bulk water and UV degradation. If you look at as many European wall details as I have, you will see slight variations on this, but they are all largely done like this. Another thermal bonus with mass timber – on larger, compact projects, the amount of insulation needed to meet Passive House is not significantly greater than code minimum projects.

One of the biggest premiums for a Passive House + mass timber building will be switching from double pane to triple pane insulated glass units. Triple pane Passive House windows have benefits beyond better thermal comfort and reduced condensation risk – they are generally also much quieter than code minimum ones – ideal for urban environments, schools, and anywhere near highways or airport glide paths. Currently, the thermal performance of most North American windows and curtain wall systems leaves a lot to be desired, but this is slowly changing. Wolfgang Feist told me that the latest Passive House-certified window is made in the U.S.!

Firms with no experience in either method may find it best to tackle one or the other before combining both. Another issue is that the embodied carbon savings of mass timber versus traditional construction may not be significant – much depends on sourcing, and end of life for panels. In most cases, the operational carbon savings of meeting Passive House will be more than the embodied carbon savings of mass timber. In this case, Life Cycle Analyses are your friend, and we should be modeling, and measuring, to validate our assumptions.

The result of a Passive House + mass timber building is a win for everyone involved. For the end-user, a higher quality building, with less external noise, more comfortable working/learning/living environments, better indoor environmental quality, and the woody goodness that comes with a biophilic design. For the building owner, a durable building less prone to mold and moisture problems than a code minimum structure, significantly reduced operating costs, happier and healthier employees/students/residents.

Bring On the Revolution!

To me, it is incomprehensible that very few of the mass timber projects built or under construction in the U.S. and Canada have been designed to meet the Passive House standard. As a Passive House nerd, and a connoisseur of fine, mass timber buildings – it pains me to see this. If you want to see examples of what has been done abroad, I have been curating a list of high-performance mass timber projects over on twitter. These are not passing fads, they are literally pushing the envelope. Pairing Passivhaus with mass timber is an almost unparalleled solution to mitigate climate change, while increasing livability and comfort. I am drawing a line in the sand – this is the only type of building I will be working on from this point forward. Bring on the revolution!

Previously on Treehugger by Mike Eliason: Why Is Architecture and Building So Different in Europe?