136: Thermal Mass Is Not That Useful

I recall visiting one of the fist 8 Homestar homes in Christchurch and being surprised that this new show home had a suspended timber floor. The double layer of insulation gave the floor a very impressive R-value, far better than any concrete floor slab I’d seen recently. All well-and-good, I thought. But thermally, surely it couldn’t be that great because it had no thermal mass. Right?

Lloyd Alter is Managing Editor at Treehugger and Adjunct Professor at Ryerson School of Interior Design. If you're interested in sustainability and green design trends, follow him!
Lloyd Alter is Managing Editor at Treehugger and Adjunct Professor at Ryerson School of Interior Design. If you’re interested in sustainability and green design trends, follow him!

Later when I looked at the modelling numbers, I just couldn’t figure out how this timber floor house could, in theory, performed so well. Was this what I’d heard some experts describe as the ‘light and tight’ method of design and construction?

Since then, I’ve questioned the ideology of a concrete slab as the best floor. The theory of the all that thermal mass sounds good, but the devil is in the detail. Despite my research into the topic, I’m still yet to find the perfect insulated slab that leaves the concrete exposed as a useful and comfortable thermal flywheel (with the possible exception of MAXRaft now that they have an XPS option for more rugged edge protection).

Trombe Walls and Other Thermal Promises

In other conversations I remember getting very excited when I first heard about trombe walls, only to have my dreams of the perfect passive heater dashed when a building scientist explained to me that he’d never actually seen a trombe wall work really well here in New Zealand, or one that didn’t just take up what would otherwise be a nice view.

And finally, there was my experience working under the hood of Homestar. I recall questioning the calculations that gave scarce recognition to any internal exposed concrete, brick or other heavy material that didn’t get anything but direct sunlight for a significant part of the day. This scientific assertion that only that part of the inside of a house (usually only the first 2 m of floor) that is hit by direct sunlight, will really contribute anything useful to passive heating, was hard for me accept. I wanted to believe that all exposed thermal mass must be good.

Local architect and fellow Homestar Assessor, Keith Huntington has similarly cautioned the dogma of thermal mass in his writings on eboss, which I also found jarring the first time I read it.

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Everything I knew was probably wrong

So when I read Lloyd Alter’s headline on Treehugger – ‘Evertyhing I ever knew or said about green sustainable design was probably wrong‘, I knew that it was time to get some answers.

Does thermal mass matter, and if so how much?

And what about those all important eaves that I’ve ranted about in the past?

How much of this commonly used ‘passive solar’ design theory is really true and useful? Source: Treehugger

Passive House vs Passive Solar

Much of this comes down to two design principles which are similar in name and have some overlap in their detail, but are fundamentally quite different. As Lloyd points out, the ‘Superinsulated’ house was really the origin and (along with airtightness) is the foundation of Passive House.

‘Passive solar’ design is what’s in question here, in particular the reliance on thermal mass and sun angles.

What Matters?

I don’t think passive solar design principle should be written off completely. It’s definitely good to design for the sun.

My main out take from this disruptive commentary is that airtightness really matters. I’m now starting to see the logic and potential of the ‘light and tight’ method of building.

Also in this episode…

Lloyd also makes mention of:

Saskatchewan conservation house, source: Treehugger.com
Saskatchewan conservation house, source: Treehugger.com

Lloyd Alter

I highly recommend following Lloyd Alter. The best place to start is Treehugger. Then take your pick of social media and other channels from there.


  • The headline for this got me excited and I wasn’t disappointed, excellent topical podcast Matthew. Lloyd is always good value and has wonderful stories to share.
    While thermal mass in the floor might not be that useful (and can even be uncomfortable if is stays cold), it does avoid the issues of ventilation (or lack of it) under suspended timber floors. “Crawl Spaces” as they are known in the US get a lot of attention from Building Science Corp as problematic aspects of houses. Eg http://buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces

    • mcutlerwelsh

      Good point. Thanks Elrond. And thanks for the feedback. I really appreciate it.

  • Jessica Eyers

    I think its important to remember that one size does not fit all. Passive solar design definitely works better in some climates better than others. Particularly here in Central Otago, with cold nights all year round and sunny winter days, it has a place. People need to be thinking about houses in terms of gains and losses, and often that will mean modelling to get it just right. It is definitely possible to have too much glazing (even if it is north facing) and too much thermal mass, even in the ‘ideal’ climate. I know many a ‘passive solar’ house that is actually too cold or too hot for a lot of the time.

    • mcutlerwelsh

      And who better to do that modelling Jessica, than someone with a good knowledge of Passive House!

  • Jessica Eyers

    The other two points I would make after reading http://www.treehugger.com/green-architecture/everything-i-ever-knew-or-said-about-green-sustainable-design-was-probably-wrong.html is that it shouldn’t be a choice between mass + north facing glazing OR super-insulation, it needs to be both. The other point is that there are lots of reasons to avoid too much glazing on the South and West sides of a building, so why not put it on the North? With adequate consideration to shading the sun to prevent overheating of course 🙂

    • mcutlerwelsh

      Yes, agreed. Doesn’t have to be one or the other. I think the usefulness of the ‘light and tight’ concept though is that in New Zealand the ‘tight’ bit usually gets missed.

    • You’re right Jessica, the best performing designs include the optimal amount of insulation, glazing & thermal mass, not an excess of any one of them. Glazing, even on the north, can actually be sized for optimum daylight & views (without overdoing views by just glazing the whole wall as some architects like to do!) as the priority rather than an idea of “free heating” (you have to pay for the window, the shading, the heat loss…) Prioritising solar heat gains can lead to oversizing the glazing which becomes difficult to control even with good shading. Low sun in the east and west will still come in through north glazing below most shading. And of course, over glazing costs more & needs more shading, which in turn also costs more!

      • mcutlerwelsh

        I like the concept of good design using smaller windows to ‘frame’ a view, rather than just having a wall of glass. I’ve heard this described by a few architects, but I’m not sure how true it is.

  • Jessica Eyers

    ps. I really enjoyed the post Matthew!!

  • Talina Edwards

    Great discussion once again! Thanks Matthew for continuing to bring insightful guests to your show that are relevant experts and help wade through the ‘greenwash’ in the sustainable building industry.

    • mcutlerwelsh

      Thanks for the great feedback Talina. You’d be a welcome guest on the show. People in Melbourne should definitely contact you if they’re wanting a good home. http://talinaedwards.com.au/

  • Everyone always describes this as the “Perfect Enclosure” but build this instead.

  • Duncan Firth

    Although I think Lloyd is right about the strengths of Passive House and what it has given the industry (which is ultimately achieving correct insulation and air tightness through specific technical design) he’s describing the worst case performing Passive Solar House. There’s no reason why a Passive Solar House can’t have any of this either, which I think in any type of high performance environmental house whether that’s Passive house, Homestar, Leed, Passive Solar is the expectation these days- if the owner can foot the bill.

    When thermal mass is correctly designed for it’s an amazing material and can adequately heat a house during winter, in temperate Climates.

    One of the main reasons why I like Passive Solar is it forges a direct connection with the environment
    and reminds us we are part of it, through the ever presence of sunlight and natural airflow. It seems to make sense to harness these in our designs.

    I find Passive Houses very internal and disconnected from the environment, the entire experience
    becomes about insulation, air-tightness and maximised energy efficiency (which I think you
    can achieve in Passive Solar when its well done), but in saying this,houses need to lift the spirit and what not is a
    better way than directly engaging with nature, I think Passive Solar does this very well.

    • mcutlerwelsh

      Duncan, I’m very pleased to hear from you on this because I was thinking a lot about Solarei (http://homestylegreen.com/designing-environmentally-functional-homes/) and your design philosophies while I was talking with Lloyd.

      I agree that designing for a specific climate and location is important. And I can see that this approach, along with a good connection with the environment can be achieved with good design, whether is passive solar, Passive House, zero energy… The key is a good designer (of which you are definitely one!)

      • Duncan Firth

        Yes Matthew, key to pulling off any good project is a good
        design team, with a specific site location conscience.