Windows and the biological thermostat
What’s the best way to manage a home’s indoor climate?
As I type this blog post, our house’s fan is running behind me, circulating air – and directly in front of me is a lovely bank of fifteen-light windows through which I can see our back garden.
Why don’t I just turn off the fan and open the windows?
Wonder what my energy loss rate is?
Throughout history, people have craved windows in their dwellings – for air, for light, and for view. Windows let in bugs and water. They also leak heat out and cold in, or vice versa.
And into the basement …
That’s why, for the last fifty years, the trend in evolving modern buildings (homes and otherwise) has been to seal the building’s airflow even as we increase its overall transparency.
Is that wise?
Windows are the bane of a homeowner’s existence – and even more the bane of a landlord’s existence. They’re surprisingly expensive to make, assemble, and install. If movable, they’re a fairly complicated installation that rattles and usually leaks. Painting the window trim is a bitch.
It’s a bitch of a job
For all these reasons, modern buildings have windows you cannot open. In previous posts, I’ve talked about the architects’ tendency to design complicated structures, which tend to leak, or have unexpected consequences. Via reader Matthew Healy, that led me to building airflow expert
Abstract. The ventilation requirement for a house is a compromise between the need to keep the indoor relative humidity down and the need to minimize energy consumption.
* The first requirement is itself a short-circuited train of reasoning about how to prevent condensation in the structure with consequent mould growth.
* The second requirement was until recently defined as the permitted energy used to keep the occupants physically comfortable.
Aside from being unsightly, mold is extremely toxic of a small subset of people, and that means litigation, lawyers, and money.
Got a little problem, have we?
Mould growth in the occupied part of the house is only indirectly a consequence of high indoor relative humidity- it is usually a consequence of uneven wall temperature gradients within the structure.
I never knew that – it’s not the humidity by itself, but rather how temperature changes within a structure, because with a wide temperature gradient, somewhere is an environment that allows mold to grow.
Designing a structure that is robust enough to be badly built and intermittently maintained is just as important to the life cycle costs as is the energy consumption from day to day.
I absolutely love that line, for it’s the reality – maintenance is imperfect. In Tthe risk of complicated structures, I’ve previously quoted Padfield’s Law of Construction:
Trends in building design have been towards complicated, lights structures optimized through the use of computer models but not always performing as expected when released into the care of the occupants. In particular the intricate detailing and tendency to use non-water-absorbent materials brings considerable risk of condensation in the northern European climate, with subsequent mould growth. Air barriers have not provided the expected security from condensation within the outer envelope of buildings.
We saw that in dreamers versus plumbers. Then there’s this admission:
There are some rather fundamental holes in our knowledge of the dynamics of biological growth. In particular the ecology of microorganisms has been relatively neglected in favour of experiments based on sterile media inoculated with purified organisms in a constant environment. The influence of air flow and air exchange on growth has been neglected.
It’s imaginative, but it’s not the world
Just as Korzybski said the map is not the territory, the model is not the reality.
“This is not a pipe”
In finance, we love our models, but the word ‘model’ implies approximation, not the full complexity of reality. Some things are hard to model, so we omit them – and sometimes that’s a mistake. And when I hear phrases like “not well understood,” my mind immediately drifts to “tort litigation” and then “expert testimony” and even something called the “Daubert standard.” Wikipedia’s summary is quite good:
In Daubert, the Supreme Court held that federal trial judges are the “gatekeepers” of scientific evidence. Under the Daubert standard, therefore, trial judges must evaluate proffered expert witnesses to determine whether their testimony is both “relevant” and “reliable”, a two-pronged test of admissibility.
And the UFO’s light were right along here …
The relevancy prong: The relevancy of a testimony refers to whether or not the expert’s evidence “fits” the facts of the case. For example, you may invite an astronomer to tell the jury if it had been a full moon on the night of a crime. However, the astronomer would not be allowed to testify if the fact that the moon was full was not relevant to the issue at hand in the trial.
The full moon had nothing to do with it
The reliability prong: The Supreme Court explained that in order for expert testimony to be considered reliable, the expert must have derived his or her conclusions from the scientific method (Daubert v. Merrell Dow Pharmaceuticals, Inc. (1993) 509 U.S. 579, 589.) The Court offered “general observations” of whether proffered evidence was based on the scientific method, although the list was not intended to be used as an exacting checklist:
* Empirical testing: the theory or technique must be falsifiable, refutable, and testable.
* Subjected to peer review and publication.
* Known or potential error rate and the existence and maintenance of standards concerning its operation.
* Whether the theory and technique is generally accepted by a relevant scientific community.
With illness due to mold rising as a source of class-action litigation and tort liability to owners, the cause of mold growth can be a multi-million-dollar issue.
The movement of water in hygroscopic materials is not understood, to judge by the failure at the many models to account convincingly for water and dissolved salt movements in porous materials and their difficulty in dealing with lateral irregularities in real walls.
Water runs along cracks.
Water leak inside a ceiling (not Photoshopped!)
There is continued reliance on mathematical analogy between heat and material transport through porous materials, even though the movement of water molecules through the mesh of different sized pores is unlikely to be purely diffusive phenomenon.
The erratic ventilation caused by undisciplined use of windows is a considerable further inconvenience for these modelers.
Programmers and economists tend to ignore what we cannot model, and of these things we wish to ignore, human behavior is among the most complex.
It is perhaps easier to believe that one has control when all the windows are sealed shut and a computer controlled ventilation system is installed, but experience does not always support this conviction.
People, bless their illogical hearts, do odd things; sometimes the wrong thing, sometimes the right thing. Can we construct incentives to do the right thing? Why, yes, we can – individual metering is the most basic example. Further, people are observant: recent pilots that have installed usage meters in houses – or hooked them up to computer monitors – have shown that people reduce their energy use when they can see where it comes from.
These material investigations, together with physiological studies of people’s tolerance to draughts and uneven temperature indoors, promise to re-activate the discussion on the use of traditional ventilation through windows as a perfectly reasonable way to control to indoor climate without unreasonable waste energy. (Pg. 2)
In other words, the biological thermostat might just be better than the mechanical or electronic ones.
One way to keep the temperature where you want it
Another approach
If houses could be designed to work well with windows that are opened when it is convenient for the occupier, one could minimise energy loss and unpleasant draughts. For example, a bedroom with water absorbent and carbon dioxide permeable walls will hold a good microclimate throughout one night with windows closed. The windows can be opened for a short period during the day to regenerate the buffer capacity of the wall.
That morning sense of needing fresh air isn’t your imagination. the air does need recirculation.
This is also the time when the ambient temperature is highest, so the energy loss is less than if the windows were slightly open all night to ventilate away moisture and carbon dioxide. (Pg. 37)
When I was kid, you were supposed to sleep with a window open, even when it was cold. Maybe they knew what they were doing.
If only I could open windows too
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