A "Leaky Thermos Flask" bird house design

Prior to building the bird house, I did a fair amount of homework. There are more than 10 iterations and reviews. Fortunate to know someone who think alike to share and brain storm together. Yes, we have different opinions but the outcome of 2 heads was definitely better than one.

Let me cite a case of poor thermal design to begin with - seen in Krabi, Southern Thailand. Installation of plastic sun screen was an after-thought in fixing a problem recognized after months of operation. Evidently it is still not a solution as it cannot withstand weather forces over time. There are so many similar cases seen everywhere.















Before going further may I share this paradigm "Quality is doing it right first time" from Philip B. Crosby, a father of quality philosophy with a zeal for zero defects. The picture above speaks otherwise!

Of all the bird houses I've seen in S.E. Asia, except in northern part of Thailand, nowhere employs double-wall design. No mention of it's use in any literature I browsed at the time. The idea is nothing new in building industry. Perhaps it's the perception of higher building cost in people's mindset. If one fix a budget and focus primarily on costs, the builders wouldn't encourage it either even if they knew better. Why should they if it eats into their bottom line profit? Else that is a good reason to up the cost. But does it really cost much more?

If single wall designs are egg shell in comparison to massive walls of limestone caverns. Double-wall design is one step up from the design perspective.

I like to compare that to the Thermos Flask because it is the best case of economical thermal design. The whole bird house is conceptualized with a thermal mass model - the walls, internal airspace and everything else within.

Light color walls & roof provide some degree of radiation heat reflection similar to the silver reflection lining of the flask. The vacuum space is ideal in blocking thermal conduction. Next best of course is to replace by "cost free" airspace.




















It is not uncommon to use double-brick walls to increase the thermal mass as well as for added security of a stronger wall. So cost of adding airspace in between is not significant.











In colder climate, construction practice even filled space between walls with insulation materials.












How good is the insulation property of air space?
We all know Styrofoam is a low cost insulating material and widely used. A comparison can be insightful. This is also an example a little maths provides more depth in understanding.

Recently a certain "consultant's blog" featured a new building with Styrofoam embedded inside walls. Did I remember correctly also photo of a bird house wrapped externally with Styrofoam "skin" (posted years ago) labeled as "creative"! (Forgot the weather resistant factor? Amateurs can be ignorant, not Professionals)

Perhaps those approaches may need some rethinking after review of following numbers.

Relevant thermal property to consider are:
Thermal conductivity of Styrofoam, k(Styrofoam) = 0.033 W/m.K
Thermal conductivity of still air, k(air) = 0.024 W/m.K
Lower values means less conductive and hence better insulation property.

Units of k are W/m.K where
W is rate of energy flow (heat) in Watts (or Joules/sec) across 1m^2 surface.
m is 1 meter material thickness
K is temperature in deg. Kelvin, analogous to deg. C for practical understanding.

N.B. Total thermal energy (Joules) transfer across 1m material thickness over a period, t (sec),
is to multiply k, by surface area(m^2) x t(sec) x T(deg. K)
where T is temperature difference across surfaces at 1m apart.

Taking the ratio of 0.033/0.024 = 1.375
It simply means that "still air" is more effective than Styrofoam insulation by 37.5%. Surprise?
Or the use 10cm thick airspace is equivalent to 13.75 cm Styrofoam! So the final choice depends on economics of building methods in use.

Conversely, the air space within my double walls is 90 cm wide (enough for staircase). Multiplication of 90x1.375 = 123.75 cm; that is the equivalent of "Styrofoam filled space". See the comparative advantage trapped airspace.

So a little basic science and maths give much clarity - no rocket science as you can see, only a matter of practical engineering considerations.

In my bird house, a matrix of air-holes punctuated the walls designed to provide a combination of both "Stack" and "Cross" ventilation. With the air-flows at entrance holes the bird house design simply becomes a "Leaky Thermos Flask". The effectiveness of air space insulation should be derated due to air movements through vents.

As often cited in literature, best to orientate the bird house along "east-west" direction to reduce foot print to sun's heat. I have a long wall facing south-west. It don't bother me, for that is a solar wall receiving thermal energy to create humidity; by sprinkling water to the wall from inside. Humidified air gets into the bird house as part of air change.

Few more examples of maths and physics application:
1) Simple trigonometry calculation to know the effective surface area, normal to west, receiving sun's heat.

2) Calculations of the energy needs at various rate of air exchange while maintaining constant humidity within.

3) Use of water as a thermal reservoir inside the bird house.

With in-depth understanding, one can design to limit and control amount of energy inflow during the day with an acceptable daily temperature cycle, regardless of building orientation. Fluctuations of energy flows due to external and uncontrollable factors that affects temperature and humidity stability with air change is a matter of balancing with extra energy input with electronic controls.

In essence that's my concept of a minimal energy bird house. Think I did enough work on bird house designs, with depths more than I've read in books and forums.