Basically it was working well, but the 40w heat lamp (infra red) was too hot for the chicks in such a small cosy space. At night they were under only a towel and that was plenty warm. But during the day, the lamp was too hot, and the chicks weren't able to sleep comfortably in the igloo, yet being outside was just that little bit too cool.
So this morning I found a reptile heat mat at a nearby pet shop for around $20. Power usage is 5 watts! That's a huge energy saving (not that 40w is all that much in the scheme of things, but power matters).
I've installed it in a kind of wire mesh 'pocket' attached to the underside of the lid at the igloo end. Last but not least I've attached a timer so that power is only on for 15 minutes with 15 minutes off (as recommended by the heat mat manufacturer, in case the plastic it's embedded in overheats).
It works because the inside of the igloo only needs to be a few degrees warmer than outside for the chicks to want to huddle inside when they get a little chilly. Once inside, their own body heat is largely trapped by the straw surrounding the igloo, and they create their own cosy space. The 5 watt mat is really just a lure to teach the chicks to go inside when cold. At about 14cm x 15 cm it easily fits into the niche area between straw sections. The mat itself isn't touching anything flammable, as there's wire mesh above and below.
Lastly I've draped a small piece of cloth over the whole lid area, helping trap the heat and deflect it downward. Here are some shots without the towelling: one of the top of the closed lid;
... and one looking at the underside of the lid when it's raised.
One last benefit of cold (or lukewarm) brooding: you don't get high numbers of coccidiosis oocysts building up around feeders and waterers. Why not? Because only the 'igloo' is heated! Since coccidiosis thrives in warm wet conditions, a cool run helps limit their numbers and reduce coccidiosis outbreaks.
Lastly, chicks under a mother certainly don't spend their whole time being heated externally. Having chicks learn when they need to find warmth is part of raising healthy chicks.
IMPORTANT UPDATE:
Training chicks to use the cold brooder after they've learned to associate warmth with light (via a lamp, whether infra red or plain) can be hard! I found while using the 5W heat pad that it didn't provide enough of a lure if there was a light source nearby, because the chicks would hurry to any source of light hoping to get warm. The process of training them to cold brood themselves would probably be quicker if the first 2 days after hatch involved a *lightless* heat source such as a ceramic reptile heater, and probably also a thermostat (so chicks don't overheat while crammed inside). But that's probably all getting a bit high-tech (and expensive).
This is a work in progress. But as a last caution, in case people are using any of this as a guide: whatever you do, don't use a 5W heat pad expecting day old chicks to know what to do to keep warm. That's a recipe for chick death. All cold brooding, and low-wattage brooding (as above) involves a lot of manual effort in the first week putting chicks into the warm compartment and letting them out for short periods until they learn where to go.
