View Full Version here: : My flip top observatory physics question
11-10-2011, 06:24 PM
First the pre-amble.
Just built a flip top observatory (link here http://www.iceinspace.com.au/forum/showthread.php?t=81602 ) and the question about how to minimise the heat build up inside it arose. PeterM suggested painting the roof using silvered bitumen paint to reflect sunlight. This is the stuff here.
I agree with peter that it's a good idea to do that and I thought of another way to use it but I'm not sure if it would work.
What if I paint the inside as well? The question is “would the silver finish minimise the infra red radiation from the inside wall to the inside of the observatory”? My backyard physics expertise says it would but I'm not sure.
Another question is “would the bitumen component of the paint stop dew forming on the inside of the roof and walls”? That would be a serious bonus if it did.
Thinking caps on folks! There has to be a materials expert out there somewhere!
11-10-2011, 06:32 PM
I don't have an observatory but I do have a big shed, a carport and an under-cover entertainment area.
My experience is that the only effective solution to condensation is something which incorporates an air gap and a decent amount of ventilation.
Paint it white. It is the most efficient reflector of heat. Silver on the other hand will absorb heat, depending on how dark it is.
11-10-2011, 07:00 PM
You would be wasting your time painting the inside of the observatory.
The paint is acting as a reflective barrier not as an insulator.
I use Dulux "Silvasheen" which is an aluminium enamel paint for the roof and white paint elsewhere.
Silvasheen forms a mirror finish which is highly reflective.
Daytime temperatures are reduced by 3-5 degrees C in summer.
A word of warning. If your observatory is under a flight path make sure you are not violating any air safety codes.
11-10-2011, 07:00 PM
Not such a simple problem unfortunately. The colour of paint is no guarantee it won't emit IR. The emissivity of paints and materials is independent of the visual colour. You need to know what its emissivity is in the IR band to make that judgement.
Regardless of emissivity in the IR band, painting the outside of your observatory white helps keep it cool because all the visual colours are reflected and not absorbed and turned into heat. That's why all the professional observatories are white. Now if you can find a white paint that is also a reflector of IR you are really in business!;)
No... well I serously doubt it. To stop dew you need to keep the material of the shed above the dew pojunt temperature, or you need a vapor barrier (a sealed dry air gap between the outer surface and usually sisalation).
Have you thought about a solar ventilation fan or a whirly bird? I bought my solar vent fan from Whitworth's.
For dew control you need to include a waterproof membrane somehow. I used Thermach Coverup - it works really well.:thumbsup:
11-10-2011, 07:48 PM
This stuff looks interesting.
Seems as though you can paint on insulation to a degree.
12-10-2011, 06:41 AM
If you're considering the Thermilate additive don't use it in a reflective paint, in particular for interior applications. The metal flakes in the paint will act as a heat conductive pathway and counter the insulation effects of the micro spheres.
12-10-2011, 09:51 AM
What if I glued this to the inside roof and walls?
Any idea what glue to use?
12-10-2011, 10:11 AM
Cheapest insulation is styrofoam it also has the highest delta.
An air gap on your roof is very effective. a few rows of battens and a tarp tacked tautly to it will give you a 5 degree change for next to nowt in cash.
Air flow - remove the heated air. whirlbird, black chimney (this needs to be at least the same height as the shed so not really practical unless it derigs when you open the top for observing.)
Cooling - put the coil for your pool heating on the roof - hot pool, cold observatory.
Evaporative cooling - do the same as above but make some pinholes in the pipe on the roof .... or spray water as a mist from an upwind source.
Forced cooling - air conditioner.(this also dehumidifies until you open the top and get condensation from the dew
12-10-2011, 05:54 PM
Ok. Decision time.
1/ Paint the north wall with silver bitumen paint to reflect heat.
The west wall is shielded and the east wall only gets an hour or two
of sunlight before the house shields it.
2/ Foil bubble wrap insulation fitted to the inside roof and walls.
I'm going to try to stick it directly to the metal with no air gap so I've got a
full moisture barrier. Will try using “quick grip” spray adhesive on a wall first
to see if it sticks.
3/ Fit passive vents on both sides at the top and bottom to see if I can get some air flow going. That should help equalise the inside and outside temps on a hot day and cool it down at night.
That'll keep me going for a week or so. Will see if I can get some temperature data over summer to report back.
Thanks for the help.
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