Mylar vs. foil

[khronik]

I'd also like an example of an application of mylar "precisely because of its ability to reflect heat!"...

That's easy: mylar thermal blankets. I hear recommendations to people to use these to reflect light all the time. If these didn't reflect heat, they'd be useless for their stated purpose. There's also that stuff that looks like mylar bubble wrap that's used for insulation.

Mylar reflects heat. Therefore, it can create hot spots.

As for aluminum foil, it is an aluminum alloy, not pure aluminum, but it's still at least 93% aluminum. Aluminum doesn't mix well with other metals, so it can't go under that number and still make a useful alloy.

I'm kind of skeptical about using a light meter because light reflects differently off of different surfaces. When lights strikes a surface, one of three things happen. Either it's reflected, transmitted, or absorbed. Metals can't transmit light, so it's either reflected or absorbed. Any light that is absorbed would warm the reflector. I think an ideal experiment would put reflective containers under light and see which one absorbed the most heat.

I'm willing to accept that surface finish has a large impact on reflectivity, since smooth things are shiny. This may be the reason aluminum foil isn't recommended, although it does appear quite shiny. Still, I think it's safe to say the standard answer "aluminum foil is bad because it creates hot spots" isn't really accurate.

[TMusic]

Damn dude, you need to smoke a joint.

[Opie Yutts]

Don't use a light meter, use a laser thermometer and do a side by side comparison. I would, but I don't have a laser thermometer.

[ProGroWannabe]

This is getting more interesting by the day. Come on kronik, you can't stop now. Establish an experiment and test the theory.

[khronik]

Heh, I may do an experiment if I ever get some free time. I realize it's possible to draw some bad conclusions from what I've said, so I'll try and summarize the facts here:

• Mylar is actually clear, and made from the same type of plastic as 2-liter bottles. The stuff we use is actually "aluminized mylar". Aluminum is deposited on it in order to increase its reflectivity and decrease its gas permeability.
• Pure, polished aluminum is probably the most reflective thing there is, or close to it. Most grow light reflectors are made from polished aluminum. Aluminum, like other metals, reflects infrared radiation as well as light.
• There is no fundamental difference between the way aluminum and aluminized Mylar reflect light. Both depend on the high reflectivity of aluminum.
• Surface finish has a large impact on reflectivity. Aluminum foil is not optimized for reflectivity.
• Aluminum forms a thin layer of aluminum oxide on it upon exposure to air. This may change its reflective properties.

Anyway, I think it's safe to say that first, aluminum metal can be made to be an excellent reflector, and second, aluminum foil is no more likely to create hot spots than aluminized mylar. Actually, if mylar is a better reflector of light, it's probably more likely to create hot spots.

This doesn't actually change much. Mylar is still easier to work with and probably a better reflector than aluminum foil. But it does mean that people using mylar need to watch out for hot spots, and people using white paint are probably the real winners.

[the image reaper]

just to make you happy, I will turn on my 400-watt HPS lamp, (I'm currently between grows, doing maintenance) and use my Infrared Thermometer (accurate to within .2 degrees + or -), and search for 'hot spots' on my Mylar walls, and report back ... :smokin:

[xcrispi]

Weedhound and I both screwed around w/ our laser thermometers some,
but never comparing aluminum foil / mylar . Just mylar / flt. wht. pnt. and found there to be about an 11 deg. dif. measuring the temps at the same distance from said surface . Ask real nice and 1 of us might do the same for you guys foil / mylar arguments .

Peace
Crispi :jointsmile:

[xcrispi]

Dejavu huh Reaper ,
Folks need to go drop the 20.00 and buy some neat new toys like the big kids have .
Peace bro.
Crispi :jointsmile:

[the image reaper]

:wtf: ... well, I just now discovered something of importance ... I had heard many times, that Mylar reflectance is affected by it's overall condition (scratches, smudges, etc) ... but, I never knew how much ... my Mylar is (I thought) in pretty nice condition, I clean it only with Windex and soft papertowels ... looks very shiny, not dull, BUT ... the corners of my cabinet are flat-white painted 2x4s ... the walls are all quality Mylar ... my white 2x4s are reflecting twice as much light as my 'one-year old shiny Mylar' ... damn it ... :wtf:

[the image reaper]

it solved a problem, I'm building a new cabinet soon, but can't afford Mylar (even being a mega-rich drug kingpin :wtf ... so, now that I know the Mylar isn't holding up well, I will paint it flat white, like I did for years ... :jointsmile: