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Source: Low Tunnel Construction: How to Build a Mini Hoop House

I have a small greenhouse that I want to keep as cold as possible in the spring. I plan to cover the greenhouse with a polyethylene tarp to shield it's contents from the sun.

However, I'm not sure what colour of tarp would be the most effective at shielding heat.

I suspect that light coloured tarps would allow for solar gain. And I wonder if a black tarp might also retain warmth by absorbing heat from the sun.

Question:

Are there specific colours of material that are known to let in the least amount of heat?

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As a practical and cost effective solution, I'd advise using aluminium foil, or tin foil as it's also called. Please realise that it's rather fragile, so it's best to glue it to a plastic foil or something to keep the wind from ripping it apart. It doesn't really matter if cracks get in the foil. Use the most shiny, reflective side of the foil as the outside. This maximises reflection of heat in the form of radiation.

Make your roof wider, so it covers more ground surrounding your crops. If the sun is able to heat up the ground right next to your crops, the heat in the ground will travel to the ground under your crops, and ending up also heating the crops themself. You don't want that.

You can't get or keep it any cooler than the surrounding air, so make sure it's ventilated, to keep temperatures as close as possible to outside air. That's the lowest you'll be able to go. Therefore, don't hermetically insulate your crops like you did in the picture, but enable it to ventilate. No insulation can keep all the heat outside. It will let through more or less heat. Any heat that is let through by the roof is removed by ventilating. If you cover them like in your picture, any heat let through by the roof is instead kept under it.

I'd advise to set up your roof like this. You'll also keep the ground surrounding the crops as cool as possible this way, minimising any heat conducted to the crops.

roof

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  • $\begingroup$ Other options for reflective material might be survival blankets or reflective film. $\endgroup$ – Wilson Feb 16 '18 at 19:39
  • $\begingroup$ Or reflective sheet insulation. $\endgroup$ – Wilson Feb 16 '18 at 19:43
  • $\begingroup$ @Wilson That's right. There are much better alternatives for tin foil, but all of them would be harder to obtain and more expensive. And they would barely give a better effect in this situation, since you can't insulate the ground beneath. My goal was to give you a practical and cheap solution. If you would to be totally insulate something, other materials will certainly give more resistance to radiation, but in this situation that won't reap any benefits. $\endgroup$ – Bart Feb 19 '18 at 10:07
  • $\begingroup$ Aluminized Mylar is not particularly expensive or difficult to find. $\endgroup$ – Eric S Apr 6 at 12:09
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I would suggest a reflective mylar film such as those used for survival blankets. That would be the most effective single layer solution. Adding additional layers of plastic would further insulate the ground from the warmer air in the tunnel.

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White and silver are the most reflective (not absorb heat from the sun). There is some debate about which is more reflective. A white tarp is probably going to be cheaper.

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    $\begingroup$ Silver reflects the most radiation ; However , white re-emits more. So the net affect of white is slightly better , until it gets dirty. $\endgroup$ – blacksmith37 Feb 15 '18 at 16:29
  • $\begingroup$ It seems strange to me that there is a debate about which colour is more reflective. Wouldn't this be easy for scientists to test? $\endgroup$ – Wilson Feb 16 '18 at 15:07
  • $\begingroup$ @Wilson Then test. Good luck getting a true silver in a plastic tarp. $\endgroup$ – paparazzo Feb 16 '18 at 15:11
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@donald Gibson is correct, i will add this note: a black tarp will exclude heat because it absorbs it. but after absorbing it, its temperature goes up and it begins re-radiating the heat which includes off the shaded side of the tarp- which means that any tarp that excludes the transmission of infrared (heat) radiation by absorbing it will inevitably fail to block heat because that absorption inevitably gives rise to radiation. the incident radiation must be reflected away, as pointed out by @Donald gibson.

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  • $\begingroup$ So, does a black car in the sun stay cooler than a white one in summer? $\endgroup$ – Solar Mike Feb 15 '18 at 5:28
  • $\begingroup$ of course not. the heat gets conducted through the metal roof and re-radiated and convected into the interior of the car. This was a lesson I learned while struggling to concoct a "heat shield" to prevent the exhaust muffler on my tractor from melting the top of the battery nearby it. the heat shield works only until its temperature approximates that of the radiant source, at which point it is no longer a heat shield. my point is that the heat shield would need to reflect as much heat as possible away from itself to keep it from heating up. Sorry I did not make that more clear, will edit. $\endgroup$ – niels nielsen Feb 15 '18 at 5:40

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