A peek under the covers.

[edds]

I think the idea is that the fleece warms up in the winter sun and retains some of this heat.

This idea has always intrigued me (and I've heard it a fair few times). If this is the idea then why not make the fleece black so it heats up more quickly, rather than white?

Black cools faster than white

I thought that was only marginal in the infrared end of the spectrum? Though it does seem the ideal might be black on the outside and mirrored on the inside!

[el48tel]

This idea has always intrigued me (and I've heard it a fair few times). If this is the idea then why not make the fleece black so it heats up more quickly, rather than white?

Black cools faster than white

I thought that was only marginal in the infrared end of the spectrum? Though it does seem the ideal might be black on the outside and mirrored on the inside!

In brief

Energy transfer is always
Hotter to colder
Shiny slower than matt
White slower than black

Rate of transfer
Faster when the temperature difference is larger
Pretty complex maths, and rate of transfer is different at different temperature differences and temperature start points.

[Conrad]

I think the choice is between fleece with slightly higher temperature and high humidity/ very poor air circulation on one hand, and no fleece and lower temperatures/ better air circulation/lower humidity on the other. Given that the plants you've been losing look like botyrtis cinerea deaths, I bet you'd have lost many fewer with no fleece and lower temps.

I've been sub zero frequently this winter in the big polytunnel, despite 11kw of gas heating. And as low as minus 4 for periods, but even the melocacti and baja mamms are currently holding on, though showing some severe stress coloration. Everything else looks fine (several 1000 Mexican and S American plants). For me the key is making sure you dry everything out very properly, like seriously, seriously dry. I also have a damp proof membrane under terram on the floor, as I'm straight onto a soil base. Also all doors open if above 2c, drier is better than warmer in my view.

[Smel]

Hi Ed,
Sorry to hear of you lost or perhaps only damaged plants, I too have had my share of disasters this Winter, perhaps lost 100 plants, not only did I not get around to renewing my cheap bubble insulation (With the proper stuff, not expecting such a cold Winter), bit I also had a couple of leaks in the roof, which kept some plants quite wet. This as only just come to light, after removing what was left of the old bubble insulation...

Working on my photographic theory, White reflects all colours (Wavelengths) while black absorbs them especially IR and heat? Their all part of the same spectrum of Energy and bounce around like radio waves.
For the time being I have used some clear plastic sheet to cover my plants with.

Mel.

[el48tel]

Just extending the thoughts and comments.
As Conrad points out, one needs to consider the air flow and balance that thought with the bacterial and fungal problems.
At low temperature differences between the air and the object, the energy flow is not so much colour and nature of surface which is the major factor in energy loss as radiation loss is a smaller feature at lower temperatures. The major factor in heat energy flow across the temperature gradient we are discussing, is convection currents and conduction by direct contact with the bench or the floor. The conduction can be managed with a standard insulation block such as foam. With regards to convection losses, and this is where most of the heat energy transfer will occur, one needs to look at the design of the surroundings. Maximum convection loss occurs when there is turbulent flow. Stop the air movement and you minimise the energy loss by convection. But .... stop the air movement and one maximises the environmental condition for bacteria and fungus. Laminar flow, where the air flows parallel to the surface, has lower thermal transfer, and will prevent the stagnant conditions which are ideal breeding ground for bacteria and fungus.

[Keith H]

I thought the idea was to trap airspace which acts as an insulating layer, cooling down more slowly after warming in sunlight, rather than the fleece itself insulating. The same principle as wearing lots of layers when winter hillwalking (or summer, up here)

My Dad was always a fan of putting newspaper over the tender plants in his greenhouse which seemed to work as that greenhouse benifited from an un shaded position and caught any available winter sun. I can not see it working in my rather shaded urban garden because it warms very little due to the lack of direct sun in winter.

[habanerocat]

It wasn't until I started putting bubble wrap laid directly on my plants like @smel above that I realised just how much condensation was dripping on the plants.
Just to big a job to string it up on the roof and sides any more.

[Chris L]

Reminds me of the discovery I made one spring a number of years ago. Our garden slopes and the water butt was up stream of the greenhouse so that it was on the flags behind the greenhouse and out of sight.

I distinctly remember it being full as winter set in and was very puzzled come first watering in the spring that there was no water in it ..... at all.

Close inspection revealed that the bottom had a 8 inch split in it and the 40 gallons of water had leaked out under my greenhouse... , naturally when the weather was cold enough to freeze the whole barrel of water.

That explained why I had suffered a number of plant losses and a large proportion of them had got covered in mould and botrytis.

A sealed floor is much better if it can be acheived.

[Mike P]

So I took a different approach to insulating the two greenhouses at home which are heated to just above freezing……I figured it would preserve as much of the expensive heat as possible there being no air gaps. The bonus has been that it has eliminated drips entirely. I can hopefully remove it without too much difficulty come the end of March…..

[el48tel]

So I took a different approach to insulating the two greenhouses at home which are heated to just above freezing……I figured it would preserve as much of the expensive heat as possible there being no air gaps. The bonus has been that it has eliminated drips entirely. I can hopefully remove it without too much difficulty come the end of March…..1693B089-5511-4BEA-A7BC-BB3F648753B6.jpeg

That is ..... thinking outside the box ..... neat.
Does it work as well as you anticipated?