Just what is the point of bubblewrap?

Blackthorn. Your original question is a good one. Whilst bubble wrap does contain a bit of still air, I suspect that it adds very little extra insulation relative to a well installed sheet of polythene. In fact last year, when I installed it first, I looked for clear polythene sheet and couldn't find any so used bubble wrap as second best. I think there is a marketing side to this as well. Garden centres can charge more for bubble wrap without the buyer really knowing what he is buying and why.

I have installed it, using lots of drawing pins, in a wooden summerhouse, where the beams create a natural air gap, and there are no draughts at all. But in a green house I imagine it is more difficult to install it well and there will be more draughts, so the extra insulation of the air bubbles may be more inportant.

 

I have found another use for it. The roof of my greenhouse leaks, and with bubblewrap in place the water is channeled away from sensitive areas like plants and electrics.

 

Well thanks for all the comments everyone and to Hex for the technical info, I think some sort of trial is called for.
PeterS, I think you have a point about the suppliers wanting us to buy what suits them rather than what may be the best option for us. I don't know whether that applies in this case but I do have my suspicions.
Maybe next year I will split my greenhouse in half, one site bubblewrapped and the other with PVC. It won't be a perfect trial as the greenhouse halves will not be big enough to take full advantage of the fact that the PVC comes in larger sections, therefore leaving less gaps to be sealed.
Environmentally, both of these materials are bad news, I suppose the only difference would be that the PVC should last longer.

 

Also about solar panels, I think the best way of using them for greenhouse heating would be to use solar panels to store heat in bricks sealed in a container or under the floor of the greenhouse, like a storage heater, that can then be opened and released at night.
You can even make the panels yourself. We have recently obtained a book called 'Practical Solar Heating' by Kevin McCartney with Brian Ford, that details how to do it (we got it for a couple of quid from Amazon). It was published in 1978, so so much for 'new' technology. Just got to get round to making it.

 

The 70`s was a great decade for "green" ideas, which don`t seem quite so wacky now we have pretty much a throwaway society and the odd climate problem

Underfloor heating is a good plan, plants like bottom heat even if the air is cool. The smaller temperature differential between greenhouse air and the cold outside means less heat loss via the glazing.

The sun provides approx 1000kw/hr per square metre per annum.
An area just 4m x 2m would collect enough energy to run 915w (24/7) for an entire year!
(That`s 8000kw/hr or at least �£560 worth of electricity provided by nature for free)

The problem is storing it for later use.. you`d need a lot of bricks for 27.3 million btu`s. A "pond" 48ft square x 8ft deep would be perfect but impractical

There`s no reason it can`t be done on a smaller scale but requires a bit more effort than just plugging in a fan heater ..maybe the biggest hurdle

 

Hex is on the right track. No one seems to have realised that the greenhouse glazing is not sealed. The laps are separated by the stainless steel lap clips so air can get through the gap. Therefore the gap between the glass and any type of secondary glazing will pass heat to the outside. A curtain of bubblewrap will help to retain heat as does a heavy velvet curtain in a house. A perfect seal would cause ventilation problems and possible mildew.

 

That is right. You must have ventilation even in the coldest weather and that's even more important if you have a paraffin heater

 

If you look closely, when the greenhouse has condensation inside, those gaps created by the overlap glazing strips are blocked by water, so they actually provide no ventilation during damp weather.

 

I think all points are equally valid.

The real question is why we need the ventilation in the first place.

For paraffin heaters it`s straightforward enough.
Other reasons may be to remove excess humidity or to replace co2 perhaps.

All perfectly reasonable except that it`s a foregone conclusion you must expel the valuable heat from the greenhouse in the process.

You can buy all manner of useful gadgets for greenhouses these days..heaters, auto vents, misting systems, exhaust fans.. the list is endless.

Someday it will occur to someone that a simple air to air heat-exchanger will recover most of the heat from the outgoing stale exhaust air to preheat the incoming cold fresh air in winter.

A greenhouse can be very efficient with some creative thinking.

Unfortunately, most greenhouse suppliers don`t think creatively and would rather you vent your expensive heat into the garden during the winter!

Just imho, as a creative thinker of course

 

Blackthorn. If you think about it a greenhouse is a solar panel. You don't need another one. But if it's full of air it has very little storage capacity. Thats why some people have drums of water in the greenhouse to store the heat. A pile of bricks would do the same. Same also if you fill it overwinter with large containers full of earth (and tender plants). Exactly how big the effect is I don't know.

With referance to the ventialtion problem, I was interested in Scotkats comments about drenching Fuschia in Jeyes fluid then enclosing them in a polythene bag with no ventilation. http://www.gardenerscorner.co.uk/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=002382

Surely a sealed greenhouse is the same as a giant polythene bag. So could we just douse everything in Jeyes fluid and never ventlate.

 

The heat sink idea is a good one if you can put a large tank of water in your greenhouse (the bigger the better) the heat stored in the water during the day is leached back out during the cool of the night if you use plastic water bottles you can stack them in all the nooks and cranies

 

Mmmm, that's interesting, I haven't thought of using water as heat storage.

When we constructed the greenhouse, we sealed the glass in the panels with a sort of grey plasticine stuff, it came on a roll of long strips. It was placed all around the frame and the glass pressed onto it. Apart from being an air seal it helps to keep the glass in place. The only gaps are at the overlaps at the bottom of each pane but these are as Pete says, usually sealed with water. We have auto roof-vents and side louvres which are opened during the day for ventilation.

 

If you calculate how much heatloss to expect on the coldest day you can size the thermal mass accordingly.
Bear in mind you will have to "charge" the mass to make it effective.
The amount of energy transferred (both into and out of the mass) is proportional to surface area and the temperature diffence.

Ideally you need a large surface area and high temperature differential.
As the greenhouse is used to charge the mass the plants will be the limiting factor..they (and most poly covers) simply wouldn`t survive the 90C air temp of a typical solar panel

Working with a lower differential you will need a larger area and a larger mass to move or store the same amount of heat.

 

You are right about the surface area Hex, which is important as the temperature differential is never going to be that great in winter. Thats why I haven't made any special provision other than completely filling my summerhouse with pots, large and small, of tender perennials. Also putting my large empty terracotta pots (ie full of soil but no plant) undercover to prevent frost damage to the pots themselves. The surface area is pretty large - but I am still not sure how much warmer it keep the inside. At the moment the inside minimum is a consistant 3 or 4 degrees warmer than the outside minimum. But last winter after a few frosty days and nights, any heat stored had been exhausted and there was often no difference between the minimum temperature inside and out.

Even under those circumstances, I am convinced that the shed did offer protection. The air was perfectly still and the minimum temperature would probably have lasted for a shorter period.

 

Hi Peter
I think insulation and airleaks should take priority as they will give the best returns for minimal effort

Thermal mass works both ways..heating and cooling so it`s definitely worth looking at.
If nothing else it will help dampen huge temperature swings.

Surface area is very important, if you consider a 2ft cube tank of water holds 500lbs and a 4pint milk carton hold just 5lbs of water.

You will get more heat transfer (over 4x as much) from 100 milk cartons than from the tank due to the greater surface area (100 sq ft vs 24 sq ft), even though the amount of water and btu`s stored is exactly the same.

Could be a good way of recycling those pesky cartons, not sure about everyone else but we buy quite a few a week.
Ask your friends to save them too and you`d probably have a few thousand in no time :D