Not posh enough for greenhouse,

I only have a polytunnel, does that count for this board? 30' x 14' on a slight slope, 3' higher at one end. the floor area is totally membraned. I have a large population of frogs and toads and very few snails. It's very full of perennials, potted to sell, and a load of seeds sown, eg aliums and perennial grasses. In the summer it's full of veg, including sweet corn in gro bags. It runs se to nw. Has doors either end for easy access and full ventilation. My query is that i expect the polythene to eventually give in a couple of years or so, (it's 3yrs old now)Would it be advisable to remove the original sheet, or place the second over, and would it give greater protection in winter?

 

Compostee - I have no first hand knowledge, but I would be tempted to remove the original sheet. The two sheets will let less light through, but I doubt that they would give better protection in winter. The insulation from two sheets of plastic, or double glazing in the house, does not come from the sheets themselves but from the still air trapped between them. Still air is a very good insulator. That is what keeps our clothes warm and the duvet warm at night - its the trapped air. In your case there would be no trapped air between the two sheets.

They might however be a bit stronger physically, ie to wind damage. But that is a different matter.

 

P is right i have been around these tunnels a number of years and have never seen anyone put a second layer over,and some of these owners have, how should i say been tight with there money,even if you had space between the two layers you would in fact get a great deal of condensation to say the least,and this in turn would cut out your sun light,even more than usual

 

Hi Compostee
It is a bit of a dilemma really

You will need to inflate the space between the 2 poly layers to create a "still" air situation.

Condensation between the layers shouldn`t be a problem if you use outside air as it`s cooler and less humid.

A certain percentage less light transmittance versus a higher insulation value and extended poly life due to the air pressure taking up any slack.
The combination of embrittlement by UV and flapping in the wind will destroy a poly covered greenhouse in pretty short order

 

Sorry Hex but i beg to difer,to do as you sugest takes time and money,he just wants to cover his tunnel in the best way he can.with the best results,i have never seen a poly with two skins in all my years and i have seen quite a few in fact at one place my friend had 20 tunnels each 120 ft long never bothered with a second skin always took it off and replaced with new,on that sort of scale one has bow to there knowledge,i hope i didnt offend you

 

What some do is line the polytunnel with bubblewrap as you would a greenhouse the reduction in light penetration is offset by the benefit of the added insulation.I would think about changing the present cover no use waiting untill it starts to break down

 

Hi macleaf
No offence taken, I like to look and think outside the box
The cost of the second skin isn`t a lot really considering halving the energy bill over the extended poly life would offset or even negate it.

Looking at it another way, it may extend the growing season by several months without any heating.

Inflated double poly quonsets/tunnels (domestic and commercial) have been in use for over 40 years, just not in the uk

Obviously no benefits are gained from just doubling up the poly, it has to have a still air gap created by a small blower.

Consider that that every winter countless people routinely add a layer or two of bubblewrap to the greenhouse.
The reduction of light intensity from 2 layers of horticultural poly will be quite negliable in comparison.

Remember that any greenhouse will be mostly in the dark on 21st december (18 hours) so the light transmittance of the covering is of little importance compared to the insulation value.

Just imho of course

 

OK Hex see your point of view,and respect it,lets just say i beg to difer.i will leave it at that i hope we can remain friends as i have no wish to fall out over such a matter

 

Thanks for the replies, all are very interesting. The amtter hasn't arisen yet, and on looking at the polythene this morning, it's looking very good, if even a bit slack. A tighten up will sort that out.

 

there you go no problem

 

I personally wouldn`t tighten it..once the temp drops lower still ..it`ll contract and tighten up like a drum.

I guess you could loosen it up a bit at that time if it hasn`t taken a set and permanently stretched..still the preverbial never ending job

The more times you stretch it..the thinner and weaker it gets.

 

I think both Hex and Macleaf are right, under differant circumstances. It depends on how much you want to heat your tunnel. If you are happy to have an unheated tunnel, which is probably the general case, it wouldn't be worth the cost of having a second skin. But if you wanted to maintain quite a high temperature the extra insulation would be worth it. Its like double glazing your house because you are heating it, but not double glazing your garage.

The Eden project in Cornwall has desert and tropical areas, which need considerable heating in winter. They use a double skin method, and as Hex says its inflated to maintain the space of still air.

Incidently, in this country we talk as though double glazing is the ultimate. A long time ago I wrote a little computer program with all the dimensions of my house in it. The program allowed me to alter the insulation value of any part that I wanted and told me how much heat was lost through every part. Once I had installed cavity wall insulation and double glazing, both in reallity and in the program, The program told me that the greatest heat loss from my house was through the double glazing. In Russia everything is triple glazed.

 

Hi Peter
I think you`re right in that it all depends on the main use for the polytunnel.

Compostee did mention he has "perennials, potted to sell" so extending the season may be quite useful in his situation.

Poly has a finite lifespan, so sooner or later you`ll have to replace it.
If you can extend it beyond normal then you`ve saved both cost and labour,even though it may cost more initially.
If in the process you can increase the thermal efficiency of the tunnel then you have a win win situation.

Assuming heat is a requirement, the cost of buying and running a heater for basic frost protection in a polytunnel that size would probably equate to quite a tidy sum over several winters.

The other option is relocate from Wales to the warmer climate of Cornwall

Eden uses 3 layers in the pillows with 2 air spaces in each..so it`s triple glazed like the russians :D

[ 06. November 2006, 06:55 AM: Message edited by: Hex ]

 

I had forgotten that Hex. I went just after it had opened - which is some time ago. I would love to see it again now. it must have matured a lot. I don't fancy their heating bill - even with triple glazing.

 

I don`t think it`d be all that bad considering the size.
Given the shape,size,insulation and the enormous amount of thermal mass involved..once it warms up it would probably take a fair old while to cool down again.

No poly to replace either for at least 20 years (unless the seagulls destroy it!) but at around �£12 sq m (6mil) it`s expensive stuff