Avoid the pitfalls

Hi everyone.I am new to the group and have recently moved to the countryside on a half acre site. Lots of work to do in the garden and I have just bought my first ever greenhouse. Is there a list of does and don'ts or golden rules to follow etc. The greenhouse is a modest 8x6 with horticultural glass. It will be used mainly for starting seedlings and cuttings later to be potted and grown on outside, growing small number of tomato plants and also for storing fuischas and other more delicate plants over the winter. So any advice on flooring, insulating, heating, watering, pest control, basically anything that may avoid me saying if I were to do it again I would **************.
Thanks,
John.

 

Hi, my first greenhouse has just completed it's first winter. Like yours, it's 6x8' and intended for the same things. Here's a list of things I did wrong!

Get a decent heater with fan if you're going to heat it! Mine is a 'frost-free' heater intended for areas like unheated utility rooms etc. It worked but did not spread the heat so I had areas getting below freezing.

Get lots of shelving for seed trays, which you can move later for larger plants and in winter.

Make sure your greenhouse has not got lots of gaps in the structure! Mine was put up by two gardener/ handymen and they have left lots of gaps which I shall go round plugging before next winter. I'm cross because it wasn't cheap and plenty of insulation was provided.

Have AT LEAST one self opening ventilator.

Try having a big tray with soil heating cable for germinating seeds. I have got several small heated propagators and am constantling juggling things- but if you have indoor space for seed trays this isn't so important. I have one small window ledge!

Last of all, have fun- I have!

 

Thanks Liz some great advice there. I was considering paving slabs on the floor then considered polystyrene then slab over it to prevent chill coming up through the ground. Not sure it is worth the extra expense. The heater with fan sounds good I was just about to buy a parrafin heater but as you say I could easily end up with cold spots.
John.

 

Mainly due to indecision, I left my floor earth with a path of boards up the middle. I thought I might like a vine or something in one corner, but I don't know if I have room!

 

I`d leave the polystyrene for use as perimeter foundation and northwall insulation.

The glazing and airleaks will be where most heat is lost in winter.

The soil under your greenhouse will heat up through the season, so heat loss will be minimal through the floor as the temperature difference will be small.

Without the perimeter insulation (2ft deep will do) your greenhouse floor will be thermally connected to the rest of your garden in winter.

Looking on the brightside that soil will still be a lot warmer than the air temperature next to your glazing.

Do everything you can.. as every little helps

 

please don't laugh if I am asking the obvious but I am a complete beginner to greenhouses.
The perimeter polystyrene 2ft deep I assume is outside and placed vertically ?

John.

 

Most people seem to use bubble wrap [large bubbles] held on by clips on the inside of the greenhouse. My greenhouse is so full of stuff I find this very awkward to do, so I was wondering too if I could put it outside.

 

Hi John,
you are correct that the polystyrene is installed vertically.

Liz
Bubblewrap adds about R-1.1 to your insulation value.
which is about the same value as adding a 2nd pane of glass.. but only if it is sealed airtight.

Alternatively, fixing 2" polystyrene to the north wall and north roof section adds about R-8 to R-10 to a fair percentage of your greenhouse surface area on the coldest side (ie: the north)

Example: a 40 sq ft (8ft x 5ft) section of greenhouse wall with 30F temperature difference (inside to outside)
Single glass: 350w heatloss per hour.
Single glass + bubblewrap: 167w per hour.
Single glass + 2" polystyrene: 32w per hour.

It also reflects the low angle winter light coming from the south back into the greenhouse

 

Thanks Hex. Are these figures assuming mininum air gap between glass and wall, ie polystyrene directly in contact with glass, which would be easier?
How do you know all these things?

 

Hi Liz,
It`s pretty basic thermal dynamics

A thin layer of polythene by itself has very little resistance to heatflow.

The gap itself contains "still" air which does the actual insulating, hence it has to be sealed airtight (like double glazing units)

If you put the polystyrene tight against the glass any humidity in the air could condense on the cold surface and cause problems.

Adding a radiant barrier facing into the greenhouse airspace could be useful too as polystyrene or bubblewrap won`t help prevent radiant heatloss.

 

I must say I like the idea of insulating the north side with polystyrene in winter.

 

Hi Hans,
Try to find a moistureproof type with a foil face.

As the sun in much lower in winter the internal north slope of the roof will act like a reflector bouncing the light back down into the space.

To be honest the UK seems years behind when it comes to greenhouse design.

If house design progressed at the same rate we`d still be waiting for double glazing and cavity wall insulation

 

There is a solid type insulation used in a lot of house building called Celotex it is a semi solid insularion with a reflective outer layer- this or similar would be a great material for use in this senario.