[jrko]

2 quick questions if I may

Why is the twin wall poly always 4mm?  It it because its the equivalent of glass

Would 10mm make a huge difference (other than cost!) in the warmth of the greenhouse?

BTW I build my green houses to fit rather than use a standard frame

Thanks J

[sunshineband]

The airspace would be wider so I guess therefore it would have better insulation properties

[jrko]

I have it in my head that light transmission is slightly better with 10mm.  The larger gap should allow more light through as the 2 layers and internal sidewalls are farther apart.  Also the joints, where internal sidewalls and layers meet, are smaller a % of the unit (than 4mm) and these lessen light transfer.

I think I'll stop being anal now.

Legit questions:  

Does anyone have experience with larger gap poly sheet?

Does anyone use end tape so seal the tubes  on 4mm?  (Otherwise heat can escape the tubes lowering heat retention)

[jrko]

no one used anything bigger than 4mm or end tape?

[jrko]

Ok i found some info:

The thermal insulation of a material is the “R” value. Horticultural glass is usually 3mm thick and a single layer has an R value of 0.93 while clear, single-layer polycarbonate retains slightly less heat with a value of 0.83. However, 4mm twin-wall polycarbonate has an “R” value of 1.42

3mm Horticultural glass - 0.93R
4mm twin wall              - 1.42R     52% better than glass
6mm  "       "                 - 1.59R    71% better than glass
8mm  "       "                 - 1.67R    79% better than glass
10mm "      "                 - 1.79R    92% better than glass

[mumofstig]

You still  need to check how light penetrates, as you mentioned earlier 

[jrko]

Updated numbers:

3mm Hort/glass   - 0.95R Factor                   98% light trans   1.05 U-Factor 

4mm  twin wall   - 1.43R  50% >glass    84%    "        "           0.70    "         50% >glass

6mm   "       "     - 1.54R  62%  >glass    82%    "       "           0.65     "        61% >glass

8mm   "       "     - 1.60R  68%  >glass    81%    "       "           0.63     "        66%  >glass

10mm "      "      - 1.89R  98%  >glass    80%    "       "           0.53     "        98%  >glass

R Factor - the measurement of ‘insulating ability’ of the material. The higher the R Factor number the greater the insulation value.

U Value - the measurement of ‘heat loss’ through the material. The lower the U Value the less heat is escaping

R & U Factor info taken from http://www.greenhousecatalog.com/greenhouse-covering-information

For those who like to build with old windows a 6mm double glazed unit has an R Factor of about 2!!  Cant find the rest of the numbers I'm afraid.

Although light transmission is lower in twin wall poly the light is diffused rather than direct as with glass and so plants receive a better spread of light, instead of being spot lit directly.  Also most UV rays are reflected so you can sunbath naked after a couple of drinks and not get burnt 

[JayG]

It would appear then that in winter a twin-wall greenhouse would lose less heat but also gain less from the available radiation from the sun.

In summer it would potentially get much hotter than a glass greenhouse due to the higher insulation, but again that would be tempered by the reduced light transmission.

Maybe someone has one of a similar size of each type and can give us their views (not that it makes much difference to me - I'd go for twinwall if it wasn't so darned expensive, despite having serious concerns about what happens when the moisture gets inside it......)

[jrko]

Use end tape JayG.  Its always used on conservatories to stop the ingress of moisture but no one ever seems to bother on greenhouses.  If the volume of air is trapped it will retain heat.  If its allowed to escape then you lose almost all he benefit of having 'twin walls'.

Its like going to the Arctic with all the modern gear and not tucking your thermals in

[JayG]

I won't be using tape jrko - mine is common-or-garden horticultural glass (which by the way does absorb rather than reflect most UV radiation, although I have yet to do any "safe" sunbathing in it!) 

[jrko]

Not so much sunbathing, but pretty much everyone knows I am a fan of liquid fertiliser.  NEVER pick up a full Lucozade bottle in my shed.......it ain't Lucozade.....

I've got a great collection of double glazed units all of which were free.  But I can get 4mm twin wall for a good price.  It's only matching the windows up for construction thats will be the bother.

This greenhouse made me want to redo mine WeUh74s2m30