[qwerty565]

Hi,

I'm going to rebuild my old Dutch style greenhouse later in the year and I've been given the chance of 6'x6' upvc double glazed units for free to make up the lower level, I'm going to stick to the old 2'x4' panes from the existing one for the roof which are ok. The question I'm throwing your way is the light diffusion created by double glazing, is it worth the hassle of stripping the units into single glazing or should I just leave it double glazed? Will vegetative growth be significantly reduced by a double glazed unit, i.e. weaker or drawn plants? I understand there will be pro's and cons and the roof will be single glazed anyway so insulation will be affected but that isn't really an issue as the greenhouse isn't heated. My mate told me how to strip the units and fix the single panes in if I go ahead with it and fixing them in is easy enough and it will leave me with spare glass for the odd brick the local kids give me every year.

Thanks to anyone leaving advice on this one!

[JayG]

Standard single glazed horticultural glass is usually quoted as around 95% light transmission, and plain double glazed units at around 80% (that may sound like a serious drop but is actually only a bit lower than many twin-wall polycarbonate sheeting products.)

Should be fine as long as they are not high-spec double-glazed units whose transmission figures can be down to about 30% (which is what they are designed to do of course for applications other than greenhouses!)

[qwerty565]

Cheers JayG..

[jrko]

pulled from my post back in Feb - might help

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