I was worried you were going to vent in the winter. I was trying to figure out how things wouldn't freeze. It all made sense in the end. ANother great vid! Cheers!
It's interesting to do a stack (chimney) calculation for this. This is where the value of low and high-level vents becomes apparent. The higher temperature in the greenhouse compared to the surrounding air creates a pressure differential that drives the flow through your vents. The greater the height difference between the low (inflow) vents and the high (outflow) vents the faster the flow through the vents. For ease its best to take the area of the smaller vent (ideally the inflow and outflow vents should have about the same area). Bernoulli's equation converts the pressure differential into a flow speed and that times the vent area gives you the volume flow rate. Using the 2% rule you have gives a temperature rise inside the greenhouse of about 15 degC (27 deg F) for 500 watts/sq.m solar heating with a 2 metre (78 inch) height difference between the vents. So, as you say, I would treat the 2% rule as an absolute minimum and shade cloths would almost certainly be required in summer. Having struggled to keep our old greenhouse down to a reasonable temperature for tomatoes (even here in the UK) I'm building a new one with about 12% vent/floor area ratio in the hope of keeping temperature rises down to about 6 deg C when needed.
Great wisdom from experience! Thank you for sharing this with everyone, when I build a solid frame and wall greenhouse I will surely be taking into account venting for summer and locking down tight for winter, since we’re in a colder zone I chose to go with a very low ceiling build to optimize heating in winter, but for summer our shade cloths at 40% surely do the trick. Thank you again for sharing with everyone!
@@Earthdwellershomestead Many thanks for the reply. Yes, making a greenhouse serve well in summer and winter seems a real challenge. Keeping the ceiling low is certainly a good way to help keep things warmer in winter. In addition to the greenhouse we use cold-frames made using 10mm twinwall polycarbonate and they are quite effective in winter for plants small enough to go in there. We have used the 10mm twinwall for the greenhouses as well because of its good insulation properties. It's also very tough which is important here as its a very exposed site (shortly after we moved in one of the pole barns blew off down-wind in a storm and took a chunk of the house roof off on its way so we had quite a heads-up to how thing need to be built around here!). The oldest of our polycarbonate is about 10 years old and is still clear and flexible so the claims for a 30 year life for the stuff seem reasonable. I'm really going to town on sealing the new greenhouse for winter as I'm convinced even small air leaks have a significant effect on over-night cooling. I'm also looking at ways of storing some of the heat of the day in the 1000 litre IBC bottle that holds the roof capture water and releasing it back to the greenhouse over night. BTW - an old Victorian trick to help greenhouses in winter was to darken the soil with ash!
Finally a video that actually sums it up. Thank you sir. Glad I found this video!
Always helpful. Something, as someone new to greenhouses, I would have never thought about.
Yes, I wanted to help in the planning and design, and overall knowledge! Thank you!
I'm gathering info.on a diy greenhouse for 2022.thanks man,Merry christmas
Thank you! And merry Christmas!
I was worried you were going to vent in the winter. I was trying to figure out how things wouldn't freeze. It all made sense in the end. ANother great vid! Cheers!
Thank you! And merry Christmas!
why do I keep hearing a rap song in my head about greenhouse ventilation! Seriousy though - thank you. I too was wondering the same!
I figured I would share what I've found for info and how I apply it to my diy greenhouses. thank you for watching and checking out the channel!
Ty! I just built my first 6ft x 9.5ft greenhouse and this was the exact information I needed
Awesome! Glad it helped at the right time!
It's interesting to do a stack (chimney) calculation for this. This is where the value of low and high-level vents becomes apparent. The higher temperature in the greenhouse compared to the surrounding air creates a pressure differential that drives the flow through your vents. The greater the height difference between the low (inflow) vents and the high (outflow) vents the faster the flow through the vents. For ease its best to take the area of the smaller vent (ideally the inflow and outflow vents should have about the same area). Bernoulli's equation converts the pressure differential into a flow speed and that times the vent area gives you the volume flow rate. Using the 2% rule you have gives a temperature rise inside the greenhouse of about 15 degC (27 deg F) for 500 watts/sq.m solar heating with a 2 metre (78 inch) height difference between the vents. So, as you say, I would treat the 2% rule as an absolute minimum and shade cloths would almost certainly be required in summer. Having struggled to keep our old greenhouse down to a reasonable temperature for tomatoes (even here in the UK) I'm building a new one with about 12% vent/floor area ratio in the hope of keeping temperature rises down to about 6 deg C when needed.
Great wisdom from experience! Thank you for sharing this with everyone, when I build a solid frame and wall greenhouse I will surely be taking into account venting for summer and locking down tight for winter, since we’re in a colder zone I chose to go with a very low ceiling build to optimize heating in winter, but for summer our shade cloths at 40% surely do the trick. Thank you again for sharing with everyone!
@@Earthdwellershomestead Many thanks for the reply. Yes, making a greenhouse serve well in summer and winter seems a real challenge. Keeping the ceiling low is certainly a good way to help keep things warmer in winter. In addition to the greenhouse we use cold-frames made using 10mm twinwall polycarbonate and they are quite effective in winter for plants small enough to go in there. We have used the 10mm twinwall for the greenhouses as well because of its good insulation properties. It's also very tough which is important here as its a very exposed site (shortly after we moved in one of the pole barns blew off down-wind in a storm and took a chunk of the house roof off on its way so we had quite a heads-up to how thing need to be built around here!). The oldest of our polycarbonate is about 10 years old and is still clear and flexible so the claims for a 30 year life for the stuff seem reasonable. I'm really going to town on sealing the new greenhouse for winter as I'm convinced even small air leaks have a significant effect on over-night cooling.
I'm also looking at ways of storing some of the heat of the day in the 1000 litre IBC bottle that holds the roof capture water and releasing it back to the greenhouse over night.
BTW - an old Victorian trick to help greenhouses in winter was to darken the soil with ash!
Are you going to use solar to run the fans?
Great question! I will as we don’t have any power hooked up out in the greenhouse. I’ve been browsing around for what I’ll use