Keeping the greenhouse from overheating in late spring / early summer when the temps begin to rise into the 90's is sometimes a bit of a challenge. Usually by mid May, the seedlings have all been transplanted into the garden, so it isn't a concern, but lately we have been getting some pretty warm days in early March and April so it is becoming an issue.
I do have an automatic window vent on one wall and last year I added screens to the door which seemed to help keep the temps down with cross ventilation, but on those sunny 80° days it can easily top 100° inside the greenhouse.
It seems a bit strange talking about this today when we have overcast skies with temperatures in the low 50's and the thermometer in the greenhouse now reading 53°, but the past several days have been in the mid 70's and it has begun to reach 90° which can stress the seedlings causing low productivity and smaller yields.
This year I decided to add a shade cloth to the greenhouse to see if I can better control the temperature and add some protection to the seedlings vigorously growing inside.
Shade cloths come in different size panels or you can purchase custom lengths and add your own clips.
I purchased a 6' x 10' Sunblocker Knitted 70% shade panel from Grower's Supply for just over $50.00.
My greenhouse faces east to west with the side facing south receiving the most sun. I installed the panel hanging mostly to the south side and attached it to the north side of the roof so I can flip it to the back when it is shady or over to the front when it is sunny.
A few days ago it was sunny and 70° and the greenhouse started to heat up to over 90° with the vent fully opened. I quickly installed the shade cloth and within minutes the temp started to drop and came right down to 83°.
The greenhouse was still warm and humid with filtered sun still coming through the cloth. The seedlings are still growing vigorously and seem to need less water now that they have a little more protection form the heat.
We seem to be getting warmer days earlier in the season which can really do damage to the seedlings inside the greenhouse that I work hard to grow each year. Using the shade cloth seems to be an easy way to better control the temperature and I will probably be able get more use out of the greenhouse during the hotter summer days.