my guess is that having the north wall insulated with rock is not going to significantly increase the nighttime temperature in the greenhouse. R value is cumulative for the whole structure, and the majority will be very low.

if you want to get tricky, I would encourage you research trying a N wall and lower 3 feet of S E W wall with a high thermal mass construction.

http://en.wikipedia.org/wiki/Thermal_mass

Twin-wall poly is a very durable material.

How are you going to ventilate and summer-day-cool the greenhouse?

Thank you! Planning to do the lower 2 feet of exterior walls wood as the inside will have 2 foot high garden boxes built around most of the perimeter. Does this sound like it might help keep things warmer?

Right now researching automatic venting, fans and vents. Have an old house fan thinking of using too.
Thanks much,
Patti

I would not go overboard - so to speak - with wood, it can be problematic in a greenhouse.

jus sayin' wood is not good for thermal massing either. if you don't mind the permanence i would go CMU as much as possible.

Ok thank you, this is what I need to learn.

The cinder block foundation is done with 4x4 wall posts for 6' walls and gravel floor is in. Ready to finish framing.

So what I hear you saying is that three insulated walls would not give enough solar benefit to be worthwhile; better to use clear panels for light. Is this correct?

Appreciate your help.

Have you considered of a pvc pipe frame covered with clear greenhouse film?

Yes we did consider the hoop house style but decided we wanted a more permanent structure.
Thanks,
Patti

A good reference book about building greenhouses in our climate is
Gardening Under Cover: A Northwest Guide to Solar Greenhouses, Cold Frames, and Cloches by William Head.

It discusses siting, styles, and heat sinks. Also shading during summer if you want to use it year round. You need clear for winter use as we have such low lighting to keep things growing.

The KCLS has it if you're in King County. I scanned the greenhouse structure page, so email me if you'd like me to share it. It talks about pitch of roof, type of roof & functions.

Best wishes & post a photo for us to see.

Corrine

Thanks Corrine,
Yes, I just purchased that book; it has been a great reference.
Never used a greenhouse before so hard to imagine needing to provide shade:-)
Probably be another month or two but will be sure and post pictures...will have to show it off!!!
thanks Again