Greenhouse Foundation Question

[Becca]

Hi Everyone! It’s been a loooong time sine I have been on here! I’ve purchased a Sunglo Greenhouse and we are about to start the foundation. The instructions from Sunglo show a detail of a gravel foundation using 4x6 rot resistant lumber with the 4x6s and about 5 inches of gravel below grade, and at the bottom of that is 6mil vapor barrier. My question is what purpose does the vapor barrier serve or what is its function. It seems to me that it would prevent water from draining through but perhaps there is something I am not aware of? Any help would be greatly appreciated!TIA!

[Mike H]

Probably just to keep the dirt and gravel mixing together over time and reducing drainage.
If it's windy out there do some research on securing against wind, I've heard of folks having their greenhouse blown down in high winds.
Plan for the maximum possible weather event in your area.

[raybark]

The densest thing I've ever seen used on the ground under gravel was landscape cloth... Mike is right, I think a vapor barrier would impede drainage.

[Dorsetman]

Surely it is the opposite ? To prevent damp travelling up ? Even ‘rot resistant’ will suffer if it stands on wet ground.

[raybark]

Good thinking Geoff. I was thinking that we were discussing the entire floor, not just the foundation barrier.

Having said that, I had treated lumber benches sitting directly on a gravel floor in my greenhouse for over 25 years, and the damage to the legs was more by the sharp edges of the gravel than anything. Most "ground contact" lumber sold in the US is guaranteed for 40 years.

Becca - one more suggestion: See if you can make the foundation taller. Every increase in height increases the interior volume without appreciably affecting the critical surface area (solid wood won't transfer heat anywhere nearly as well as the greenhouse cover), making it a lot easier to stabilize the conditions within the structure.

[Mike H]

Good idea, I built a greenhouse designed to sit on an 18" base on a 36" base and now have lots of head room. Most of the foundation is below grade so the structure doesn't look disproportionate, not something everyone may want to undertake but very effective energy wise as well as space wise.

[Brutal_Dreamer]

I would love to see a photo of your new greenhouse, Becca! AND - great to see you on here again.


Cheers,
BD

[Becca]

Mike, if it would prevent damp from traveling up, wouldn't it also prevent the water used for watering the orchids to drain through it? Where would all the water go?

---------- Post Merged at 01:53 PM ----------

Good thinking Geoff. I was thinking that we were discussing the entire floor, not just the foundation barrier.

Having said that, I had treated lumber benches sitting directly on a gravel floor in my greenhouse for over 25 years, and the damage to the legs was more by the sharp edges of the gravel than anything. Most "ground contact" lumber sold in the US is guaranteed for 40 years.

Becca - one more suggestion: See if you can make the foundation taller. Every increase in height increases the interior volume without appreciably affecting the critical surface area (solid wood won't transfer heat anywhere nearly as well as the greenhouse cover), making it a lot easier to stabilize the conditions within the structure.

Hi Ray! Yes I plan on raising the greenhouse by 18", it's not much, but better than nothing! Also I decided to go with a 240v electric heater, I was told that if the minimum temp of the greenhouse in the winter would be around 60 that it shouldn't increase my electric bill by more that $30.00 or so.

[Mike H]

Becca, .what I meant was it might be there to prevent dirt mixing with the gravel.
I have no reference to say if it's effective one way or another.
I'd be interested in what the manufacturer of the greenhouse says about their reasoning, makes no sense to me really.
Definitely post some pics when completed.

[Becca]

Becca, .what I meant was it might be there to prevent dirt mixing with the gravel.
I have no reference to say if it's effective one way or another.
I'd be interested in what the manufacturer of the greenhouse says about their reasoning, makes no sense to me really.
Definitely post some pics when completed.

Mike, I'm sorry, it was Dorsetman that made the comment!

---------- Post Merged at 03:11 PM ----------

Surely it is the opposite ? To prevent damp travelling up ? Even ‘rot resistant’ will suffer if it stands on wet ground.

If it would prevent damp from traveling up, wouldn't it also prevent the water used for watering the orchids to drain through it? Where would all the water go?