Warmest place in greenhouse

[womble]

Last year sometime, I had a discussion with Snadger with respect to which was warmer, the bottom of the GH, or the upper bit.

I now have 2 identical sensors for my temp unit, so I did some tests.

In a unheated GH, no insulation. On the floor (soil, but close to slabs) it was consistently warmer than just over halfway up the GH on a staging.

One night it was -1.2 on the floor and -2.8 higher.
On the next night something like 5 on the floor and 3 higher.

Seems to be a solid result.

Of course you get more slugs on the floor

[womble]

This was all at night btw. In the daytime when the sun is on it, it's the other way round.

[Bren In Pots]

Thanks for that Womble I'll moving my seedlings on to the floor, then back up on the staging when the suns out.

[Potstubsdustbins]

Womble,

It would be interesting if you could do the same comparison but with 1 sensor on the floor and 1 in the appex of the roof. I can see the ground being warmer than midway up as it would hold the heat more. But as we all know cold air pushes hot air up and it would be good to know if this is happening.

Colin

[womble]

How often do you put seedlings in the apex of the roof?

[chris]

I just put my sensor above the doorway, wedged in between the triangle pane of glass and frame.. I didn't think until after it's probably cooler there over night.

[rana]

In a reasonably airtight greenhouse one would expect the temperature to be greater higher up because heavier cold air will flow downwards. It is probable that initially the soil (or slabs) radiate heat for a few hours after the sun has gone down thereby temporarily raising the temperature at ground level, Also if the temperature is falling throughout the night a temperature gradient would persist between the glass at the edge of the house and the interior. Further complications occur down to the insulation values of the materials in close proximity to the sensors (eg open wooden staging vs concrete) and the air flows around the sensors.

So many factors are involved that each greenhouse will respond in a different way depending on materials, geometry, volume and the temperature differential from outside to inside. It is therefore difficult to generalise as to where the warmest part of a greenhouse is.

[Two_Sheds]

It is probable that ...the soil (or slabs) radiate heat for a few hours after the sun has gone down

That's what I was thinking too.

I have my max min thermometer on the lowest level of staging (about 4" off the ground) where it stays shaded and out of the way

Oh, just remembered something! I had two tender geraniums in the gh all winter, one on staging and one on the floor. The one on the floor died, the other one is OK, looks really healthy. So I guess the floor was coldest overall

[womble]

In a reasonably airtight greenhouse one would expect the temperature to be greater higher up because heavier cold air will flow downwards. It is probable that initially the soil (or slabs) radiate heat for a few hours after the sun has gone down thereby temporarily raising the temperature at ground level, Also if the temperature is falling throughout the night a temperature gradient would persist between the glass at the edge of the house and the interior. Further complications occur down to the insulation values of the materials in close proximity to the sensors (eg open wooden staging vs concrete) and the air flows around the sensors.

So many factors are involved that each greenhouse will respond in a different way depending on materials, geometry, volume and the temperature differential from outside to inside. It is therefore difficult to generalise as to where the warmest part of a greenhouse is.

Yes it's very complicated, with so many variables. I tried to keep it as simple as possible.

The sensor on the floor isn't that close to a flag stone, maybe 8" away on a plastic pot, just off the soil. The sensor above is hanging from a wire from thin metal skeletal shelves, which I don't think make any difference to the temp of the sensor.

Both sensors are the same distance from the glass. It's a metal greenhouse with safety glass.

I did it on the lowest temp overnight, not on constant temp.

I think more work needs to be done, but I think you'll agree, that it's all very interesting.

[womble]

Oh, just remembered something! I had two tender geraniums in the gh all winter, one on staging and one on the floor. The one on the floor died, the other one is OK, looks really healthy. So I guess the floor was coldest overall

I think more long term research in different weather conditions are needed. Thanks for the info

[rana]

using a couple of sensors to record minimum temps of areas in a greenhouse could be useful when it comes to protecting plants. For a really good temperature profile however an infrared camera would be ideal. This would reveal where the cold spots are and how the temperature flows and changes over time.

[womble]

using a couple of sensors to record minimum temps of areas in a greenhouse could be useful when it comes to protecting plants. For a really good temperature profile however an infrared camera would be ideal. This would reveal where the cold spots are and how the temperature flows and changes over time.

Maybe a bit beyond a few tomatoes in the back garden though

[Potstubsdustbins]

Womble, I have a small shelf at high level (above head height) at the moment it is crammed with seedlings to ensure they get maximum light.

Rana infra red camera I happen to know a man who as got one of those, I will see if I can blag it of him for a few days. He uses it to find leaks behind false walls and in concrete floors so it will depend on his work load how long I can keep it for. Maybe HMRC could buy me one will look into it.

Colin

[rana]

..... infra red camera, I happen to know a man who as got one of those,
Colin

If you do get one I'd be most intrigued by the results

[womble]

Womble, I have a small shelf at high level (above head height) at the moment it is crammed with seedlings to ensure they get maximum light.

Rana infra red camera I happen to know a man who as got one of those, I will see if I can blag it of him for a few days. He uses it to find leaks behind false walls and in concrete floors so it will depend on his work load how long I can keep it for. Maybe HMRC could buy me one will look into it.

Colin

Her Majesties Research Company? I'd be very interested in the results as well.

Do you not get enough light low down in your GH then?

For the record. Last night was 0.4c up high and 1c below. But I did move the sensor off the floor to the 1st level of staging as I lost the signal to inside.