Confused by condensation

[BayPointArchitect]

Given situation:
A sunroom had old brittle plastic roof panels that had been walked on and cracked. The edge of the panels were buried deep under the adjacent roof structure and could not be removed without extensive demo work. So a portion of the existing panels were left in place and new panels overlap/underlap the existing.

Questions:
(1) Why is condensation forming only on the underside of the new plastic roof panels and not the existing panels?
(2) Any suggestions on how to solve this problem?

As always, thanks in advance.

ICC Certified Plan Reviewer
NFPA Certified Fire Plan Examiner

 

[cda]

Thicker layer?? Two layers of panel.

Add some air into the room??

 

[classicT]

It probably is condensing b/w the two layers.

And is that snow on top of the panels in the first two pictures? If so, the snow will chill that plastic down pretty quick, so if there is warm air with any moisture, it is going to condense pretty quickly on the cold surface.

 

[cda]

What is. S N O W?

 

[Robert]

The old panels are under a roof overhang = less sun exposure = cooler temps which are probably closer to the temp inside the space (uninsulated). Therefore less differential in temp = less condensation.

 

[BayPointArchitect]

The original panels are slightly thicker but not by much. And maybe a different material. The previous layer of roof materials leaked water through the brittle cracks around the screw heads. But no problem with condensation. With all other things being equal (24 degrees outside every winter and 35+ degrees inside the sunroom) it baffles me. There is something different between the first / original translucent panel and the second / replacement translucent panel.

Regardless of what the significant difference is, I agree that the solution involves a second layer. Just like you don’t see frost on a double pane window right next to the frosty single pane window.

Thanks everyone!

ICC Certified Plan Reviewer
NFPA Certified Fire Plan Examiner

 

[tmurray]

Condensation always occurs on the coldest available surface: the condensing point of interest. So, the question as to why condensation is on the new panels and not the old is simple: the new panels are colder than the old ones.

The real question is why is it colder? Different thicknesses in material, build up of dirt, and differences of material can all effect the thermal resistance between the outdoor and indoor space.

Put a couple ice cubes in a glass of water on a warm day and you will note that the condensation will first appear where the ice is. The water in the glass is not all the same temperature. It's close, but it is just a little bit cooler around the ice. Eventually, given the proper conditions, the whole glass gets covered with condensation because there is no more surface area for condensation to form in the condensing area of interest (this creates a new condensing area of interest).

The same is true for this structure. The condensation first will appear on the new panels, then the old panels, then the structure supporting the panels if the air is still saturated.

To control the condensation you have to make it so the air is no longer saturated. You can reduce the amount of water in the air using mechanical dehumidification, prevent it from getting there in the first place (control water ingress, remove plants, source contaminant removal where water is used), or increase the temperature of the surfaces of the building either through increasing the actual temperature in the space or replacing the panels with thicker panels with a better insulation value.