Answer: Pressure Differentials in Double Glazing Units

[sainty]

Pressure differentials in Double Glazing Units

Hi

I hope I'm in the right place. I'm also hoping that this problem will appeal to someone! I'm a joiner and I make windows. I am in a discussion on a woodworking forum about the physics of a double glazed unit and it's starting to go above my basic understanding!

The crux of the discussion is that DG units fail when condensation appears on the inside of the unit. A DG unit is made up from 2 pieces of glass separated by a spacer, to create the gap which contains a desiccant. A sealant is then applied around the unit to make it "air tight". We believe that temperature fluctuations cause pressure in the units that make the sealant "fail" and leak (tiny holes?), this then exchanges the dry air in the unit to be replaced by new moist air. This exchange continues until the desiccant becomes saturated. At this point the failure occurs and condensation appears.

Firstly, does this sound feasible? Given a 20 degree C temperature differential, what sort of pressures are exerted on the unit and what volume of air is required to equalise the pressure in the unit. Lastly, what could be done to prevent the premature failure of DG units?

Thanks in advance for any help received.


Rgds


Stuart.

 

[marcusl]

You aren't the first person to build windows that fail due to temperature swings and changing pressure (including wind). One of the most famous is Cobb's John Hancock building in Boston, which was literally a disaster when the specially glazed windows started popping off their mounts and crashing onto the sidewalks 30 stories below. That may not be your problem, but in my opinion you should buy premade DG units from a reputable supplier and install them into your frames. Your customers deserve that much.

 

[pantaz]

FYI: They aren't filled with a desiccant, they are filled with an inert gas, typically Argon.

 

[GeorgeRaetz]

20C temperature increase on a sealed gas increases pressure by 1 psi. That's 144 pounds force per square foot.
The interior is sealed air tight; some (not all) will use a high molecular weight gas (argon,krypton) fill to increase insulation.
The dessicant is used to remove moisture sealed in during manufacture. The tiniest defect allows moisture laden air to saturate it. (driving force is mostly diffusion though, not pressure)
Download the brochure from the well known manufacturer of wood framed windows and you will see they use an interior bead of rigid vinyl to insure seal integrity.(The design and manufacture of which is not trivial.)
To ensure long life:
Buy a window with a twenty year warranty.
Avoid excess room humidity.