Insulating glass units: installation, sealants and storage
An insulating glass unit lasts for decades as long as its edge seal stays dry and nothing attacks it. A good share of misted units are not a factory fault, but the result of the wrong silicone, water standing in the rebate or poor storage.
How the edge seal of a unit is built
Between the two panes is a spacer bar (aluminium or TGI) filled with molecular sieve desiccant, which absorbs any moisture trapped in the cavity.
The primary seal is butyl; we use Kömmerling GD 115. It is applied to both sides of the spacer and stops moisture getting in and gas getting out. The life of the unit depends on it.
The secondary seal is polyurethane; we use Kömmerling GD 677 NA Plus. It holds the panes together and protects the butyl. Polyurethane does not tolerate sunlight (UV), so the edge must be covered by the frame. Where the edge is exposed to the sun, for example on structural glazing and frameless glass, the secondary seal is silicone.
Silicones: never acetoxy, and not every neutral-cure is the same
You can tell acetoxy (acid-curing) silicone by its smell of vinegar. As it cures, it gives off acetic acid, which attacks the edge seal of the unit. Sanitary silicones are usually exactly this type. For sealing glass into a sash, whether PVC, timber or aluminium, use neutral-cure silicone only. Glazing guidelines expressly prohibit sealants that release acetic acid or amines as they cure.
Neutral-cure silicones are not all the same either. Cheaper ones are often “extended” with mineral oil or plasticisers. Over time, that oil migrates through the polyurethane to the butyl.
The butyl then swells, softens and starts to creep into the visible area of the glass, as in the drawing. The edge no longer seals: gas escapes, moisture gets in and the unit mists up inside. A unit like that cannot be repaired, only replaced. The damage does not show straight away, but over time.
Glazing packers or gaskets made from materials containing plasticisers can do the same, as can a joint that is too deep for the silicone to cure fully.
How to avoid it: use a silicone that the sealant manufacturer has tested for compatibility with the unit edge seal. Our secondary seal is Kömmerling GD 677 NA Plus, and Kömmerling keeps a list of silicones tested with it and with their butyl. For sealing glass into a frame, see table 2.1.
Download the list of tested silicones (Kömmerling TI 200, v3.0, PDF)
The list is Kömmerling’s own, in English. These are the sealant manufacturer’s recommendations; for any silicone not on the list, ask us before installation.
The edge must not stand in water
The edge seal of a unit can cope with humidity, but not with standing water. If water sits in the frame rebate, the seal breaks down over time and the unit mists up.
That is why the rebate has drainage (weep) holes to the outside to let the water out. Glazing packers must not block the drainage.
During installation and servicing, check that the weep holes are not blocked by dust, plaster, silicone or leaves.
Storage
Upright, never flat. The glass stands on a rack, leaning back at about 6°.
On timber or rubber supports, never directly on metal, concrete or stone. Use non-absorbent spacers between stacks.
Dry and in the shade. Outdoors, cover the glass with an opaque, breathable sheet. The bottom edge must not stand in a puddle or under drips from a roof.
If a stack gets wet, separate the units and dry them on the outside. Water left between neighbouring units in a stack leaves permanent marks on their surfaces.
In the sun and in transit
A stack of coated units left in the sun heats up unevenly and can crack, and sunlight also damages the edge seal. On site, always cover the stack or keep it in the shade.
Glass is transported upright, on two padded supports, so that the units do not touch each other or any metal. If the glass will cross mountains, with more than 500 m difference in altitude, let us know: we then make it in a special version, because the gas inside expands.
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