Shims and Low-Expansion Foam Around a New Window

The foam is supposed to seal the gap. Often it just bows the frame and jams the sash. The breakdown sitting on austin window installation greentopia fest lists the shims and the foam in order because a new window in Austin, TX only works when both are done in the right sequence. Shims set the frame plumb, level and square inside the rough opening, and the foam fills what is left without pushing anything out of position. On a window installation that goes wrong, the cause is usually sitting in that gap.

Setting the frame with shims

The window goes into the opening on a level bed, with the sill resting on solid shims at each end and under every mullion. Pairs of tapered shims are tapped in from opposite sides so the pair makes a flat, even wedge. Next come shims at the top corners and at the jamb locations beside each fastener. The frame is checked with a four foot level on both jambs and the head, then diagonals are measured corner to corner. A double hung sash that drifts when tested is telling you the frame is out of square.

Why low-expansion foam and nothing else

Standard expanding foam grows hard enough to pinch a vinyl window or a thin aluminum window frame inward. The sash then drags on the track, or a slider will not close on one side. Low-expansion foam made for windows and doors cures soft and stays put. It goes in lightly, about a third of the gap deep, with the can tilted so it fills from the bottom. A casement is the least forgiving, since a pinched frame shows up as a sash that will not latch.

What older frames do to the plan

Houses on Austin clay and limestone soils move in dry spells, and the Balcones Fault runs through the region as a reminder that the ground here is not uniform. Openings in older homes are often out of plumb by more than the new window can hide. The answer is a thicker stack of shims, tied to the framing, not a heavy hand with foam. Where the sill dropped, a full stack under the new sill, carried to the framing below, replaces what the old wall lost.

Checking the work before trim goes on

Once the foam cures, the sash is cycled several times. A single hung or double hung unit should move with one hand and stay where it is left. Casement and awning sashes should close with an even reveal around the whole edge. Cured foam that pushed past the frame is trimmed flush with a knife, never torn out. Insulated glass with a warm edge spacer is checked for pressure marks, because a bowed frame will squeeze the unit and start a seal failure years later.