upvc window - seal the bottom or not?

IKO

The white UPVC windows in my old house — which I bought last year — are sealed with silicone on the top and sides, but not at the bottom, where there's a noticeable gap. That gap sits about below the weep holes, right where the window meets the sill.I’m confused — why would the bottom edge be left unsealed? Is this standard, or should I seal it myself with silicone? Thank you! I'm new to DIY.

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10-10-20

Hi! There's an old justification for not sealing the bottom of a window which was valid at the time, but is less-so these days.

In the past window framers would seal a window with the objective of keeping rain from penetrating the wall and causing damp within the living space, and that was achieved by sealing the top and both sides of the frame to the reveals, and the sill was left open so that "any rain which accumulated behind the window could drain away". Rain was kept out and Mrs Buckley was happy with the lesser amount of condensation on the new windows and Mr Buckley was just happy that he didn't have to go around drying off the inner sills.

As energy prices rose and technology advanced the focus of homeowners then moved to lessening the effects of energy loss through air penetration and one of the biggest culprits for that are the doors and windows of the 80's and 90's. In the 00's it then became more common to seal under the frame, but it was typically done with a squirt of silicon along the up-stand of the sill (hidden in the back behind that bottom lip of your windows). That made a rudimentary seal but was never good enough to provide the levels of air-tightness that Mr and Mrs Buckley need today and Mrs Buckley would have complained about the draft at her neck as she sat on the sofa in front of the window. So these days the windows are typically sealed all around with both silicon and expanding foams, but with a final airtight seal applied using special airtight tape applied from the inner side.

So to answer the question - yes you can seal it if you wish - and no it's not going to impinge on the weep holes above (as these are cut into the frame itself).

But three pieces of guidance:

1. Make sure that the window is in good condition with no missing or leaking rubber elements around the glass and that it's sealed well to the wall and that the opening close correctly and form a seal on the outside (you don't want excess water getting into the frame).
2. Use masking tape around the area to get a nicer finish as paint doesn't stick to silicon afterwards.
3. Use a silicon specific to the task of window/door glazing and not a general purpose sealant.
4. Lookup the method of applying silicon on YouTube as it's difficult to get right on a first attempt.

And good luck. Keep us posted.

muffler

Sealant is applied (or should be) where the frame rests on the upstand of the window sill and is covered by the bottom slip so you won't see it.

Edit/ beat to it by 10-10-20

IKO

Thanks a million, 10-10-20 — really appreciate the detailed explanation. The windows seem to be in decent shape; I believe they were replaced a few years ago, though the house itself is about 70 years old. The bathroom window doesn’t have any weep holes on the outside, which seems unusual. It’s the type with two handle positions — where the top tilts inward in one position and opens normally in the other [The other windows are not like this type] . I've bought replacement weep hole covers for the windows that were missing some.

The house tends to be cold, so I’ll go ahead and seal them to prevent any warm air from escaping. Last winter, I could actually feel a cold draft coming through the interior window sill in one the sitting room. Cheers!