Q: There’s a lot of contradictory advice on the internet about caulking. In particular, some sites are quite explicit that metal/wood junctions should not be caulked. I have a door-sized window where the framing meets an aluminum sill. Should this, in fact, be caulked?
A: It is true that some junctions between metal and wood should not be caulked. Over windows, for example, there shouldn’t be caulk between the metal flashing and the window frame. Nor should there be caulk along the flashing that often separates panels stacked on exterior walls when more than one panel is needed to cover the whole height of the wall, such as on a gable end of a house or shed. In these cases, the flashing is bent into a Z shape so one leg extends up the wall, behind where it becomes visible on the exterior. A second bend allows the metal to cover the top edge of the piece below. The purpose of this flashing is to channel away any moisture that gets behind the siding above the window or the top panel. Plugging the gap between the flashing and the window trim or lower panel would trap moisture and defeat the purpose of the flashing. Handmade Hollow Mgo Clean Room Panel
But in the situation you are asking about, the sill is designed only to allow rain to flow away from the window. Around the window, behind the sill and frame, there should be a membrane that keeps water from getting into the wall. Usually a sticky-backed, rubberlike material, this membrane should have been installed starting at the bottom of the window, with a piece long enough to extend a little way up the sides. The side pieces would have gone on next, overlapping the first piece of membrane. The membrane at the top would have been last, overlapping the side pieces.
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Buildingscience.com, a site run by experts that diagnose and recommend solutions to building problems, says the top membrane should fit under the house wrap and be sealed directly to the sheathing (the paneling that covers the framing, to be hidden later with siding). Some builders stick the membrane at the top to the house wrap, rather than the sheathing, but Building Science Corp., which runs that website, says that traps any moisture that gets through the house wrap.
If your window’s membrane is installed correctly, caulking the sill to the side framing is optional. It wouldn’t hurt, but it wouldn’t be necessary because the hidden membrane would be channeling away any water that got through that gap. The water would dribble down the wall behind the siding and eventually evaporate.
Of course, at this point you probably have no way to know how the window was installed. So caulking the gap between the metal sill and the edge of the side window trim is probably a smart thing to do. It could help keep moisture on the sill from getting into the trim, or maybe the wall. And it would help seal the pores on the end of the wood trim around the window. In the picture you sent, it appears that paint has flaked off the ends of the trim boards, probably because the wood there absorbed moisture, causing the wood fibers to expand and push off the paint.
Use a clear caulk that’s labeled as paintable, such as Dap’s alex plus clear acrylic caulk with silicone ($4.28 a tube at Home Depot). Clear caulk won’t be noticeable now, and if you ever repaint, you will be able to brush on a straight edge without worrying about whether the paint covers some of the caulk.
Caulk works best when it needs to bridge only the two side edges of a gap. If the gap is deep and you try to squeeze in enough caulk to fill it, the caulk has to stretch three ways and is likely to fail prematurely. The label on Alex Plus says it lasts for 40 years — but that’s only if it’s used to fill gaps no wider or deeper than half an inch. The gap between the sill and the trim on your window is probably deeper than that. Stuff in a short piece of backer rod, such as Frost King poly foam caulk saver. (A 20-foot roll with a 0.375-inch diameter is $3.93 at Home Depot; it also comes in a half-inch diameter for wider gaps.) Given that you need such a small amount, you could probably improvise with a little piece of weather stripping or even foam packaging. The purpose is to fill the deeper part of the gap, so the caulk just has to connect the metal and the edge of the window trim. Try to apply the caulk so it fills the gap but doesn’t bulge out beyond the trim.
Should you caulk around the window trim where it meets the siding? No, it doesn’t make sense to caulk there, even though it would be a wood-to-wood connection. Nor should you caulk gaps between the shingles on your house. They should be installed in an overlapping way that solves the moisture problem without needing caulk. And don’t use caulk to fill horizontal seams between boards that are used for siding.
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