Sealing IGUs to Trim
Insulated Glass Units (IGUs) were replaced in a Zone 5a true multi-pane bow window. It used to have putty sealing the glass of the old IGUs to the old wood frame so water couldn’t get between the glass and wood. Now it has trim strips in the 90 angle between the glass and the old wood frame. The installer installed no seal between the glass and the strips (see attachment). For durability or thermal performance should there be a seal between the glass and the trim strips so water doesn’t get between them, sit, freeze/thaw, etc.???
Also, many of the joints at the 90s where the trim strips meet each other are sealed with paint and others aren’t sealed at all. For durability or thermal performance should these be sealed against the elements so water doesn’t get between them, sit, freeze/thaw etc.??? And does paint count as a sealant for these small cracks???
Thank you (in my head all exterior joints and junctions should be sealed, and I would tend to think not just with paint, but maybe I’m not being realistic),
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
The IGUs should be sealed to the trim with glazing tape. If there is nothing there, the window frame will rot in no time. Quick check is with a garden hose, if water makes it into the house you have issues.
Trim at the corners should be sealed, typically the corners would have a big glob of silicone and the IGU is pushed into it. Anything that oozes out is trimmed off.
Technically, there should also be weep holes on the bottom so any water that makes it under the IGU can drain out. Lot of older windows, this is not there and they mostly hold up provided not much water makes it through.
Akos,
Thanks for the response. The quick check was a storm after the contractor supposedly finished the rehab - water came in.
In case I have not communicated the window properly I've included a schematic (attached) that is similar to each of the 25 IGUs in this frame (overview of window attached). The pressure cap in the schematic would be the trim (except there's no pressure - it's just presumably caulked in the 90 between the glass and the structural wood frame). I suppose if water is getting in then some of the caulk "heel beads" weren't complete seals. But right now I'm thinking the better the outside is sealed the less bad heel beads matter...
So your saying to seal trim to glass (so no water gets in the crack between), span from trim to glass with glazing tape, then paint it? Should gaps between the trim and the structural frame be spanned with glazing tape and painted over too so water doesn't get into the crack and sit (in the schematic the "sash profile" is the structural frame around each of the 20 IGUs)?
It sounds like the trim corners should be sealed too. Given the trim is already in, setting it in enough caulk that it oozes to the surface and leaves no crack is not an option. Squeeze caulk into the cracks at the 90s until flush with surface so no crack remains?
Thanks (I'm thinking water lingering in cracks and freeze/thaw would damage the trim and damage the outside of the frame even if the inside heel bead were perfect and let no water into the interior - let me know if it's actually OK to have water in these exterior spaces),
Take a look at FIG1 here:
https://basc.pnnl.gov/resource-guides/window-sash-retrofit
This is also a good info for proper IGU install:
https://www.labcwarranty.co.uk/technical-blog/timber-windows-water-ingress-around-glazing
That 5mm gap between the IGU and framing they talk about is important, if the IGU edge seal sits in water it will fail in short time.
You have a lot of individual lites there, it will be a big job to do it properly.
My perspective is there is the frame, and there is the trim[stops]
The IGU should be sealed to the frame. There should be rubber blocks under the IGU to keep it from sitting in a puddle.
THe IGU to the trim?
I guess it can be sealed, but it doesn't do much
The trim keeps the IGU from falling out of the frame when a tornado comes by
this is how I approached my IGUs in frames that are part of my house. The trim[stops if you will] are nailed to the frame. I sealed the top of the bottom stop against the glass so water running down the surface of the glass tends to run out rather than behind the stop, but there will be water behind the stops, so the stop is not sealed to the frame so that the water can get out.
When we replaced an IGU we broke in an Anderson casement, it was sealed to the frame as we had done, and the vinyl stops push up against the IGU and give the appearance of being sealed, but are not really.
I will also note this is the opposite of classic single pane glass, where the putty seals the outside and the glass is up against the wood without seal.
Akos and gusfhb,
Quick note, I'm reading the articles you linked to and working up my next response. But in the mean time I think we've had a miscommunication...
The window has already been rehabbed - it was rehabbed last Fall. You can see in my first post's pic there is no weep line at the bottom. I doubt if there are rubber blocks under the IGUs as that would shim them up but I can see the top of the IGU's interior seals in a few cases from inside (meaning I assume they're sitting on the muntins). Given all this, I doubt there are 5 mm gaps at the sides.
So this is triage now to try to seal over where it should have been sealed but wasn't, and maybe seal more (for instance, seeing there is no weep line or 5 mm gap at the sides of each IGU perhaps the stop/trim should be sealed to both to the muntins and the glass all the way around to try to simply block water from entering).
The Timber Windows article (link above) states "water... becomes trapped... enters frame joints..." (causes freeze thaw damage etc.)=why I ask about cracks and spaces...
GusFHB noted the weep can be subtle. But the pics show my contractor painted it. So we approach this assuming water will not be able to leave under the bottom stop.
The "Window-Sash Retrofit" site (link above) shows the figure attached here as "Best Practices vs This Install" & I've labeled what is/isn't in this install. Notably, the figure shows a sealant bead from stop to glass so there's no crack open to the outside. It's unclear if this suggests a bead around all four sides or just the bottom. "Timber Windows" says "The joint between... the glazing and the [stop, GB]... receives a capping of... sealant. The lowest [stop, GB] must allow drainage of water." Because they singled out the lowest stop for drainage but did not single any side out for the glazing/stop joint being capped, I believe they meant cap all four sides.
You can see directly from the interior edge of my stops to the IGU's warm edge spacer, meaning no sealant cap (see "No Sealant Cap," attached). So, would it be best to seal with a cap bead from stop to glass on ALL FOUR SIDES of each IGU seeing 1) this is often recommended and gusfhb says sealing the top three sides can't hurt and 2) perhaps it's important to prevent as much water as possible given there is no weep to allow it out??? OR, would it be better to cap only the bottom edge and leave the others open as a poor man's weep to let water evaporate from???
The reason there isn't much discussion about the sides and top is that not much water can make it in. Nothing wrong with sealing all sides up properly but you only really need drainage on the bottom. Typically the top also has air intake weeps but I think that is more important for well sealed assemblies.
Globbing on silicone will help in the short term, don't know how well it will hold up. If your IGUs are sitting on the wood frame and you do get water buildup there, the wood will rot and the IGUs will fail aventually. Really depends on how long you want this to last.
The proper solution is to pull the trim, space the IGU off the bottom, router/drill wheel holes on the bottom, install the glazing tape, sealed the corners and reassemble. I would pull all the trim but the bottom is what needs fixing. Note that the glazing tape has some thickness so unless you shave the stops, they will not sit flush with the rest if you are only installing the glazing tape on the bottom.
Not a fun project.
Akos, thanks for sticking with me. I researched IGU coatings and spacers and specified the IGUs at the time of the rehab but assumed the contractor new how to seal/drain the window, so this seal/drain discussion is all new to me.
Most best practice diagrams seem to have the outside sill sloped so water will run down it. A weep slot (or holes) following this slope makes sense in that gravity will draw water down, like in the figures from the UK web site you provided (attached as Glazing Pocket Draining in the UK).
BUT, does the flat, non-sloped weep in the national lab link you provided make sense (attached as Glazing Pocket Drainage, PNNL)? Doesn't that seem like exactly what we try to avoid - a crevice for water to get in, sit in contact with wood, freeze/thaw, and not drain because it is flat?
I'm asking because the bottoms of my muntins' glazing pockets are flat, not sloped - for all I know some slope backwards. Of course sloped is better, but if flat = much better than nothing then your proper solution still stands (though I probably can't afford it).
The devil is in the details. All that effort to get more insulated space and then you still have that tiny piece of glazing tape at the bottom acting at a heat short circuiting it all. I guess it's a design compromise.