External Window Cill (Sill?!) poorly done, what COULD happen? Fascia not high enough behind trim?

hummus_chips

Hello all, first post but I intend to contribute more from now on as we have some experiences to share now.

Recent square extension, usual thing, out the back of house. Made the big mistake (baby brain) of not getting a mid and end of build inspection, and it's been pointed out to us that the window cill is not sticking out enough (and it really isn't, the groove behind the drip lip is not exposed at all). The cill sticks out at most 20mm in places.

It's a cavity block wall, and it's faced off inside with 100mm insulated plasterboard. As I understand it, this is already generally not great as the cavity block always remains 'cold' due to not having a warm face from internal environment, and so any moisture penetration will have a tough time 'evaporating' - I have a great article I will post later on this topic. Nevertheless, does anyone have any first hand knowledge or experience of what exactly can go wrong here, if moisture creeps back inwards along the cill over time? I should say that disappointingly, there is also no DPC on the cill.. I'm imagining damp accumulating over time, and eventually a mold problem between bare block work and PIR spreading out possibly the full run of the wall.

From the back you can see the lack of DPC and also the fact that the sill is brought it too much, I have included a pic from the HomeBond manual of how a sill should sit (albeit as a wall plate). The length of the lintel also looks like it's cutting it fine..

2nd Q is that the Fascia in my view doesn't rise up high enough behind the lip of the GRP raised edge trim, to cater for driving rain. A lot of internet pics show it rising right up behind, and I've heard it should be at least 100m up behind lowest point of down trim.

In the above pic that's as bad as it gets, due to the run of the roof (the slope) the lip gradually comes lower and lower down over the fascia so by the end of that run, you go from about 10mm rise behind (left) to about 60mm rise behind (at the far right). I know the Fascia was installed AFTER the GRP went on.

Any feedback and pointers to some online material/manufacturers specs/best practices would be great, but otherwise your opinions would be just as useful!! This stuff doesn't seem covered by building regs sadly.. that I can see. There may be ISO standards but it's 170 quid for a copy of it!!

merci!

10-10-20

Hi and welcome.

From the view of a non-professional, but somebody who has done some building over time, that cill is a slight concern to me in three ways:

1. It's completely bridging the cavity of the cavity blocks and faces off internally on the same plane as the cavity blocks.
2. It's missing the wrap-around DPC and insulation at that point
3. It's missing the drip-channel.

From my reading of the construction the cill is now very effective thermal bridge at the rear side, it's also that it's a conduit for damp (as concrete has vapour permeability) and can also allow rain to track backwards from underneath to either appear just under the cill or within the blockwork under it. Additionally as it's closing off the cavity of the blocks under it, there isn't an escape route for the entrapped moisture other than to condense on the coldest surface, which in this case will be the underside of the cill!

All of these issues combined with any internal insulation - especially foil-backed insulation - mean that the rear side of the cill will be constantly damp (and probably dripping) due to the difference in temperature and the availability of trapped moisture. That moisture will propagate through any gaps around the window/windowframe, plasterboard joints and possibly even pool at floor level (there was a somewhat similar case of that here in ~2020, linked below) and you'll end up with staining on the finished painted wall, mold in and around that area and things like wood swelling and deteriorating around that point.

In relation to the fascia - I'm not too sure how much of an issue that is - others might have stronger opinions on that, but I'd have to ask whether they installed a warm roof or cold roof design, do you know? A warm roof would be king here, except the devil is in the finishing detail of a warm roof as they require some more work to get right than a classic cold roof. Maybe if you had photos of that we can also have a look.

https://www.boards.ie/discussion/2058278398/wet-patches-in-new-build

Hope it helps.

Calahonda52

OP, please post a full frontal pic of the window to see does the cill go as shown on the inside and highlighted by 10-1-20

re the flashing, what are we seeing above white facia inside the flashing, this should not be visible

bfclancy2

yeah unfortunately cill is all wrong, too long, in too far, not wrapped in dpc, wasn't a block layer that built it anyway going by the images

sydthebeat

they didnt actually install a sill thats about 650mmm too long, did they really? jesus h christ

10-10-20

https://www.boards.ie/discussion/comment/122680529#Comment_122680529

Too long on the cill and too short on the lintel.

bfclancy2

yeah and the fecking window is in now and reveals finished, pain in the butt to fix now but i can't see any other fix only take it out and start again

DublinDilbert

Get one of the insulated over cills they use when doing external insulation and fit it over the existing cill.

10-10-20

https://www.boards.ie/discussion/comment/122680792#Comment_122680792

Other than the fact that putting insulation at that location will serve no benefit, it's not a bad call. I'd embed it into the reveals so that it's not a half-job.

https://passivesills.com/over-sill/

Calahonda52

https://www.boards.ie/discussion/comment/122680777#Comment_122680777

..

dathi

the roof doesn't appear to be strapped to the blockwork either

hummus_chips

Hi everyone, sorry for slow response, it's my first post and I wasn't expecting such a big reaction, so grateful for everyone's input. Will follow up with more pics, info and address ppls Qs over the weekend, thanks!

hummus_chips

Hello everybody, and thanks again for all the comments/answers. I'll address them one by one, and also I should preface by saying we have had a good relationship with the builder up to this point.

@10-10-20 I have posted some pictures of the external, and I can confirm that it is supposed to be a warm build roof. this morning I pulled down the down lighters and I discovered PIR board in there, between the joists. All blown to bits of course during the downloader's second fix. i could not believe the strength of the draft coming through when I took a downlight out, which all of a sudden made me realize that there are vents on the front and sides, on the soffits. I can only assume the PIR board was roughly put between the joists, i.e. not at all of to a standard That would make sense in terms of airtightness and thermal efficiency. I had a poke through the sofit vents and with some I was able to poke a stick all the way through into what presumably is a cavity between the PIR between the joists and the OSB sitting on top of the joists. Essentially, We have open cavities rather than closed cavities under the warm roof.

In summary there's about a 100mm warm roof Sitting on ventilated cavity with 100 mm PIR Very loosely placed between the joists Just on top of the ceiling plasterboard. From my research online this is a complete nonsense setup. In particular this seems like terrible news for moisture making its way into the cavity and condensing on the cold spots. This also draws the dew point of the actual warm roof construction much closer to the bottom layer OSB board which is terrible news.

Now back to the sill, attached some pictures. It looks symmetrical from the outside but that is because I had asked the builder to trim off that extra long bit on the left which from the inside you can see just how overly long the sill is. Those of you with a sharp eye may notice that the windows not actually centered, it's off by between 150-170mm. Everything you're saying in terms of moisture running back along the seal and meeting the foil backed board makes sense to me it's pretty much in line with what I would have read or figured out for myself.

To start over would be a huge job I feel, as it's been pointed out the flipping sill is so damn long the effort and risk to the structural Integrity of the wall (and the short lintel) is real. you could not ask for a worst setup to rectify it feels like.

Interestingly the sill is more away from the wall to the right side, than left.

@DublinDilbert I've been thinking this the whole time, it's gonna have to be an aluminum sill chased into the walls front and sides. you can already see the moisture sitting under the windowsill and against the wall which I can only assume shows there's some capillary action happening there already.

@dathi Really useful observation there about the ties to block work, I'm not familiar if those wall ties should have been mounted to the internal or external side of the cavity wall. I've looked at other pics I have, no ties. Unfortunately I have no pictures of whether they are mounted On the outside, I do feel like giving the builder the benefit of the doubt that the joists which are notched have some sort of fixing to the block work that perhaps we can't see in pictures. I will absolutely be asking about this.

@Calahonda52 Thanks for your input regarding the fascia board, I agree, the builder didn't seem to understand the concept of driving rain and how the fashion needs to rise up behind The drip lip. also I am realizing that that's also likely contributing to the draft circling link through the layer between the warm roof and the PIR between joists.

If anyone wants a laugh, On the builder's quotes it stated that the extension is to be built to homebond standard.. not even close!

If anyone has any perspectives On remedial work, I'd really appreciate your view, the main thing to fix for me immediately would be the window sill, I have been trying to research how to chase those into the walls but doesn't seem to be much information available online For a retrofit aluminum windowsill.

I suppose the next thing is to figure out how the hell this bloody warm roof was built, Including whether a vapor barrier was used or not (even if the PIR is closed cell), And what is going on with the how the overhang is sealed off. When I had a poke around the soffit vents, I could see that there is some loft insulation wedged between some of the joists but others seem to be missing it entirely. As I say, unbelievable breeze blowing in through the Hole in the ceiling where I pulled the downlight out.

Thanks everyone, this is so helpful

hummus_chips

also this vid and channel have been so helpful to me https://www.youtube.com/watch?v=RthYYgzhMWU&t=356s

10-10-20

https://www.boards.ie/discussion/comment/122689949#Comment_122689949

Steve is the man for explaining warm roofs. Sorry to say, I'd be very concerned about your setup there. Plus if that extension is a kitchen then the risk of condensation is even greater.

10-10-20

also I am realizing that that's also likely contributing to the draft circling link through the layer between the warm roof and the PIR between joists.

I suppose the next thing is to figure out how the hell this bloody warm roof was built, Including whether a vapor barrier was used or not (even if the PIR is closed cell), And what is going on with the how the overhang is sealed off. When I had a poke around the soffit vents, I could see that there is some loft insulation wedged between some of the joists but others seem to be missing it entirely. As I say, unbelievable breeze blowing in through the Hole in the ceiling where I pulled the downlight out.

That looks like a standard cold-roof there. With a cold roof the concept is that you could effectively blow a gale through the gap between the roof sheeting/OSB and the top of the insulation and not have to worry about it as the vapour barrier and insulation should take care of the moisture and heat, so that space can be as open as you like - except you can't allow any punctures in the membrane, especially for 7cm dia downlights. That's a big no-no.

https://www.youtube.com/watch?v=LL2-8RXSYKc&t

If retaining the roof as a cold roof… Remediation there would/could be that the ceiling is pulled down, retain the existing 100mm insulation between the rafters & repair it, create voids for the electrical cable and downlights, then sheet 20mm insulation board over the whole ceiling (cut out voids for downlights), apply vapour barrier over the whole lot making sure to create little cut-outs for each downlight, adhere the VB well to the wall, then affix plasterboard… skim and cut-out downlighter holes… 🙄

hummus_chips

Yeah makes sense I've learned alot this weekend about cold/warm and I've learnt alot in the past about PIR between joists all the aspects to consider and fitting etc.. a saving grace with switching to a cold roof would be that the joists are all relatively new and straight so you would get a nice snug Fit with the PIR, possibly use gapotape..

hummus_chips

@10-10-20 a concern that remains by making the extension cold roof is the warm roof sitting atop it. As I understand it, having that bottom OSB exposed to cold vented drafts will bring the dew point right down towards it. Long term that's risking moisture travelling down to that bottom OSB and dampening it. Damp OSB on the joists, plus the swelling..

10-10-20

https://www.boards.ie/discussion/comment/122691755#Comment_122691755

Not sure I follow - are you still thinking that there is insulation above the OSB to create a 'warm roof'? I'd hope not, that would be a very unusual decision for the builder to make and make me question his sanity. Best investigate that asap.

hummus_chips

yeah that's exactly what's happpend. 100mm GRP warm roof. Open sofit vents with some basic attempt at 'tucking in' the roof as friend Steve would say, which is absolutely not working at all. And to top it off, 100mm PIR between joists above ceiling level, which has been drilled for down lights, MVHR duct vent, and most boards have been damaged in the process I would reckon, based on what I can see when I pop out downlights. Amazing knowledge gap on the builders behalf. So an unintentional 'hybrid roof'..

https://youtu.be/Lc2D5U4DJew?si=Du5LIrQekKOnclV0&t=447

10-10-20

https://www.boards.ie/discussion/comment/122692771#Comment_122692771

Oh no! What a mess.

hummus_chips

Thanks everyone and in particular @10-10-20, will update here on conversations with Builder as that's just an important and I'm sure folks would be keen to learn the builder's perspective and so on, and what remedial work we do.