Semi-structural foam for replacement window fitting

10-10-20

Folks, I have a bay window which is thermally ineffective and very poorly sealed around the frame and ceiling of the bay. The existing window isn't load-bearing but I still expect that there is some load being applied to the window through the concrete tile pitched wooden roof structure above. So for the purposes of this DIY job I'm going to support the roof with acro's but I'll not be installing a load-bearing window frame.

My plan is to fit 50mm insulated plasterboard to the ceiling of the bay, but by doing so I'll have to reduce the window frame height by roughly the same. I'm looking for a product which can be mounted to the joists in the bay ceiling to allow the new window be bolted through to secure it, but I don't want to use a piece of stud-work timber as I'd prefer a foam solution with better insulative and airtightness features, as well as being rigid and capable of taking any load which is applied without deforming.

I looked at COMPACFOAM from Partel (CF150 / CF200), but it's not supplied in smaller quantities and it's pricey for the small job that I'm doing. I'm looking for alternatives or suggestions how I could achieve the optimal solution here without going for a lump of timber. Rough dimensions are 45mm by 75mm by 4m long. Any ideas please?

Mellor

If the bay window is not load bearing, what is holding up the bay roof?

How wll this block look externally and how is it sealed?

10-10-20

Hi Mellor. The bay is using a standard joist and rafter construction with the gable facing outwards, except that the front-most joist runs on to the right where the end of it sits on the porch wall. The left side is floating and rests on the window frames, but the existing window frames are not structural and take whatever load that the left of the bay roof creates without issue (no sign of alignment issues or movement over the last 20 years). I'll grab a photo later if that is needed to clarify it.

Hence I'm taking the approach that while I'll have to support the roof to replace the windows, it's not bearing the full load on the window. Are you thinking otherwise?

The front-most joist is effectively the fascia (with the window frames screwed into it from below) and it's clad in white PVC fascia board. I was planning on installing a 60mm deep fascia trim over the new foam spacer, coming out from under the return on the existing lip of the fascia.

Sealing at the moment is very poor so I will be fitting airtight window tape and a membrane above the PIR plasterboard to stop air penetrating. For sealing against the new 'block', I'm thinking that if it's foam based then it's likely to have a small amount of elasticity and I'll apply an airtight sealant to both sides of the block (joist and then window-frame side), but that the joint should become airtight once the window frame is screwed into the joist and compresses against the block anyway…

What are the odds that somebody has a few offcuts of a similar product?

Our window was very similar and here's a heads up, the roof was supported with steal I-beams which came right down to the top of the window.

10-10-20

https://www.boards.ie/discussion/comment/122573178#Comment_122573178

Thanks. No steel in mine, I checked. 👍️

10-10-20

Alright, I think I have a lead on this. Compacfoam is also supplied in bar form, except the local supplier here was quoting me for a large sheet (350 EUR) even though I provided bar dimensions. Here it is in the UK, I just need to find another supplier in Ireland.

https://odoo.weare21degrees.co.uk/shop/compacfoam-74#attr=106

Option #2 is probably phonotherm, which is available in much smaller sheets.

https://www.ecologicalbuildingsystems.com/product/phonotherm-200#downloads

I'll probably go with an expanding tape then instead of a silicone sealant.

The sill is the next challenge - not sure how I'm going to neatly cut the required 45 degree ends. 😣

Lumen

I don't totally understand what you're doing (a photo with lines drawn where you think the loads are going would help) but just be sure you're not confusing compressive strength with beam loading.

Certain types of insulation can take small compression loads whilst being completely useless in tension. A beam has both compression and tension loads, which is why concrete lintels have steel reinforcement.

Or more concisely

If the bay window is not load bearing, what is holding up the bay roof?

10-10-20

Ok, I fluffed the description. 😁 I'll return with a photo.

hesker

Triotherm is a similar product and available in Ireland.

10-10-20

So, to clarify. 😀

It's a replacement of the bay windows but the intent is to decrease the height of the new window frames by 50mm and fit a suitable filler into the gap (at the red line) so that I can fit 50mm of insulated plasterboard to the ceiling (also at the red line). The filler needs to take any load generated by the roof and transfer it to the window-frame, but the existing window frame is not structural (it does not have a steel element other than the U-channels within). The outer joist in the bay's gable runs to the right of the photo and rests on the lintel over that small window in the porch.

Triotherm does also look like an option, I'll read into what options are available for a 50mm depth.

hesker

Supplier will provide custom cuts. It’s easily cut with a band saw.

Lumen

https://www.boards.ie/discussion/comment/122581106#Comment_122581106

OK, just to be clear

I'll not be installing a load-bearing window frame

You will be installing a load-bearing window frame, whether it's intended to be so or not.

I guess you can put some scales under the acros when the old window is out, that'll at least give you some data to give the engineer if the new windows fail. 😀

10-10-20

https://www.boards.ie/discussion/comment/122582015#Comment_122582015

Hmm. ok, I get you. I suppose what differs between us the definition of a load. All of the houses on the road have a similar bay except mine is the only one which has lateral support where the porch was added. Windows have been upgraded and none that I know of used acro's when doing the job. 😄 And all were fitted with standard windows with no steel supports.

So I'm betwixt and between now. I think I just need to measure-up and not over-think it.

Lumen

https://www.boards.ie/discussion/comment/122582082#Comment_122582082

Are you there isn't a simple fix for this? Have you spoken to the window supplier?

Bay poles and jacks are a thing, e.g.

These bay pole round jacks are designed for most upvc window and door profiles

https://www.primrox.co.uk/products/adjustable-round-bay-pole-jacks-for-baypole-upvc-bay-windows-shield?srsltid=AfmBOorUhcnQHWrN1AezPaQF4pg1Y5ajO8GWJcl2gh0TkTq8rZ5AP6eT

https://www.primrox.co.uk/products/aluminium-bay-pole-reinforcement-42mm?_pos=6&_sid=ba8c31383&_ss=r

10-10-20

Thanks for the pointers here Lumen. The conversation with the supplier went along the lines of the window not needing supports as the existing doesn't have any. I see the point that the roof isn't fully cantilevered, so I'm looking as options now to support it.