Insulate bay window

Rek88

i have a 1930s house that has cavity wall insulation. The bay window doesn’t as it’s a solid wall and is noticeably colder than the rest of the house, obviously the glass will be lowering the temp too. The render is blown on the bay and we are getting small damp patches. So I have to re-render and whist at it I am thinking of adding external wall insulation. Any advice on what I need and how to install it. Insulating from the inside would be more hassle as would include moving plumbing. The wall isn’t curved but nice and straight so hoping this makes it an easier job. Any tips.

FreeBear

Rek88 said: Any advice on what I need and how to install it. Insulating from the inside would be more hassle as would include moving plumbing. The wall isn’t curved but nice and straight so hoping this makes it an easier job. Any tips.

Tips - Do not block any air bricks. Don't bridge the DPC. Depending on how much insulation you fix to the wall, the window cills may need to be replaced or extended to give you 30-50mm of overhang. You can get uPVC extensions designed for use with EWI systems.

Celotex/Kingspan type insulation boards will give you a higher performance than polystyrene boards. Although, on a solid brick wall, you do need to be careful not to trap damp withing the wall. To avoid that, woodfibre or cork skimmed with a lime render may be more appropriate. The downside with those products is you need a thicker layer to achieve the same u-value.

Bendy_House

Hi Rek.

I wouldn't discount the internal approach - I think you might find it cheaper, less hassle and more effective overall.

Your external render has blown, so that clearly needs redoing. Absolutely consider adding ext insulation while you are doing this, but check that the added thickness won't cause hassle - altho' just extended the cill should be straight-forward, if that's all it takes. Once the ext-insul has been added, then what? Mesh and render? Weatherboard cladding? If you can work that out, and whether it could also add to the kerb appeal, then that might well make 'ext' the way to go. But, if not, then I think I'd just be looking at a quick re-render and then focus the insulation on the inside instead.

(Does the whole front need re-rendering, or just the bay?)

For internal insulation, I presume you mean you'd need to move a radiator outwards? If so, that's now't in the scheme of things, and you might even want to consider replacing it with a larger output equivalent if you can, so you can heat that room to the same level, but with a slightly lowered water temp = energy savings.

For the inside, remove rad, take off skirtings, remove any blown plaster, if you are concerned at all about any future penetrating damp - tho' you shouldn't be once the outside is rendered - then give it a coat of tanking slurry, and then D&D insulated p'board to the thickness you can happily get away with; even 1" will be transformative. 2" would be awesome. More than 2" would be 'silly'.

Fit it snug to the floor, and run a bead of foam or frame sealant under it to block any draughts coming through, assuming you have a suspended timber floor? Either crank the rad pipes, or lift a 'board and reposition them correctly. Fit new skirting.

Entirely your call.

But unless the ext insul would be simple, unobtrusive, and/or be part of an aesthetic revamp, then I know which I'd choose. 

FreeBear

Bendy_House said: For the inside, remove rad, take off skirtings, remove any blown plaster, if you are concerned at all about any future penetrating damp - tho' you shouldn't be once the outside is rendered - then give it a coat of tanking slurry, and then D&D insulated p'board to the thickness you can happily get away with; even 1" will be transformative. 2" would be awesome. More than 2" would be 'silly'.

For building regs compliance (which should be regarded as a minimum standard), 50mm of PIR/PUR is not going to be enough. Crunching the numbers through an online calculator - https://www.changeplan.co.uk/u_value_calculator.php - and making some assumptions (15mm plaster/PB, 200mm brick, & 15mm external render), 50mm will get you a u-value of 0.38W/m²K. Going up to 65mm will get closer to the target of 0.3W/m²K, 75mm will get you 0.275W/m²K.

Use the warm batten method to secure the insulation boards to the wall, and a thickness of 75mm works nicely with 25mm thick sheets. Offcuts can be used to add a single layer under the internal window cills, and if the windows are uPVC replacements, the increase in height will be barely noticeable.

Bendy_House

I know I know I know... But that's just 'silly'.

If Rek fits the thinnest TLB to that inside, it'll be as well insulated as the rest of their house. If Rek goes for 2", it'll be far superior to the rest of their house.

For the sake of that 'bay', just how far do you go? If you go to current regs, then that's silly imo.

Unless Rek can fit that without compromise on internal space, in which case fill yer boots.

I have a 1930's build too, and the cosiest room in the house is the single-skinned attached garage which now has 1" of Jablite and 1" of TLB on the inside.

Rek88

Lots to think about, thank you. The bay is the only part of the house that is rendered so aesthetically nothing will change hence my preference for external insulation.

My hesitance to insulate internally is due to the radiator but also the original bent skirting is still there, and there’s no way that will go back on or be replaced easily. 

My thinking is that surely adding anything is better than what is there currently. At some point, prob not for a few years, the whole room will need to be replastered so can revisit at that point. 

i have put up thermal blinds and thermal curtains which have already made a huge difference and were an easy option. So again I am looking for easy options as I will be doing the work myself.