Hello,

I am new to this forum and also to house renovations. I am looking on advice for the best way to insulate some of our external walls, focusing on the northwest facing front room first. The house is non-standard construction, downstairs is brick and thermal block with a very small cavity (<50mm). The upstairs is timber frame mansard and it looks like the mansard sits on the upper floor joists. We have been advised against getting the cavity filled due to how small it is and also it is not capped at the top so any pumped insulation (beads or similar) would migrate into the cavity. We will be getting the mansard insulated with PIR insulation. The downstairs floors are concrete.

For downstairs I am considering the PIR backed plasterboard (37.5 cm thickness) just straight onto the wall, but worried whether this will cause any condensation issues. The room is quite narrow and the door frame is only 5cm from the front wall so we are quite limited on the thickness. I also looked at the Spacetherm wall-liner but it is really expensive so not sure whether it is worth it (~£800 versus £200 for the insulated plasterboard). I am also not sure whether the mansard roof insulation will overlap the upstair floor joists at all and whether we would need to do something about that space if it is left completely or partially uninsulated.

On the front room wall we also have a bow bay window with an unfilled canopy and this seems to be quite a large source of cold air too - any advice on how to insulate this from the inside and also the window sill from the outside?
Also to complete the picture, for the concrete floor, we will probably add minimum thickness solid insulation again and wood floor on top.

Ultimately due to the odd house construction I don't expect it to be perfect but hoping there are ways to make it a bit warmer without creating condensation issues. Also my DIY skills are not great so looking for the least complex solutions that would still make a difference please :).

Thanks in advance for your help.

Violet3 it is great that you are thinking along these lines, we got started in a similar way with our first renovation.

As Tony said, you would be better with thicker insulation because once you go to the trouble of fitting it, you won't want to go back later and add more.

You can use thicker insulated plasterboard and taper it thinner just adjacent to the door frame, eg 62.5 or 72.5mm give twice the insulation as 37.5mm.

You ideally want the roof insulation to join to the wall insulation. If you fill the space between the ends of the first floor joists with insulation, you can also block up drafts between the joists while you are in there, which really helps.

Bay windows are difficult, there are a few threads on GBF where people have worked on them.

Concrete floor: add as much insulation as you can, because cold or warm feet make a huge difference to perceptions of comfort.

These kinds of changes will reduce your U value by a lot, maybe an improvement of 0.5 or more. Many people on GBF sweat to shave 0.05 or 0.02 off the U value of new-builds, so what you are doing is definitely worthwhile.

Unfortunately you're going to find that there is a dearth of best practise on how to deal with early and poor cavity walls as most of the literature points to IWI solid wall or retrofit cavity wall insulation. However, here is a link to recent government best practice guidance on IWI (https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1019707/iwi-guidance.pdf). This document does suggest that IWI can and is suitable for cavity walls but it is important to understand how the hyrgothermal properties of the wall will change with the new insulation, with a particular focus on moisture.

Generally speaking the current principles suggest that if installing a moisture closed insulation system, it should be fitted with a ventilated cavity before the exterior wall. I believe this would apply also to a cavity wall because the IWI will affect the moisture and warm of the internal wall leaf, introducing greater risks of interstitial condensation.

If using a moisture open insulation system, such as woodfibre, then this can be installed directly to the wall without a ventilated cavity, but you'll need to make sure the whole buildup is moisture open, including your paints.

The best practice guide suggests moisture open systems to be of least risk.

My currently house project has some old walls with a variable cavity from between approx 30mm to 75mm and the cavity is in a terrible state so not suitable at all for any attempt to fill with insulation. Mine is therefore left as it is but I've used EWI and just ensured the cavity is well closed off to drafts and unwanted air movement.

For IWI, detailing will be key to making sure it's worth all the effort. A pretty good current guide to detailing can be found in this guide here: https://sdfoundation.org.uk/wp-content/uploads/2018/01/2015_bristolsolidwallinsulationguidance.pdf

HTH