Roof insulation/rafter changes on lean-to extension

[battenburg]

I have a Victorian terrace with a lean-to extension at the back of the outrigger. I imagine there are similar on thousands of houses all over the place. I have no idea when it was built and to my untrained eye it seemed fairly sturdy when I moved in – the roof is ‘formal’ with tiles and it was decorated much like the kitchen. I’ve since realised that a) its an extension to the original house and b) it is single-skinned brick, unheated, damp and without insulation.

 

Its really not a nice room to be in and I’m desperate to make improvements. In reality, knocking it down and starting again would be the best option, but my budget won’t allow for that. Instead, my goal is to make it warmer, better ventilated and somewhere that I don’t want to immediately leave. I also have an aspiration to maybe take a sabbatical at some point in the next couple of years, so I’d like something that I wouldn’t feel morally compromised in renting out!

 

The number one thing is to insulate the lean-to roof – currently it has no insulation at all. I’d also like to put a loft window in there to get a bit more light in. Multiple builders have told me there is no need for any Building Control involvement for ‘this kind of work’. However, they have also suggested that it would be worth replacing the timbers because its a small area and they will replace the membrane as well that will make it easier to have a greater depth of insulation, and I guess they could space the rafters to accommodate the roof window.

 

My reading of the Building Regulations is that this then becomes replacing a thermal element, so needs to conform to certain standards for insulation – fine, happy with this and happy to submit a notice to the council, even if the builders say it isn’t necessary. What’s worrying me is that by changing the timbers and making them bigger, plus installing a roof window, does this then become a ‘structural’ thing and would I need to be conforming to other elements of the Building Regulations in relation to the room’s structure?

 

I have no idea whether the lean-to has footings, or what depth those footings might be. There isn’t the budget to go about rebuilding/underpinning the room (or whatever one does if you add heavier timbers to a lean-to roof, if anything!). Obviously I don’t want the roof to cave in, but I’m getting so tied up in knots about this and wanting to do things by the book that I’m almost considering asking the builder to just do a like-for-like timber replacement and have less insulation so I don’t have to worry about it. None of the builders have expressed any concern or suggested I need structural calculations for installing deeper joists, so perhaps that just isn’t necessary. I'd imagine that its a very run of the mill job, especially as there are so many similar rooms on the streets around here.

 

Can someone help explain how a situation with this would work in respect to Building Control? I realise that many many people would just get a builder to do the work and wouldn’t think about any of this. I feel like wanting to do things by the book has to potential to open up such a can of worms, so much so that I would almost rather not doing anything. The BC team at my council are very difficult to get hold of for an informal chat, but I understand that for something like insulation I could just submit a notice rather than a 'full' application. Apologies if my terminology is a bit muddled.

 

Ultimately I want to make what improvements I can afford to a poorly built lean-to, and the whole process just seems overwhelming. I would really appreciate some helpful words that might make this situation a little less confusing to a layperson!

[Nickfromwales]

Internal wall insulation is an option, to improve the damp situation, but pointless unless the roof is brought up to a reasonable spec also.

 

The damp issue is caused by warm humid air inside the house hitting very cold sections of masonry or the cold plasterboard of the ceiling btw, and that is more of an issue than the ventilation. If this is not an utility, and has no sink or appliances in it giving off steam and humidity, then some humidity controlled extractor may suffice; caveat is this will suck heated air out of the house and into the lean-to, and then out to the clouds, but it is what it is.

 

The quicker and still effective way to better this ceiling can be to use a foil faced PIR insulation board over the current timber (remove the plasterboard first) and go for a min 40mm thickness there, then foil tape all the joints as your VCL (vapour control layer). Then install a 25mm x 50mm counter batten and then your new plasterboard. The extra air gap will help manage the changes in temp of the surfaces of each layer. If you arrest the airflow through this then even the small amount of insulation will still reap plenty of benefit. If you can go for 60mm or 75mm of PIR insulation then do so, just depends on headroom etc.

 

Use a surface mounted light fitting, and try to keep the newly boarded ceiling penetration free, to stop heated moist air from entering the ceiling void, and seal the hole where the cable comes through for the light(s), avoid spotlights if you can. Clever use of lighting will negate the expensive ball-ache of cutting in roof lights. Avoid IMHO.

 

The vapour control foil layer needs to be linked to the insulation of the outside walls, with suitable tapes / foam, so air from either voids and the room interior can't mix. Foam sealing the internal wall insulation is of paramount importance, to stop 'thermal tenting' where heated air gets sucked up the back of the insulation via convection, causing long term problems which will remain hidden for a good while. An expanding foam which is airtight and doersn't bridge moisture is required, so use a foam such as Illbruck FM330 to seal 360^o around the internal wall insulation boards. These can be pre-insulated plasterboards LINK which kill 2 birds with 1 stone.

 

I would keep this a million miles away from the council, and do it the best you can independently, as they'll want war & peace and the cost will shoot up obvs. There's not enough weight to be added here to concern yourself about foundations tbh, so set that aside  

 

A decent job of installing these things is paramount, as adding the insulation but not getting it taped and sealed against draughts etc accordingly will just make you worse off not better.

 

The floor can be made 'damp-proof' by applying a liquid damp proof membrane (DPM), and then some Marmox insulation boards, say 25mm thick as a thermal break, fixed down with flexible tile adhesive, and then a decent engineered wooden floor can go down; materials chosen to be kinder underfoot so heating is perceived as less of a concern.

 

For heating you can add a radiator, off the central heating ideally, or an electric oil-filled one will suffice (with timer and thermostatic control). You can also install an electric heated mat under a wooden floor, if you wanted belt and braces, but that's expensive to run tbh.

 

Making sure the back door leading out, if there is one, is replaced with a modern draughtproof door will pay huge dividends.

 

Others will chip in, just my 2 cents.