retrophit

Thanks @markc - something like this? I think 44x170mm C24 joists should be ok. What's the best way of attaching them to the wall plate - angle brackets? Any recommendations on bolt sizes for sistering? Presumably I need to use square washers on either side, and a dog tooth washer in between timbers?

I'm renovating my 1930s semi and have taken down the old lath and plaster ceiling in one of the rooms, as it was crumbling to pieces. I want to put plasterboard up but the ceiling joists are warped and sagging slightly, so it's not a flat surface to work with. I also want to install 270mm of insulation in the loft and board over it for storage, so I'm not convinced that the joists are strong enough to take the added weight. The existing joists measure about 40x105mm (2x4s?) and are inconsistently spaced between 350-450mm, but at around 400mm centres on average - the longest unsupported span is 3.6m. Based on these span tables for ceiling joists, if they're roughly equivalent to C16 at 400mm centres, even at light load the maximum recommended span is < 2m? I'd like to either replace the existing joists, or install some new ones alongside them so that I have a strong and level surface for a plasterboard ceiling and for loft boarding (C24 2x6 or 2x8?). I don't need them to be up to the standard needed for loft conversion floor joists, but I'd like to be able to use the loft for more than just light Christmas decorations. I'm not sure if the wooden lintel over the bay window can take much additional weight either - it's constructed from two 50x180mm timbers doubled up to make 100x180mm x 2.7m long (unsupported span is 2.25m), and is possibly already sagging slightly. I've measured everything up and put together a rough model in SketchUp (front and top views attached) - I haven't bothered modelling the ground floor, or windows other than the bay opening. Colour coding as follows: White walls are made of brick and are load bearing Yellow walls are made of cinder blocks, seemingly built after the lath and plaster ceilings were put up, so not load bearing Red joists are "short" (~3.7m) Green joists are "long" (~4.3m) Pink joists just rest on the chimney breasts, so don't seem to be helping structurally Blue is the bay window lintel Brown is the wall plate The joists are just nailed together in the middle where they overlap, and are connected to the wall plate with a single skew nail - they don't seem to be connected directly to the rafters, which possibly explains why they've warped away from them. The rafters are presumably connected to the wall plate, so indirectly supported by the joists? Would it be best to pay a structural engineer to do some calculations? Any suggestions would be much appreciated!

Thanks @PeterW! There's no external render on any part of the house - I initially looked into things like PIR and insulated plasterboard, but as @Redbeard mentioned, the BEIS guide suggests that you need a ventilated cavity for "moisture closed" construction? Yeah, if I go for MVHR then I'd like to hide all of the ducting in the ceiling/floor voids and walls. It might be possible to fit an MVHR unit in the utility room of the ground floor rear extension, but I'm not sure how I'd route the ducting to the front of the house as the floor joists run from the party wall to the side of the house. There are some big gaps in the brickwork where the floor joists enter the party wall! They will certainly need blocking up for airtightness, but also soundproofing. The ground floor seems to be an uninsulated concrete slab - also weighing up the benefits of excavating the floor to install insulation. I'd like to install wet UFH on both floors, but not sure if I can live with the disruption of screed drying times on the ground floor. Thanks @Redbeard - I'll see if I can find someone to do a WUFI calc! Thanks @Ando! I keep thinking I've settled on a solution, but then find some conflicting advice that changes my mind 🙈

Hi everyone! I recently bought a 1930s semi and after stripping off several layers of wallpaper and ripping up the carpets, I've discovered that the house needs a lot of work. Given that I'm already tearing down ceilings, pulling up floors, and removing blown plaster, I think it makes sense to do some retrofit work at the same time. The front wall of the house is solid brick (side and back walls have cavities), and in the process of reading up about internal wall insulation I've gone down a rabbit hole of learning about the risks of interstitial condensation. I think I'm going to go for a "moisture open" buildup of a lime parge coat, woodfibre insulation, lime plaster, and clay paint - not sure if I need to go as far as hygrothermal modelling (e.g. WUFI). I've also been trying to figure out the best way to improve airtightness in various areas, and the trade-off of needing MVHR if I go too far with it. I was originally considering PIV for ventilation, but the government's best practice guide on IWI warns against mixing it with PIV. Installing MVHR would be tricky as the unit would probably have to live in the cold loft, and all of the walls are either solid brick or made of what appears to be cinder blocks. I could potentially knock down some non-load-bearing walls and replace them with hollow stud walls that I can run ducting through. If I'm going to go this far though, I wonder if I ought to aim for getting EnerPHit certification eventually, and involve a certified designer from the start. I'm just trying to decide upon the best way of renovating each part of the building, with the added complication of living in the house at the same time. There's a lot of dependencies, and I want to do things in the right order to avoid having to tear things up again later! I also don't have the money to pay for it all to be done by the trades, so I'm hoping to do most of it DIY. Thanks in advance for any advice!