Iceverge

Just swap out the old rattly bathroom extractor for one of these. I have one in my office. The only thing that's wrong with them is that they're too cheap. If they cost 10 times as much people would think they were excellent.

Welcome, welcome. It's a nice bunch in here, hope we can help. You should have some proper continuous ventilation. A couple of dMEV fans would be cheap and tick the box. People often forget that these old houses had a fire going 20hrs per day pulling fresh air through the house. Trickle vents may not be the only option. You could have wall or ceiling vents.

----furiously rereads post.---------- Nope, 75m service cavity to get below 0.16W/m2K. I would prefer 220mm and 50mm service cavity as it is nail gun friendly but as we're starting with 175mm rafters we're stuck.

If you must use PIR put it outside the roof and diligently tape the OSB as an airtight layer. Far better to have all timbers inside the insulation for long term durability. etc. This is also a very easy way to make a room in roof airtight if you return a membrane to the inner wall surface through a double sandwiched wall plate.

I assume that you are thinking about PIR between the rafters. Don't! To hard to fit, too wasteful, shrinks and falls out, bad in fire, bad for noise etc etc etc.... I assume you have 175mm rafters at 600cc? Full fill, use 75mm insulated service cavity with mineral wool.

As per Daves missive above I'm not a big fan of balconies etc projecting from a wall. I'd much prefer to have it as a completely stand alone structure not in contact with the building. To this end I would. 1. Initially add horizontal support under the balcony maybe 500mm from the wall. Acro props under a plank and some cross bracing. 2. Cut the joist ends from the wall with a saber saw at an angle to avoid the joist brackets. 3. Remove the ledger plate bolted to the wall. This may be tricky as the fixings look rotten. You could get a hole saw and an accurate depth gauge to "just about" cut the thickness of the joist around the screws and then break it off the wall with a pry bar and then remove the screws with a mole grips or something. 4. Add a couple of vertical post supports and a horizontal beam under the deck to replace the wall ledger. Keep them at least 150mm from the wall. Add appropriate cross and knee bracing to stop it swaying. 5. Give the whole thing a couple of really good doses of wood preservative. Especially the joist ends nearest to the wall where they were getting wet.

Boroscope from the top of the cavity might be an option if it's not filled with insulation. I'd be slow to do too much SDS drilling if there wasn't a lintel. It may make the situation worse. Here's a random one from Amazon to give the idea.

I suppose it depends on your funds and the standard you expect from a building.

They're dear and noisy. I do like the idea though. Ideally I'd like one with no indoor unit like a Mini block A2W. Better efficient and cheaper to buy a split unit.

The TV show seems to have an uncanny ability to get the Mrs of the house knocked up with alarming regularly if that was a concern.....

Didn't know that. I just checked and it's still there for Ireland. Was there some new research/study?

Partial fill is impractical to do correctly. The beads or the mineral wool batts are good at keeping any water that gets in outside to the external leaf. Blown mineral wool is different, not encouraged with brickwork. In Ireland we have been full filling with EPS beads with 20 years and this place resembles a jetwash for 9 months of the year. The only failures I have heard of are from bad window and cavity tray detailing etc.

Step 1. Knock down old house. I'm dead serious. It'll cost less. Step 2. Ignore step 1, everyone does.. For the extension NO NO NO to PIR in the cavity. Don't bother with lightweight blocks either. Full fill with mineral wool batts or EPS beads every time and normal blocks for both leafs. SS cavity ties are fine too. Wet plaster internal for airtightness. Or timber frame. Like Redbeard says, it's lovely work. EPS for the floor is good. For the walls I would do either what you suggest or parge the walls to solve airtighness, follows by studs and mineral wool in-between. Much cheaper.

Full fill or EPS blown beads. EVERY SINGLE TIME

Could you swap out the woodfiber for dense EWI mineral wool batts and then add Intumescent fire cavity strips on the battens. This would nicely compartmentalise any fire and the mineral wool has a class A1 fire rating. What U value have you planned for the wall? Is it an on-site or factory built frame as you may need to alter the position of the OSB too.

A quick doodle. Parge the brickwork behind the mineral wool as your airtight layer. Join it to the red airtight membrane that runs between 2 wall plates to protect it and up the top of the OSB on top of the rafters. Tape the OSB as your airtight layer. Add PIR on top and mineral wool below. I would prefer something like a dense EWI style mineral wool like Rockwool Frontrock if you can source it. Add stub rafters with long screws to extend roof. This detail makes airtighess simple and extra robust for following trades to bring services up from below and route them in the rafters. I'm a big fan of having structural timber warm behind insulation too. Far less thermal movement etc. From the top 1. Roof tiles 2. Tile battens 3. Counter battens 4. Roof membrane 5. 100 mm insulation 6. 11MM OSB taped at all joints 7. 150mm rafters with mineral wool. 8. Plasterboard and Skim.

I would do full fill mineral wool batts in the cavity. Not PIR boards. It's impossible to do them right.

Why not use insulated metal panels? You could reduce the amount of ceiling timbers then too.

What SI units would you like me to quantity my reply in?

The house i grew up in winter had 2 modes. 1. Howling winds blowing through the cracks and freezing cold. 2. Stuffy as hell and damp running down the walls on calm days. Neither was a choice and neither was healthy. My dMEV airtight office and Tupperware tight house with MVHR spend much of the year with windows and doors open, kids creating havoc etc. When we do close them we still have plenty of fresh air and the icy North Atlantic gales stay outside where they belong. TLDR: 1.Airtightness is vital for comfort. 2.Mechanical ventilation is vital for fresh air. Just decide what type suits you best.

Honestly neither, unless it's needed for wheelchair access I'd put in a proper tray and a proper door. The shower stays warmers and there's no mess on the floor then.

I think my next one is going to be insulated slab too with external insulation ( problem rockwool)on timber frame with the sheathing taped as airtighess layer. I should get an exceptional envelope with that.

Yes we have it. Cost about €2500 in 2020 to DIY. Running costs are about €40/year on electricity and the same again in filters. About 2hrs labour per year I would say on average for cleaning and servicing the unit. I DIY it and my unit is very accessible, not tucked away in the corner of some attic On a new build on £££ alone won't stack up against dMEV or DCV unless you use the energy savings to omit the central heating from the budget on day 1 like we did. I would do it again for the comfort and quietness. I wouldn't recommend it to anyone who was unlikely to spend the time or money on correct maintenance. For them a few Greenwood dMEV fans and humidity sensitive wall vents would be a far better bet.

For speed an insulated metal panel kit is probably the fastest. https://www.insulatedpanelstore.co.uk/garden-rooms?srsltid=AfmBOoqRKgKk5A49VJzAw4UAI0A1Qrr_WqlDy83VP-C2g0KhavY7zK4x For cost you'll stick frame it on site yourself a lot cheaper. Liam has some tutorials here. He does a DIY metal insulted panel recently too which looks economical. https://youtube.com/@thegardenroomguru?si=F75UkGxu67eeS0py I wouldn't use PIR between timbers ever though, too fiddly to get right and lots of waste. Mineral wool is a better option in my view. What people forget to do is. 1. Add appropriate ventilation. I have a dMEV fan in my office. About £70. Same as @JohnMo. I couldn't recommend it enough. 2. Appropriately Airtight the building. If you do these things you can heat such a small room warm very quickly with a fan heater. I did a suspended timber floor in my office. Wouldn't do it again, everything rattles. @Nickfromwales did an insulated slab I believe although it was shrouded on secrecy........