Retrofitting insulation in a dormer/ story and a half house

[woodstream]

Hi all, looking for some advice on how best to insulate our main bedroom and ensuite which essentially sit within in the roof space.

These rooms have always been much colder than the rest of the house and while re-doing the ensuite it seemed like a good opportunity to punch a hole in the wall to the cavity behind to see what the situation was.

There was no insulation behind the ensuite, plasterboard straight out to the roof space, behind the bedroom on the vertical walls and 45 degree ceilings there was a thin foil blanket that doesn't seem to be doing much.

There was no insulation on the floor which is essentially the ceiling of the room below. 

My thoughts were to lay 100mm of rockwool on all the floors and walls, and use 50mm on the 45 degree slopes where ventilation needs to be maintained on the underside of the rafters, does this seem like a good idea

I'm not even sure if it will be physically possible to get to the 45 degree slopes to fit insulation.

 

Void behind shower, with no insulation, the floor is the ceiling of the room below - this would be the most accessible space.

 

Most of the void is this size with a thin foil backed blanket, I could just about crawl down here,  at the top of the wall on the left it transitions to a 45 degree slope celling. 

 

 

[Sparrowhawk]

Welcome @woodstream!

 

Parts of my house are built into the roof like this. The 45 degree bits have badly fitted PIR insulation where the cold air goes both sides of it(!); we've decided the payoff from tearing down the ceilings to redo it isn't worth it, so I've focused on the vertical walls.

 

If you need to crawl in the tight space, treat yourself to the recycled plastic bottle insulation, it's a lot more pleasant to work with. I put 100mm of this in the 1m high vertical wall on one side of our bathroom, and that alone raised the temperature by 2C in winter, so it's worth doing.

 

Where you can get to the 45 degree rafters, pop mineral wool batts between them (remember it needs supporting, with netting, plasterboard, whatever - it will sag without) leaving a 50mm gap for ventilation. It can be easier to put wooden battens along the joist to enforce this gap.

 

And then make it as airtight as possible, because draughts going round your insulation will undo much of the indoor comfort that your insulation brings. If you're up for it, put an air and moisture tight membrane on; otherwise plasterboard and seal all cracks carefully.

[Redbeard]

Hello!

 

Although the first bit of insulation is always the best one (the graph of insulation efficiency climbs very steeply, and for each subsequent unit flattens out further and further) 100mm on the ceilings is pretty unexciting. The Bldg Regs would require 270mm of 'fluff' (glass-fibre or Rockwool or - as per @Sparrowhawk -  recycled plastic bottle insulation ) or a tiny bit less of sheep's wool, recycled cotton, flexi woodfibre etc. I would not recommend lass than 300mm.

 

The stud walls are going to be more tricky for you because of the tight space. My minimum recommendation would normally be full fill of the stud depth, with a wind-tight breathable membrane stapled over the studs. If I could I'd put the same membrane over the ceiling insulation too, to reduce the risk of 'windwash' - cold air getting in among the layers.

 

Are those rafters 150mm? So you could get 100mm in and still have decent ventilation between the felt and the insulation. Access would be easiest if you took the ceiling down. That could, if the pitch is steep and headroom less of an issue, allow you to add further battens and insulation under the rafters.

 

Note  the minimum suggested 270mm on the flat ceilings and possibly the 100mm+ to the stud wall may comply with current Bldg Regs. 100mm in the sloping ceilings wouldn't. Normally I advise people to make sure they do, so that on future sale they don't get would-be purchasers trying to push the price down. Others take a different viewpoint. Getting Bldg Regs approval (or not) costs. See Planning Portal for details.

 

@Sparrowhawk has covered the rest re air-tightness and thermal by-pass.