Loft insulation sloping sections

[jonshutt2]

Hi folks,
Next task on the old house is loft insulation.

In one section of the house there is a gap between the slates, rafters and rooms (plaster and lath) we can access from the loft (green section in attached). You can see right down into them from the loft (See photo) and so we're hoping we can just push insulation down rather than take off plaster-work. The space is 10cm (height of rafters) by 40cm (gap between rafters), about 2m deep.

Will it be ok to cut blocks/sections of insulation and push down as far as we can push? How much gap do we have to retain between the insulation and the sarking. The roof is solid wood sarking with no gaps, no membrane the slates.

Between the eves on the flat section we'll just roll out glass-wool or rockwool.

Also - does this gap in the sloping bit have a technical name?

Thanks

 

[garyo]

This is a pig of a job, but the main problem you're going to have is that your lat and plaster means that you won't be able to get the PIR insulation tight up against the lats, so the air is just going to circulate around, rendering your insulation ineffective.

Also you may not have noticed that the horizontal joist on the right of your photo means that the widest piece of PIR you'll be able to get down the slope is going to be 45mm too small. It all adds to the joy of the job.

If you do a site search for 'skeilings' or 'dormer bungalow' you'll turn up a lot of good discussion on this very topic.

 

[noseall]

Also - does this gap in the sloping bit have a technical name

Raked ceiling.

 

[jonshutt2]

Hadn't thought about that horizontal joist!

...but it has just led me to the idea of cutting 2 large rigth angle triangles, first one going down, then next one cut in a way that it can slide in and wedge against the first pushing out to tough the sides...

 

[garyo]

Yep I think the sliding wedge idea has mileage, but I've not tried it myself. It was suggested on here a few years ago so if you can track down the list maybe you can ask the chap how he got on.

 

[John D v2.0]

If you're not worked about ventilation of the timbers you could always treat it as a cavity wall, ie put in poly balls covered in glue or whatever the current preferred method is. Not sure how easy to diy, but it'd solve the access issue.

 

[garyo]

Hadn't thought about that horizontal joist!

...but it has just led me to the idea of cutting 2 large rigth angle triangles, first one going down, then next one cut in a way that it can slide in and wedge against the first pushing out to tough the sides...

Let us know how you get on Jon. I still have a few of these to do in my house and I'm particularly interested if you can get the sliding wedges to work without them buckling up and causing excessive air gaps under the boards. The technique I've used so far is:

(Assuming 100mm rafter)
- Cut board to 55mm narrower than the rafter gap. This gives 10mm wiggle room to fit past horizontal joist, and when in situ gives 27mm each side of PIR board.
- Hacksaw the ends of various exposed nails sticking out of stupid places. Swear quite a bit. Get board fitted.
- Fabricate oval wedges to brace the PIR board against the back of the plasterboard and ensure there's no movement during the next step. These are a pig to get fitted.
- Use expanding foam with a 1 metre long extension nozzle to fill down either side of PIR board to a depth of around 40 or 50mm.

Gary

 

[catlad]

Surely fitting insulated plaster board to the underside of the ceiling
saves a lot of hassle\swearing

 

[garyo]

Surely fitting insulated plaster board to the underside of the ceiling
saves a lot of hassle\swearing

Agreed but if you're living in the house with a family and dont want the dust and disruption then you do what you do.