Sit joists on wall plate or use hangers?

[flanagaj]

Can anyone tell me how you decide whether to use hangers for the joists or whether they sit on the wall plate.  My initial thought here, is that by hanging them from hangers, insulation fitted between the rafters can be done in a much more air tight manner.  This obviously depends on whether you could fit the collars in a prefabricated style using a metal plate with the collar angled into the truss rather than it being bolted to the side of the rafter.

 

Insulating a cold roof roof, with both the floor joists and collars bolted to the side of the common rafters looks a pain and must provide spots where cold air can easily come trough?

[Iceverge]

I'd have thought you would have needed a structural connection between the joists and the rafters. 

 

Otherwise your rafters will end up like jean Claude van damms legs in that truck advert. 

 

 

[joe90]

4 minutes ago, Iceverge said:

I'd have thought you would have needed a structural connection between the joists and the rafters. 

+1, insulation just needs attention to detail.

[flanagaj]

So this looks like the best solution as it means you can install a door into the void for storage and also means the complete envelope of the structure is insulated, where as the image at the bottom does not look that efficient and also means you lose the valuable storage space if you wanted to utilise it.

[Iceverge]

Ok I see where you're at. 

 

Not a simple junction re airtighness to do as drawn. 

 

My preferred way for this would be to take the airtighness layer up the inside of the wall, across the top of the wall to the outside plate and up the top of the rafters.

 

Then insulate between and above the rafters. You'll need to screw on rafter tails on top of the rafters.

 

This way you've no timbers breaching the airtighness layer which makes it far far easier to construct. 

 

 

I'll do a sketch in the next hour or two if I get a chance.

 

 

Edited March 29 by Iceverge

[joe90]

Yes I prefer the top one (it’s close to what I did) and you can use it for storage.

[Iceverge]

 

 

Heres a rough sketch, 

 

1. Fix your 100*75 wall plate as normal. 

2. Put blue membrane in place as shown. Something like a good woven external breather membrane would do best. VP400 maybe.

3. Add a 100*25 board on too to protect the membrane. 

4. Roof as shown with no rafter tails. 

5. Screw 18mm osb on top of the rafters as you aritight layer, tape all joints and tape the wall palte membrane to it to complete you AT layer.

6.Add 100mm rafter tails as shown and insulate in between them with 100 mm rigid insulation. EPS or PIR. 

7. Add a second layer of 50mm rigid insulation to reach over the tails and cover the top of the cavity.

8. Add 50mm timber to the top of the rafter tails to make level with the insulation.

9. roofing membrane.

10. counterbattens

11. tile battens 

12. tiles

 

13. Below the OSB add 150mm flexible insulation. 

 

14. Allow the cavity wall insulation to join up to the underside of the 50mm top layer of insulation too.