I don't like rigid insulation between joists/rafters/studs for many reasons. I plan on putting a lambda 32 'rafter roll' between the joists on my suspended timber floor when I lift the boards. 

 

Rather than using lapped breather membrane with taped edges, any reason why I couldn't put a graphite EPS board (50mm?) on underside, as it's open cell and therefore breathable?

 

This assumes that I;

Maintained a 150mm air gap underneath

Adequately sealed and jointed with foam

Fixed them securely

Could actually get the bleeders in place

 

I've tried to find actual breathability data and struggled. I would assume that thicker the EPS the less functionally breathable it would be. 

 

Would be interested to see what others thought

It's not really breathable. There will be a condensation risk on the warm side of the sheets. You'd want a vapour control layer. 

Yeah, makes sense. I'd be reluctant about a VCL under floorboards as they need glueing. Renovation screed over the top with a liquid DPM perhaps? Bitumen paint to underside of floorboards?

 

 

 

 

 

 

 

 

I don't think you would need to tape and lap any breather membrane.  It is only there to support the insulation.

 

Any VCL (probably not required) could go on top of the floorboards.

You could use tar paper like a lot of flooring contractors use, approx 1mm thick, paper tar paper on a roll

19 hours ago, WGL said:

don't like rigid insulation between joists/rafters/studs for many reasons

 

Preach. 

 

6 hours ago, Conor said:

There will be a condensation risk on the warm side of the sheets. You'd want a vapour control layer. 

 

Poor airtighess causes almost all condensation issues with air leaks actively transporting damp air to colder surfaces. Almost none happens through diffusion through permeable materials. 

 

If you can assure a excellent airtighess layer somewhere then I wouldn't worry about condensation. 

 

@WGL how exactly are you planning on sticking the EPS to the underside of the joists? 

3 hours ago, Iceverge said:

 

Preach. 

 

 

Poor airtighess causes almost all condensation issues with air leaks actively transporting damp air to colder surfaces. Almost none happens through diffusion through permeable materials. 

 

If you can assure a excellent airtighess layer somewhere then I wouldn't worry about condensation. 

 

@WGL how exactly are you planning on sticking the EPS to the underside of the joists? 

Adhesive foam to the underside of joist and between the sheets. Insulation retaining washers screwed from underneath as well. The last rows will be fun, but I'd work something out. Perhaps wire attached to the washer, fed through the board and tied off against joist. I would add some netting so that the weight of the fibreglass isn't borne on the EPS. 

 

Doing it with the EPS and rafter roll would give me a decent u value, without having to cut mounds of thick celotex. With the best will in the world, it's very hard to get right and a horrible job. I swore a decade ago I'd avoid cutting it if at all possible. 

 

 

I think self leveller with an airtight foam around the perimeter will do a good job of airtightness now I think about it