How to prep for MVHR installation retrofit

[oliviaaa999]

Hello! We are preparing to self-install (or contract out) a MVHR retrofit in the cold loft of our bungalow, but we have a few questions for the prep stage!

 

Quite a lot of our electrics are in the loft& attached to the current boards. These will be removed for insulating, so we aim to attach the cables to the floor joist sides instead.

 

Should the order of layers go:
1) membrane > insulation up to joist level > layer of ducting with insulation around it above joists > loft boards

 

2) insulation up to joist level > membrane across joists > layer of ducting with insulation around it > loft boards

3) ducting with insulation around within and above joists where necessary > membrane above joists and ducting ( > potentially a layer of insulation above joists and ducting so the membrane is on the warm side?) > loft boards

4) something else? We’re open to any alternative ways as we’re not sure which is correct!

 

We are also not sure if we need an intelligent membrane, VCL or just airtight? For reference, the joist depth is appx 8cm and the roof is ventilated at the eaves. The floor was sealed with VCL over the subfloor but under a plywood base, and the house has been replastered/painted (still some new cracks though!).

Thank you!
 

[Nickfromwales]

25 minutes ago, oliviaaa999 said:

Hello! We are preparing to self-install (or contract out) a MVHR retrofit in the cold loft of our bungalow, but we have a few questions for the prep stage!

 

Quite a lot of our electrics are in the loft& attached to the current boards. These will be removed for insulating, so we aim to attach the cables to the floor joist sides instead.

 

Should the order of layers go:
1) membrane > insulation up to joist level > layer of ducting with insulation around it above joists > loft boards

 

2) insulation up to joist level > membrane across joists > layer of ducting with insulation around it > loft boards

3) ducting with insulation around within and above joists where necessary > membrane above joists and ducting ( > potentially a layer of insulation above joists and ducting so the membrane is on the warm side?) > loft boards

4) something else? We’re open to any alternative ways as we’re not sure which is correct!

 

We are also not sure if we need an intelligent membrane, VCL or just airtight? For reference, the joist depth is appx 8cm and the roof is ventilated at the eaves. The floor was sealed with VCL over the subfloor but under a plywood base, and the house has been replastered/painted (still some new cracks though!).

Thank you!
 

Are you making the house airtight, throughout? Need to understand where exactly you intend to stop and start any membranes.

 

If you're going belt & braces, then the ducts should lay on the plasterboard between joists, packed with insulation, with more insulation on top open to the cold attic.

 

Any membrane should be underneath the joists, as with all the cables etc above it'll be a pig to detail. Going to need some attention to details here, and a lot of time and patience. 

 

Is this part of a major refurb?

[Mr Punter]

Have you looked at loft legs?  They could raise the height to give you more space for ducting, cables and insulation.  It will reduce height though.  I am not sure why you are proposing membrane.