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.

[oliviaaa999]

17 hours ago, Nickfromwales said:

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?

Hello Nick, thank you so much for your response! We've been airtighting where possible, so a VCL airtight membrane on the subfloor taped up the wall (with plywood and finished flooring on top), as many service penetrations in the walls (sockets, pipes etc) sealed, no letterbox/external fans etc. 

 

The major refurb is mostly finished (replaster, rewire, replumb etc), we haven't been aiming for passivhaus standard as that's beyond our skill/financial level but we're definitely aiming to be as close as a regular retrofit can be! The last stage is the MVHR installation/loft work. 

 

It's semi-rigid ducting and the direction is mostly perpendicular to the joists so we wouldn't be able to go just between, it'll have to go above at some point, so we are still considering insulating to the top of the joists to get a 'flat' layer, the membrane on that layer and then the 300mm ducting and insulation above that? We'll be using loft legs to overboard for the final layer of insulation so we can use it as storage, (leaving a 50mm gap above the insulation). It definitely won't be possible to do the membrane under the joists unfortunately, but we've been recommended the airtight membrane by the MVHR designer.

 

That's a great point regarding the detailing for the cables, it's something we need to speak with the electrician about - we're not sure if the cables will be going in conduits within the insulation or if it's possible to pull them above the final overboarding layer

 

 

 

 

 

[oliviaaa999]

17 hours ago, Mr Punter said:

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.

Hello, thank you for your response! Yes, we're definitely planning on using loft legs! We're going to need 300mm insulation for the MVHR ducting and we read that we need 50mm between the insulation and boards for ventilation, so the legs should help there (we're only using the loft for storage so it should be okay r.e height). We were recommended membrane by the MVHR system designer, we've airtighted the bungalow as much as possible but the ceiling/loft hasn't been done yet

 

[MikeGrahamT21]

I also considered membrane, but decided against it, as it would have been near on impossible to install and get good enough to say its a proper membrane. Instead I opted to put rigid PUR boards between the joists with a foil membrane on the warm side, just to temper any moisture passing, and then topped this with further wool, and used loft legs. Loft is cold, as i live in a bungalow too, and can't say there has been any issues, been installed for a good few years now. 

 

 

I did cock up while taking the photos, probably as i was absolutely nackered doing it, and forgot to take a photo showing the semi rigid pipes, it went on top of the PUR, under the wool, ideally would go against plasterboard, but never going to happen when you are going across the joists, so may as well be straight. I never did do the part 1 of that, not sure i even had any photos of it! 🤣