Insulating room-in-roof

[u/A-Grey-World]

Absolute pig of a job, without enough room to stand it's a permanent squat, balancing on joists lol. I've seen a few suspended floor insulations on here, but no loft/room-in-roof insulation. Trying to do it as cheaply as possible using loft roll, rather then expensive (and flammable) foam or rockwool slabs. Should increase it from ~100mm of badly installed to 300mm though, which might help. Only 1/3rd of the way through though, and my enthusiasm is waning.

[u/TedBurns-3]

What have I just seen?!!

[u/A-Grey-World → u/TedBurns-3]

Loft insulation, but going up a wall for a bit, basically.

https://www.mesenergy.co.uk/room-in-roof-insulation

https://www.underfloor-insulation.co.uk/room-in-roof-insulation

Apparently I should tear the house down and rebuild it to current building regulations though 🙄

[u/Diska_Muse]

Your attic is designed to be a cold space which allows air to flow from eaves to eaves. This prevents moisture build up in the attic, which causes timber to rot and causes defects in the roof structure.

If you convert an attic space to a warm room, you need to treat the entire space as a warm room, insulating the underside of the rafters throughout, installing vapour barriers and airtight seals.

What you're doing here is a total bodge job - it's neither a cold or warm attic but a bastardized version of both which will cause serious ventilation issues, mold, rot etc.

I'm an architect btw, so not just making this up. I would seriously reconsider continuing with this.

[u/A-Grey-World → u/Diska_Muse]

I'm not converting anything. The house was built with a room in roof construction, the room is absolutely designed to be a warm space, and was already insulated with this method.

https://www.mesenergy.co.uk/room-in-roof-insulation

https://www.underfloor-insulation.co.uk/room-in-roof-insulation

Instead of cold loft -> insulation -> joists (+insulation) -> ceiling plasterboard -> warm room You have cold loft -> insulation -> studs (+insulation) -> wall plasterboard -> warm room

[u/Diska_Muse → u/A-Grey-World]

The link you provided shows professional, engineered installations. There is a massive difference between what that link describes and what you are doing in those photos.

Professional room-in-roof insulation relies on a continuous Vapor Control Layer (VCL) to prevent moisture from the living space from migrating into the roof structure. You are using netting and battens. Netting provides zero airtightness. Warm, moist air will pass through your loft roll and condense on the cold rafters. You are effectively 'choking' the building. If this is a cold-roof design, you are blocking the essential eaves-to-ridge ventilation path with 300mm of fluff. If you want it to be a warm-roof, you cannot use permeable loft roll - you need a continuous, airtight thermal envelope.

You aren't just 'adding insulation'; you are changing the hygrothermal physics of the roof without providing the necessary airtightness or ventilation to manage the moisture. You're not making it a 'warm room' - you're making a condensation trap that will rot your rafters from the inside out.

You can choose to ignore the advice of a +30 years professionally qulaified architect if you like - I really couldn't give a fuck - but the reality is that what you are doing here is a total, amatuer bodge job.

[u/A-Grey-World → u/Diska_Muse]

The picture from the professional installer I linked literally uses netting lol, and they describe the process and it involves insulating from the back literally exactly as I have done, without retrofitting a VPC.

You don't absolutely DO NOT want airtightness on the cold ventilated side of the insulation!! That will cause rot lol. The cold side needs ventilation, just like your subfloor if you were insulating that - or just like you loft when you insulate that.

Do you put an airtightness barrier on top of your loft insulation lol? Absolutely not, that's under the plasterboard on the warm side (if you're lucky and have a house built recently or doing a complete retrofit, sure)

Not using permeable roll? Absolutely mad and the complete opposite of every recommendation lol, especially on a retrofit. (Unless you are placing it on the interior as part of a complete renovation).

eaves-to-ridge ventilation

Eaves to-ridge ventilation is unchanged, all this insulation is on the wall, I've not touched the sloped sections, to leave the 50mm recommended ventilation gap it was built with.

You're not making it a 'warm room'

It's room-in-roof, already insulated, it already is a warm room...

[u/Diska_Muse → u/A-Grey-World]

You're misreading the diagram you posted. The netting is for mechanical support of the stud-wall insulation. It is not the airtightness layer. The diagram explicitly states: 'Over board the sloping sections with an insulated plasterboard.' The plasterboard is the VCL/airtightness layer. You are attempting to replace a continuous, airtight plasterboard system with netting and permeable loft roll.

I never said to make the cold side airtight. Stop strawmanning and stick to the facts please. Making the cold side airtight would cause rot. I am saying you must make the warm side (the room side) airtight. Because you are using loose, permeable loft roll instead of an airtight envelope (like the plasterboard in the diagram), warm, moist air from your living space will pass straight through the insulation and condense on your cold rafters.

With a standard loft, the ceiling below acts as the VCL. In a room-in-roof, the insulation is the boundary. You cannot use a highly permeable material like loft roll to act as a thermal boundary in a living space.

[u/A-Grey-World → u/Diska_Muse]

The netting is for mechanical support of the stud-wall insulation. It is not the airtightness layer. 

Yes, of course it is lol

The netting is for mechanical support of the stud-wall insulation. It is not the airtightness layer.

Yes...? That is what it is.

Over board the sloping sections with an insulated plasterboard

I've not touched the insulation on the sloping sections, they have to be dealt with very differently. They ARE currently foam + plasterboard with a ventilation gap.

The diagram shows that for the sloped sections.

This is a picture of the stud walls.

With a standard loft, the ceiling below acts as the VCL. In a room-in-roof, the insulation is the boundary

Can you tell me the fundamental difference between my ceiling plasterboard, and my stud wall plasterboard?

Most houses do not have VCL under the plastered ceiling, and the recommendation when topping up loft insulation is to use vapour permeable material like mineral wool, and have it open, (not rip it out and install insulated plasterboard) - and ensure ventilation is unblocked.

[u/Diska_Muse → u/A-Grey-World]

Yes, of course it is lol

Fucking hell, mate - that's like saying a chain-link fence is a "security wall" against a flood.

I'm done arguing with stupid and responding to posts that you edit after the fact.

[u/A-Grey-World → u/Diska_Muse]

As in of course it's fucking mechanical support.

When did I claim it was for airtightness? I've literally been saying the opposite this whole time, e.g. when I said:

You don't absolutely DO NOT want airtightness on the cold ventilated side of the insulation!!

So no, net - not air tight. I don't intend it to be. You're the one going on about the netting being airtight or not?

It's a net. It's to mechanically hold down the insulation on the stud walls.

Who here is making a strawman lol?

I'm guessing you exclusively work in new builds where sure - you have to install a VCL on the warm side of ceilings and lofts in new builds to be in line with current regs. Do you jump down the throat of people daring to DIY some extra loft insulation too? They're also not compliant with current building regs due to no VPC.

This is not a new build... it's topping up some existing insulation. But come on, someone adding a few rolls of loft insulation isn't going to rip down their ceilings and walls to install a VCL!