Second Floor - VCL/Insulation Queries

[Jammy5]

After a couple of really bad months on a personal level and also still not having my mortgage in place my build has ground to a bit of a stand still.

 

However, I’ve managed to get free up some funds to keep something progressing, albeit at a much slower pace for a while.

 

Whilst I await windows and doors I’m looking at the roof and room in the roof insulation and it’s here I need the advice of the forum members, or critique of my understanding of what’s on by building regulation drawings and also that within the SAP calc.

 

Remember I’m on a traditional block build but I think it’s predominantly irreverent for this post.

 

I’ve created the attached sketch showing my thoughts on what I’m being asked to produce within my build and I’ve colour coded for ease of explanation on what I’ve been asked for:-

• Red – 150mm PIR between rafters.

• Cyan – 63mm Gyproc Thermaline PIR

• Magenta – 12.5mm standard plasterboard

• Pink – 100mm PIR between vertical studs.

• Green – 38mm Gyproc Thermaline PIR

• White – Knauff Crown R40 Loft Roll Cross Lapped

• Dark Blue – 100mm Minimum, Acoustic soundproof quilt.

 

The main area that’s testing my understanding is the vapour barrier or VCL which according to the drawings is only being asked for on sloping ceilings (and the dormers which I’ve left off this post for ease).

 

From some research and my limited knowledge on the situation, it’s very difficult to pin down a specific and robust answer that whilst may not be perfect, at this point in life I’m looking for a…. is it “good enough” solution.

 

Q1. It appears as though Insulated plasterboard such as Gyproc Thermaline acts as a VCL when taped and skimmed, is this correct?

 

Q2. I’m struggling to find what product this is taped with, prior to plaster skim coat, to enable it to act as a VCL.

 

Personally, I also can’t get my head around the insulated plasterboard acting as a VCL as its usually poly sheet or foil backed insulation with foil tape that creates the VCL.

 

Q3. Any penetrations through the insulated plasterboard need to be sealed. However, what does this detail look like for back boxes and MVHR outlets that are positioned in the vertical wall in this space. Do I simply allow them to penetrate and use something like this:-

https://www.ecomerchant.co.uk/instaabox.html?gad_source=1&gad_campaignid=21463137325&gbraid=0AAAAADf7paMA__cG_1DR9lzBewgHEVxEE&gclid=Cj0KCQjw9-PNBhDfARIsABHN6-1VRKNMyaBChEM-7t2A21FuIqThWTN5AM_dycJL21O7TmBHMYz_4nYaAjGzEALw_wcB

and then use airtight tape around the MVHR pipe entry prior to the plaster skim coat

 

If my understanding of Q1 is correct, as there is Insulated plasterboard on the face of the vertical studs and sloping ceilings then this will act as the VCL and this then only leaves the horizontal ceiling to find a solution.

 

 

 

For the horizontal ceiling, its asking for 500mm of cross lapped loft insulation. I would then usually just put a normal 12.5mm plasterboard under this fixed directly to the joists. However, the complication comes that I intend on using the plaster in profile lights and the lack of VCL and air tightness.

 

Q4. Is it as simple as using a polythene sheet on the underside of the joists, prior to boarding?

 

The last part is the first floor ceilings directly below the cold loft space on the far left and right of the drawing that is lapped in 500mm of loft insulation. The set up will be the same as the horizonal ceiling on the second floor, that I will also be using plaster in profile lights.
 

Q5. What is the best solution here, should I just use a polythene sheet on the ceiling of any rooms directly below this space? I’m just worried that it’s potentially not good practice to have a VCL between two warm spaces with half of the ceiling being directly below the warm room above.

 

There are similarities to another post on the forum from this time last year here:-

Correct method of vapour barrier installation - Heat Insulation - BuildHub.org.uk

But it doesn't quite answer my queries.

 

It’s probably also worth saying that the two sloping areas are a lot shorter than shown on the sketch, potentially only a few hundred mm each.

 

Like I said, at this stage, I’m not trying to be perfect, I’m not trying to achieve passive house standards, I simply need to achieve my target of and 3m3/hour/m2 at a differential pressure of 50 Pa as per my SAP calculation.

Buildhub Insulation Sketch.pdf

[Redbeard]

1 hour ago, Jammy5 said:

Q1. It appears as though Insulated plasterboard such as Gyproc Thermaline acts as a VCL when taped and skimmed, is this correct?

 

Many would say so. I say not, as the VCL is there on each sheet, but not at the joint. You go on to say

1 hour ago, Jammy5 said:

Q2. I’m struggling to find what product this is taped with, prior to plaster skim coat, to enable it to act as a VCL.

'Traditionally', nothing, which is why I prefer working with insulation only (say foil-faced PIR), taping the foil face , then battening and hten boarding. However a few people have started to use 'fluff tapes' like Contega or Pavafix Win, over which you can plaster, so that would work, although strictly that 'bit' of the VCL is not in quite the 'right' place.

 

I think you can get a 'tighter' VCL if you use bare boards and separate plasterbds. I rarely use foil tape - it's too variable. I prefer to use Pro Clima or similar air-tightness tapes.