Vapour control, air permeable, confused confused

[Russell griffiths]

So I would like to go back to basics and get my head around what I need to achieve in my build and how I’m going to do it. 

My build consists of

beam and block floor

ICF walls

I joist roofing timbers

metal sheet roof covering

 

so what am I struggling to get my head around ?

my walls consist of 67mm of eps insulation then 150mm of concrete then 67mm of eps insulation, according to all the literature and my common sense way of thinking my wall will be very air tight.   YES,  NO. 

 

so I have a 5 sided box floor and 4 walls, what do I do with the roof/ceiling ?

 

what is the difference between airtight layer and vapour control layer

all my ceilings will be vaulted. 

 

I have seen that my icf company make an eps product for ceilings, so if this is used I would end up with a 6 sided eps lined box, all fully sealed nothing escaping out of here

 

is this what I want to achieve?

what is all the talk about intelligent membranes ? Are these all more aimed at timber frame

with I joists as a roof construction this portion of the house feels like timber frame construction, would it be right to have the inner side fully sealed and the outer breathing to allow any moisture that occurs in the joist area to breath out. 

 

God it was so much easier building draughty brick houses. 

 

Help @JSHarris  @PeterW and any other of you clever bods out there. 

Many more parts to this question, just want to get the ball rolling. 

Cheers. 

[Ed Davies]

14 hours ago, Russell griffiths said:

what is the difference between airtight layer and vapour control layer

They're two different functions which may or may not be combined in a single layer.

 

The vapour control layer goes on the warm side (inside in the UK) to prevent water vapour getting into the wall/floor/roof and condensing there which would make them go black and slimy.

 

There's really a need for two airtight layers. Firstly you need some sort of barrier on the outside (the wind-tight layer) to stop the wind blowing into the insulation and carrying heat away. It should be decent but doesn't need to be completely airtight.

 

Also, for more modern builds you need a properly airtight layer somewhere in the build-up to prevent warm air in the house being lost through draughts.

 

Also, you need some sort of layer on the outside to prevent liquid water which gets past the outer rainscreen or condenses on the back of it from getting into the structure.

 

Typically for timber frame you have a “breathable” membrane on the outside which serves to prevent liquid water getting in but allows water vapour out and also serves as a windtight layer. Then there's a layer on or near the inside which provides airtightness and the vapour barrier.

 

Intelligent membranes go on the inside and are supposed to stop water vapour from the house getting into the wall/roof while, somehow, allowing any water which does get into the wall/roof to evaporate out into the house as water vapour. I don't know any more about them. Anybody?

[Declan52]

Look at the difference in roof felts. The old tar black stuff any moisture getting that far condensed and dripped onto whatever was below. The new felt will let that moisture through. That's your breathable membrane.

 

The below link will explain the intelligent membrane better than I will.

 

http://www.greenspec.co.uk/building-design/airtightness-membranes/

[Jeremy Harris]

I can't add anything to @Ed Davies description, it sums up perfectly what's needed.  Pity some of the mass produced house builders can't get their heads around what is, in essence, a pretty straightforward thing to get right.  I'm particularly concerned by some of the SIPs builds, where I fear we're going to see a repeat of the Barratt fiasco with their first foray into timber frame and a failure to understand interstitial condensation that caused so many early (1970s/early 80s) homes to have problems.

[Russell griffiths]

Ok cool that lot has sort of confirmed what I was thinking 

lots of timber frames being built here with timber sheathed walls and ceilings with taped joins to provide the airtight layer. 

 

My build is icf so a layer of polystyrene insulation on the inside walls  all sealed up nice and tight, what is to stop me having this layer of eps extend up and under the ceiling joists all glued and taped to make a complete airtight box. 

[PeterW]

EPS won’t give you the uValue as well as PIR and it doesn’t tape well. PIR provides the vapour membrane too if it’s taped on all the joints. It’s also half the thickness for the same insulation value 

[Russell griffiths]

Sorry. This would not be the only insulation full fill insulation between I joists as well, trying to find a solution to create a warm roof and provide an air tight layer. 

 

 

[Declan52]

What about spraying the insulation in like @PeterStarck did with his.

[PeterW]

23 minutes ago, Russell griffiths said:

Sorry. This would not be the only insulation full fill insulation between I joists as well, trying to find a solution to create a warm roof and provide an air tight layer. 

 

 

 

So that is PIR - foil creates the airtight layer. 

[Russell griffiths]

11 hours ago, PeterW said:

11 hours ago, PeterW said:

 

So that is PIR - foil creates the airtight layer. 

I am really trying to avoid pir, I hate it I think it is massively overpriced, 

 

Getting back to the airtight layer at the ceiling, in a perfect world I would fit an airtight membrane then a service batten and then the finished ceiling, 

 

I think I need to add up the cost of pir versus an external insulation with a airtight membrane on the inside, I know some of these membranes can be costly so it may be that pir taped will end up cheaper. 

[Russell griffiths]

With an mbc type build up I believe the roof timbers are spread apart with a plywood gusset and then loose fill insulation is pumped in to the roof void creating a no cold bridge construction, is this correct @JSHarris

would I joist construction do the same or would the osb web transfer heat to the outside needing additional insulation over or unde the joists to prevent a cold bridge. 

[PeterW]

An airtight membrane can just be a sheet of plastic nailed to the timber - it is 100% air and vapour tight and comes on big rolls .... it doesn’t need to be complex ..!

 

Would an I Beam / cellulose pumped roof no also take out some of the noise from the roof with rain etc ..??

[Russell griffiths]

1 minute ago, PeterW said:

An airtight membrane can just be a sheet of plastic nailed to the timber - it is 100% air and vapour tight and comes on big rolls .... it doesn’t need to be complex ..!

 

Would an I Beam / cellulose pumped roof no also take out some of the noise from the roof with rain etc ..??

Why are people buying the fancy membranes if I could just use clear polythene 

[Triassic]

8 minutes ago, Russell griffiths said:

Why are people buying the fancy membranes if I could just use clear polythene 

I used clear polythene as the internal vapour control layer when I recently built our cabin. Easy to use and tape. Cheap as chips!

 

I now intend to use it on my main house build.

 

 

Edited September 21, 2018 by Triassic

[Nickfromwales]

16 minutes ago, PeterW said:

Would an I Beam / cellulose pumped roof no also take out some of the noise from the roof with rain etc ..??

110%. That would be my number one worry with metal roof and no dense insulation. That coupled with a 30mm layer of PIR foil faced and taped would make the roof a lot easier too detail too imo. Plus you really would want to clear the cold bridge, again imo. 

[Russell griffiths]

My first idea was 240mm of a soft roll type insulation installed between the I joists, with 40 mm ridgid wood fibre insulation laid over the top, then a breather membrane and a vapour control layer on the inside taped to the walls. 

Roof to be held off the wood fibre with a counter batten providing an air gap. 

[Ed Davies]

49 minutes ago, Russell griffiths said:

Why are people buying the fancy membranes if I could just use clear polythene 

 

i think it depends on how vapour open the outer layers of the build up are. If they allow the wall/roof to dry out easily then polythene on the inside is fine. If they put up a bit more resistance to water vapour then you need to be able to also dry out inwards into the house.

 

My roof build up as specified for the building warrant:

Perihelion Roof Buildup.pdf

[Jeremy Harris]

1 hour ago, Russell griffiths said:

With an mbc type build up I believe the roof timbers are spread apart with a plywood gusset and then loose fill insulation is pumped in to the roof void creating a no cold bridge construction, is this correct @JSHarris

would I joist construction do the same or would the osb web transfer heat to the outside needing additional insulation over or unde the joists to prevent a cold bridge. 

 

 

My rafters are I beams, with an 10mm gap and then and additional length of stud timber nailplated to the lower member.  The outer face of the I bean rafter is clad with 22mm OSB as sarking and to give racking stiffness, and then counter battened, a layer of breathable membrane and then battened for the slates/PV panel frames.

 

Internally, there is a layer of Isover VCL fastened to the underside of the additional stud with 50 x 50 battens.  The plasterboard is fixed to these battens.  The Isover is taped with Siga tape and run partway down the internal VCL board on the inside of the walls and then taped.  The cellulose was pumped in through holes in the membrane on the inside which were then taped over with patches and Siga tape.

 

The thermal bridging through the OSB web is negligible, and is reduced by only being a 2D thermal path, because of the insulation that's blown in tight to the sides of it.  Overall our roof U value (corrected for thermal bridging) is a fraction under 0.1 W/m².K

[Jeremy Harris]

2 hours ago, Russell griffiths said:

Why are people buying the fancy membranes if I could just use clear polythene 

 

Main issue with polythene is that it has an extremely low surface energy, which means that it's really difficult to get any adhesive to stick to it for very long.  The membranes used for VCLs have a surface that has been made from a material (or specially treated) to increase the surface energy and allow much better long term adhesion.  My worry would be that even the very best airtightness tapes may not stay adhered to non-surface treated polythene sheet for the decades that's expected for something embedded in the structure of the house.

 

All the approved VCL materials have an inner surface that is either made of a material to which tapes will bond very well, or have a surface treatment that does the same thing.  Ordinary polythene sheet won't have any form of surface treatment, so isn't likely to allow adhesive tapes to remain well-bonded for decades.  You can get around this by choosing to use mechanical sealing, perhaps, like using some form of mastic on any joints and a batten to secure them and keep the two bits of polythene under pressure, but I'm not sure how you would deal with things like door and window openings.

[Russell griffiths]

Thanks for all the response guys @JSHarris would there be any chance of a cross section sketch or a. Photo I am wondering how the 10 mm gap was created

 

also @Ed Davies any chance of a sketch or photo 

 

thanks guys. 

[Ed Davies]

18 minutes ago, Russell griffiths said:

also @Ed Davies any chance of a sketch or photo 

 

The PDF contains two images from a SketchUp sketch. There's a photo in my profile header of what exists so far in reality, not enough to help with this.

 

As @JSHarris says, the problem with polythene is sticking to it. My plan is to overlap across the I-beams, staple and tape those then rely on the mechanical fixing of the PIR to keep it sealed.

[Jeremy Harris]

Just now, Russell griffiths said:

Thanks for all the response guys @JSHarris would there be any chance of a cross section sketch or a. Photo I am wondering how the 10 mm gap was created

 

also @Ed Davies any chance of a sketch or photo 

 

thanks guys. 

 

 

Here's a cross section through our house, showing the detail:

 

 

The gap was created when the rafters were put together in the factory, by nailplating a bit of timber to the lower member of the I beam with a small gap.

[Russell griffiths]

They say a picture paints a 1000words

thanks to you both @JSHarris @Ed Davies I owe you both a beer. ???

[Triassic]

So if a modern vapour  control membrane was to be used, which one would you recommend