Do I need VCL, Vapour Barrier or nothing

[MortarThePoint]

I know this is really important for timber frame construction and something that looks to be evolving. I've read an article that draws a distinction between VCL and vapour barrier and suggests that understanding has moved on and that the layer shouldn't be completely impervious, like sheet plastic is, and calling into question foil backed plasterboard.

 

These are what my Architect has called for at ceilings:

   

I had assumed it was everywhere, but I'm now unclear if the polythene vapour barrier in the first paragraph is only for wet areas.

 

That's me sat up there looking confused, though sadly I don't have a comfy chair yet or a floor to put it on.

 

Skimmed plasterboard is good enough for airtightness as I understand it. I know skimmed plasterboard won't be completely moisture proof, but there won't be significant quantities of moisture passing through will there and there won't be a temperature difference across the plasterboard? Consequently, why would a vapour barrier be needed at ceiling level in a normal room?

 

Without a vapour barrier, I suppose the humidity under the plasterboard is going to be higher than above it, presuming the air above has a consistent moisture content per m3 but obviously varying temperature as you move through the insulation. That would create a moisture gradient in the plasterboard which would serve to draw moisture through it, but it wouldn't increase the moisture level, it would lower it. Is there a concern of the moisture transport degrading the plasterboard by migrating water soluble 'goodness'. I have heard of that happening with masonry wall mortar when people have ventilated subfloor voids in the US.

 

[JohnMo]

That's some pretty thin insulation especially at the blue line.

 

Plasterboard skimmed will give you reasonable airtightness, but only if no sockets or lights, these will leak like a seize.

 

It is not moisture tight, so water vapour will pass straight through.

 

If it was my house

1.  VCL the whole roof fully tape all joints.  If the walls are timber, same applies.

2. Install service Battons (counter directions to roof joists), 45 x 50mm, once services are installed insulate with 50mm mineral wool. This will help reduce the repeat thermal bridges, caused by the rafters.

3. Normal plasterboard in dry areas, wet areas with moisture resistant.

4. The wall that is blue, double the insulation thickness.

 

Consider more insulation everywhere, you only insulate once, heating costs all the time.

[nod]

I’ve been installing and REMOVING vapor barriers for the last 40 years 

I wouldn’t instal polythene on my own build But I do install both breathable and poletdhene on a weekly basis 

I would prefer to use a foil backed board or no vapor barrier 

Polethene  does sweat and I’ve stripped out 100 m2 over the years 

Mostly with poor or no airflow

Opinion amongst designers seems to be split 50 50 

[MortarThePoint]

1 hour ago, JohnMo said:

That's some pretty thin insulation especially at the blue line.

....

My plan has been to do pretty much as you suggest except 75mm under rafters.

 

1 hour ago, nod said:

I’ve been installing and REMOVING vapor barriers for the last 40 years 

I wouldn’t instal polythene on my own build But I do install both breathable and poletdhene on a weekly basis 

I would prefer to use a foil backed board or no vapor barrier 

Polethene  does sweat and I’ve stripped out 100 m2 over the years 

Mostly with poor or no airflow

Opinion amongst designers seems to be split 50 50 

Interesting. Polythene sweating must mean it's cold no? Does that mean there is some for of draft cooling it?

 

I've already bought my plasterboard (SoundBloc F) and it's not foil backed. Would you omit polythene all round then (orange blue pink green)?

[nod]

Yes I would 

 

[Radian]

The cold flat roof vent is scribbled out - why?

[markocosic]

There's no reason to install polythene (air and vapour barrier) unless you're needing to stop vapour getting into an assembly that absolutely isn't able to dry itself. Use a breathable air barrier as your airtightness layer. Plasterboard is pretty worthless thanks to all the holes through it. Easier to stick up a breathable "tent" that's airtight and line that with plasterboard for structural integrity so to speak.

 

 

We had a local plasterer come to quote for dry lining and either taping or skimming our seasonal cabin build over here. "Are you going to heat the building all year round?" was the first question he asked on the phone; followed by "So you will want to use moisture rated plasterboard everywhere?" as a confirmation type question and was horrified at "No" as an answer.

 

When he arrived on site and saw OSB as the (vapour open) air barrier instead of the usual polythene it was a case of "Oh thank christ for that" or words to that effect. "You're not trapping the moisture." [between the (vapour retarding) paint and the (vapour closed) polythene in a building where there isn't always a strong drying force (from reducing winter humidity by heating it all year round) to suck that moisture back through the paint]

 

 

If there weren't howling gales through most buildings I think we'd start to see fun as people air conditioned UK builds and moisture backed up against the polythene on the structural side or tiles / wallpapers / vinyl type paints on the interior side of the plasterboard...

 

[MortarThePoint]

15 hours ago, Radian said:

The cold flat roof vent is scribbled out - why?

There is a continuous length of vent along each edge of the flat roof instead.

[MortarThePoint]

15 hours ago, markocosic said:

There's no reason to install polythene (air and vapour barrier) unless you're needing to stop vapour getting into an assembly that absolutely isn't able to dry itself. Use a breathable air barrier as your airtightness layer. Plasterboard is pretty worthless thanks to all the holes through it. Easier to stick up a breathable "tent" that's airtight and line that with plasterboard for structural integrity so to speak.

With the exception of back boxes and light fittings, plasterboard and skim should be a continuous air tightness barrier shouldn't it?

[JohnMo]

In theory, but in practice the holes you make in it, means it's not airtight, plus you would need to seal skirting level.

 

Instead of guessing do if I need this or need that, get an interstitial condensation assessment.  Either download a free trial copy or get your house designer to do it.

 

Are you going for an airtight build? 

Have you looked at ventilation?

 

 

 

[Radian]

50 minutes ago, MortarThePoint said:

There is a continuous length of vent along each edge of the flat roof instead.

I would pay extra attention to that flat roof area and its ventilation. Really would have wanted it to be done as a warm roof with ply deck on firings or use tapered insulation.

[markocosic]

It's a waste of time attempting to seal backboxes etc. Put the airtight layer behind them.

 

The amount of moisture carried through a small air hole in a backbox etc into the roof will vastly exceed the moisture going through the plasterboard itself.

 

Pressure will be highest in the attic too.

[MortarThePoint]

2 hours ago, JohnMo said:

In theory, but in practice the holes you make in it, means it's not airtight, plus you would need to seal skirting level.

 

Instead of guessing do if I need this or need that, get an interstitial condensation assessment.  Either download a free trial copy or get your house designer to do it.

Good thinking, I think my SAP assessor may have done that already so worth asking him.

 

2 hours ago, JohnMo said:

Are you going for an airtight build? 

Have you looked at ventilation?

Reasonably air tight. This plan is roughly as follows:

• Make as air tight as practical
• PIV system for fresh air in
• dMEV for stale air out

The PIV and dMEV will never be perfectly matched so I need to decide which to do deliberatly:

1. Add passive vent(s) (i.e. no fan) that equalises the pressure
2. Ensure excess dMEV and use fabric to provide additional inflow (could this draw in VOCs/particulates from the building fabric voids)
3. Ensure excess PIV and use fabric to provide additional outflow (could this result in moist air getting where it shouldn't)

[MortarThePoint]

2 hours ago, Radian said:

I would pay extra attention to that flat roof area and its ventilation. Really would have wanted it to be done as a warm roof with ply deck on firings or use tapered insulation.

 

There is a bit of an air gap that is hard to see in that section drawing

 

The planned make up is:

• 15mm WallBoard type plasterboard
• Sheet plastic VCL (<0.3mm thick)   ???
• 100mm OmniFit Slab 35 (R=2.85) against VCL/plasterboard and between 47mm wide timbers at 600c/c
• 150mm Loft Roll 44 (R=5.00) perpendicular to timbers so complete layer
• 75mm OmniFit Slab 35 (R=2.10) between 47mm timbers at 600c/c
• 101mm air gap
• 25mm Plywood (fitted)
• Sika Trocal roof membrane (fitted)

I'm content with the U-value (0.115W/m2K), but need to be sure about moisture.

 

There are firings on top of the flat top chord to create slope. I had asked chippie to cut notches in them, to allow perpendicular ventilation as well as in parallel to the truss, but he didn't and I was in survival mode at the time. I have wondered about adding a length of perforated pipe all the way along perpendicular to the trusses to blow dry air down if need be. Easy to add the pipe now and a Godsend if needed.

[JohnMo]

Not much of a fan for the ventilation you are producing, you will end with the worst of all worlds.

 

2x power required, for extract and supply, but no heat recovery to offset the ventilation losses and electric use.

 

I would ditch the PIV.

 

Then three options, vent fans with heat recovery, such as these (link below) you will need one at least in each room, wet and dry rooms - dMVHR.

 

https://www.ventilationland.co.uk/product/40954/decentralised-mvhr-unit-o100-mm-with-switch.html?utm_source=googleshoppingUK&utm_campaign=googleshopping-FeedUK&gclid=Cj0KCQjwzqSWBhDPARIsAK38LY9sv8y5tPeyEPH6lJdO42VQvR7AagTCv5jhXEym6Mpwy0W1M5whOLAaAv7EEALw_wcB

 

Full MVHR.

 

dMEV, but condition based ventilation, so passively controlled vents inwards that response to humidity and the same for fan extract side .  This way ventilation will be limited to what is needed.  This will limit ventilation losses, and only require power to vent fans in wet rooms.  You need to get good cross flow for this to be effective.

[MortarThePoint]

On 10/07/2022 at 13:11, nod said:

Polethene  does sweat and I’ve stripped out 100 m2 over the years 

When you see the sweat is it on the interior (plasterboard) side or the insulation side? If it's on the interior side, doesn't that suggest the insulation isn't working well as the polythene should be at the same temperature as the interior of the house. That said, I can imagine situations where the humidity in the house interior is so high that the when the internal temperature of the house drops condensation forms in a trapped space between the polythene and plasterboard. Do you think that's what is happening?

 

In the summer, the temperature outside can be higher than inside the house (e.g. today). That could mean condensation forms on the outside of the polythene. The roof space may be hotter still, but there won't magically be more water in that air, unless it has been drawn out of the fabric of the building. That could then condense on the polythene that is at the cooler interior temperature.

[MortarThePoint]

Based on research below, I suspect:

• Condensation below polythene would be due to moist air pockets in a dropping interior temperature (interior RH more than 80% and temperature drop of more than 4C) or very poor insulation/ cold bridges
• Condensation above polythene would be due to fabric of the building drying out or leaks (airtightness or water)

Obviously a holiday home type scenario could well have the very high humidity and cold interior temperatures that would be a real concern.

 

Does that match your experience @nod

------

I'm not a weather expert by any stretch, but as I understand it the dew point is the temperature of a surface at which condensation will form at the current air temperature and humidity (and pressure). Cambridge University publishes weather data (with some easily detected errors) back to 1995 [1]. I analysed it and 99% of the time the dew point is below 17.3C. Only 0.18% of the time was it above 20C when the exterior temperature was below 25C and that appears to be in the early evening following a high 20s or above day when the roof temperature would be high.

 

A reasonable approximate for dew point is Td = T - ((100 - RH) / 5) [2]. That means at 80% humidity, the polythene needs to be 4C lower in temperature than the air.

[JohnMo]

You will get a small temperature drop across the plasterboard, but not much.  So the polythene will be at room temperature all the time, except in the case of poor insulation, a high degree of wind wash and thermal bridges, then it could be closer to outside temp.  But shouldn't happen in a well designed house.

 

With a decent ventilation system your internal humidity should be above 60% RH (dMEV or MVHR).  In winter with building standards ventilation flow rates (with heat recovery) you may struggle to get much above 30 to 40% RH in really cold weather.

 

As said do an interstitial condensation calculation, you can mess with lots of different materials, different temperature and different ventilation rates to see the effect.   What I found interesting when you start to control max humidity at no more than 65%, it doesn't really matter what the internal temperature is you don't get condensation.  If you don't control humidity, you start getting condensation at 18 deg, which could be the temperature of your bedroom.

 

I used on from really useful software, which is free to use in demo mode.

[nod]

It does 

Bathrooms are a real no no 

A lot of the problems Particularly in the 80s was that it was put in without properly understanding what they where trying to achieve 

Ive striped PB back after what appeared to be a leaking roof Only to find the wool inside was soaked due to condensation 

Our Architecht has put it on the drawing for ours 

 

[MortarThePoint]

1 hour ago, JohnMo said:

You will get a small temperature drop across the plasterboard, but not much.  So the polythene will be at room temperature all the time, except in the case of poor insulation, a high degree of wind wash and thermal bridges, then it could be closer to outside temp.  But shouldn't happen in a well designed house.

 

With a decent ventilation system your internal humidity should be above 60% RH (dMEV or MVHR).  In winter with building standards ventilation flow rates (with heat recovery) you may struggle to get much above 30 to 40% RH in really cold weather.

 

As said do an interstitial condensation calculation, you can mess with lots of different materials, different temperature and different ventilation rates to see the effect.   What I found interesting when you start to control max humidity at no more than 65%, it doesn't really matter what the internal temperature is you don't get condensation.  If you don't control humidity, you start getting condensation at 18 deg, which could be the temperature of your bedroom.

 

I used on from really useful software, which is free to use in demo mode.

 

Hi, I presume that was DewPoint 3.0. I've just downloaded a copy and tried it out. For one of my makeups (effectively pitched roof with insulation at ceiling and for vapour resistance of the VCL of 250(standard) or2500), it seems to say "No surface condensation is likely" and "No interstitial condensation is predicted" as long as I keep the internal temperature 18C or above and the RH 79% or lower:

 

Insulation between rafters needs >=18C and <=78% RH:

 

The flat roof @18C and 78% RH is a problem if the air space between the insulation and plywood is only slightly ventilated, fine is "well ventilated". RH has to go down to 45% to not have interstitial condensation risk. I have used "asphalt" as the outer layer as seemed semi suitable as a replacement for the Sika Trocal I've actually got:

 

As @nod and others suggest, removing the VCL solves the interstitial condensation problem with the slightly ventilated case.

 

Edited July 12, 2022 by MortarThePoint

[MortarThePoint]

Need to be careful with DrewPoint 3.0 as it can lose some of the data fields and make erroneous calcs.

[MortarThePoint]

How to identify the difference between well ventilated and slightly ventilated becomes pretty critical:

 

BS 5250:2011+A1:2016 page 38

 

Air vent is 10mm equivalent height so 10,000mm2/m. The width of the flat roof is 3m and there is one vent at each side so that works out as 2*10,000mm2/m / 3m = 6,700mm2/m2.

If I consider just the eave vents, the distance round from eave to eave is about 4.5m + 3m + 4.5m = 12m so that works out as 2*10,000mm2 / 12m = 1,670mm2/m2 still OK (just).

I think the roofers actually used 25mm vents so better by a factor of 2.5.

 

Should be OK to consider as "well ventilated" then.

Edited July 12, 2022 by MortarThePoint

[ADLIan]

You need the relevant BS for the definition of well and slightly ventilated. Try BS 5250 and/or BS EN 6946. In a cold flat roof the airspace should be well ventilated - this affects both the U-value calculation and the condensation analysis.