MortarThePoint

Thanks, that looks like an option. It has a driver too, but I can probably bulge the vapour barrier slightly where that goes.

Is that creating a 50mm void under the vapour barrier and above the plasterboard then? I'm using Resilient Bars. So I have stuck the polythene to the underside of the truss bottom chord and then screwed the Resilient bar underneath. The resilient bar is 16mm high so there is a 16mm void.

I'm keen on having downlights in the bathrooms like the JCC V50 shown below. I am installing a polythene vapour barrier in the ceiling as there is insulation and loft above and a bathroom is obviously one of the most humid spaces. How to I deal with the downlight as that will make a pretty big hole in my vapour barrier?

I guess, depends how well the mortar sticks to the tray. The weakness is in tension or shear I suppose. It's how it's typically done though.

I don't know what prices you are seeing, but foil backed plasterboard seems to cost about 60% more than standard so adds something like £1.75/m2. The common alternative is to put up polythene sheeting vapour barrier (green 500ga, not cheap dust sheet) which is around £0.40/m2.It's extra work to do the vapour barrier poly sheeting, but at leas you can see what you've got before adding the plasterboard as you can never inspect the back of the plasterboard once it's up. I don't know how often the foil is damaged when you receive the sheet as I have had plasterboard with torn paper. That's interesting. Can you recommend any brands? I suppose one tricky bit is how you get continuity with any poly vapour barrier you might be using. I guess one option is to overlap by a good 300mm or something.

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.

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

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.

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.

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.

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.

Good thinking, I think my SAP assessor may have done that already so worth asking him. 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: Add passive vent(s) (i.e. no fan) that equalises the pressure Ensure excess dMEV and use fabric to provide additional inflow (could this draw in VOCs/particulates from the building fabric voids) Ensure excess PIV and use fabric to provide additional outflow (could this result in moist air getting where it shouldn't)

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

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

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

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.

Here's an option that uses the existing noggins at the base of the dwarf walls. Before putting the ceiling up I can apply the lower VCL. Before putting the floor down above the joist, I can apply the upper VCL, lapping the joint. This creates a much larger volume outside the airtightness though. Not a problem if the joist is filled with insulation there.

I wonder about using expanding foam tape under the noggin, so between the noggin and the plasterboard. That wouldn't sort circuit the resilient channel so much [Note the minimum R=1.2m2K/W can be achieved by 53mm of 0.044W/mK Loft roll, so 150mm is plenty to meet that).

I agree no point in having isolation valves as long as it's not a regulations requirement like it appears to be in the US.

I'd love to use compression fittings for all sorts, particularly isolation valves as the Hep2o ones look rubbish and are expensive (£7 Vs 70p), but not at the cost of introducing a possible issue. I guess there is little point in being able to isolate a basin or shower inside a room if I can isolate the whole room at the main manifold.

I wondered about using UFH pipe for mains water but: am not sure its WRAS (think some of Wunda's old stuff may have been) 16mm so wouldn't fit the fittings probably a nuisance to 'cable' Is heating the pipe and forming it sanctioned by the manufacturer?

I'm content to avoid anything that someone with more experience than me is concerned by.

Has anyone tried using UFH pipe formers with Hep2O? Annoyingly I can't find anywhere that says what the bend radius of these formers is, but they are massively cheaper that the cold bend formers that nobody stocks for Hep2O. I've copied the closest thing I can get to useable data below. I guess the 14-18mm one probably has a bend radius of around 125 - 25/2 = 112mm. Not in spec for the 15mm pipe's minimum bend radius of 120mm. The 20-22mm one is probably around 140 - 30/2 = 125mm but would probably be a bit loose/risky.

If I make the holes in the OSB/plasterboard 19mm and have the clip holding the Hep2O pipe about 150mm up from the holes it should leave a couple of mm wriggle room in each direction which will be nice.

I think this all works out as first fix being as the image below. It's pretty tight to drill the screw holes for the wall mount fixing kit.