Triassic Posted December 10 Share Posted December 10 I have a couple of damp patches in a cathedral ceiling in the main bathroom. Initially I thought it was a leak in the roof membrane, but having stripped the roof slates and re-felted the area, its still getting the occasional damp spot on an oak beam at the bottom of the sloping ceiling. I’m now wondering if it’s a dew point issue, resulting in condensation within the make up of the ceiling/roof. Can anyone point me in the direction of information on how to investigate the problem further, or suggestions on how to fix it? ps. The whole house has cathedral ceilings and the problem is only associated with the main bathroom. Link to comment Share on other sites More sharing options...
JohnMo Posted December 10 Share Posted December 10 Post up you roof buildup, what materials air gaps etc Link to comment Share on other sites More sharing options...
Triassic Posted December 11 Author Share Posted December 11 Slate covering, with 50mm of battens and counter battens, 9mm OSB sarking boards, 10mm air gap, then 125mm Kingspan, air tightness membrane, 12mm air gap then 12mm plasterboard. Link to comment Share on other sites More sharing options...
SBMS Posted December 11 Share Posted December 11 (edited) 1 hour ago, Triassic said: Slate covering, with 50mm of battens and counter battens, 9mm OSB sarking boards, 10mm air gap, then 125mm Kingspan, air tightness membrane, 12mm air gap then 12mm plasterboard. Where is your roofing membrane?? Should be under your counter battens/on top of your sarking boards? Edited December 11 by SBMS Link to comment Share on other sites More sharing options...
joth Posted December 11 Share Posted December 11 1 hour ago, Triassic said: Slate covering, with 50mm of battens and counter battens, 9mm OSB sarking boards, 10mm air gap, then 125mm Kingspan, air tightness membrane, 12mm air gap then 12mm plasterboard. Are there any penetrations through the kingspan that maybe creating a thermal bridge? How is the membrane and plasterboard supported? You could get THERM modeling done to identify thermal bridge condensation risks, but that's normally pre build activity. A thermal camera maybe quicker and cheaper at this stage, especially with some nice cold days coming up Link to comment Share on other sites More sharing options...
SteamyTea Posted December 12 Share Posted December 12 Start by taking some internal and external temperature and RH readings. It may just be bathroom condensation. Link to comment Share on other sites More sharing options...
Triassic Posted December 12 Author Share Posted December 12 12 hours ago, joth said: Are there any penetrations through the kingspan that maybe creating a thermal bridge? How is the membrane and plasterboard supported? No penetrations through the Kingspan. I have a feeling its condensation as Steamy suggests. How do I cure condensation? Maybe a RH triggered Boost switch for tye MVHR system? Any ideas/ suggestions? Link to comment Share on other sites More sharing options...
SBMS Posted December 12 Share Posted December 12 Is your ‘airtightness membrane’ a VCL? I think I read the differential of the vapour resistivity between the inside (membrane) and outer (OSB sarking) needs to be around 5:1 to ensure vapour migrates out. It’s worth just checking this because if it’s at risk of condensation at least you know it’s down to makeup and not poor workmanship (design not build). Knowing doesn’t fix your problem though. I’m surprised that even with MVHR you still have a problem as that should help. I would suggest your nuclear option is to fit a dehumidifier and keep that running. You can get ones that are wall mounted and the condensate outlet can be plumbed in. Link to comment Share on other sites More sharing options...
Triassic Posted December 12 Author Share Posted December 12 The makeup is as designed and the workmanship 100%, we did most of the work ourselves during lockdown. I’m thinking that the shower is the problem, users don’t press the Boost button. Any ideas how to fit a RH Boost switch, that will boost the MVHR if the shower is used? Link to comment Share on other sites More sharing options...
Kelvin Posted December 12 Share Posted December 12 1 hour ago, Triassic said: No penetrations through the Kingspan. I have a feeling its condensation as Steamy suggests. How do I cure condensation? Maybe a RH triggered Boost switch for tye MVHR system? Any ideas/ suggestions? I had something very similar in a bathroom at our previous house. Not a cathedral ceiling but quite a high ceiling with a large step that ran the full width of the room. Moisture was getting trapped in this area and sitting on the wall upstand. The extractor was on the lower part of the ceiling. I moved the extractor to the stepped bit and this solved the problem. Link to comment Share on other sites More sharing options...
joth Posted December 12 Share Posted December 12 7 hours ago, Triassic said: No penetrations through the Kingspan. I have a feeling its condensation as Steamy suggests. How do I cure condensation? Maybe a RH triggered Boost switch for tye MVHR system? Any ideas/ suggestions? Just to clarify, I also think it is condensation. This normally forms at the coldest spot on the wall/ceiling, and a thermal bridge is most likely cause of such a cold spot - hence my question. (I wasn't meaning to suggest there's a penetration that moisture seeps in through, just any break or simply a thinner patch of insulation would be prime cause of a slightly colder spot ) Link to comment Share on other sites More sharing options...
SteamyTea Posted December 12 Share Posted December 12 (edited) Water vapour can change to liquid water for a number of reasons. Temperature, air pressure and particulates in the air. That last one is often overlooked and it had a cousin. A solid surface will have a roughness to it, this can cause water vapour to transition to liquid. Also worth noting that if you see steam, the water vapour has already condensed. Just thought of more. Temperature is a measure of mean 'free path' speed. So when a pan boils, some of the water will be below 100°C, and some above. Water vapour is the same, but a larger temperature range (up to about 374°C I seem to remember). There is also a triple point where water can be a solid, liquid and a gas. Edited December 12 by SteamyTea Link to comment Share on other sites More sharing options...
Alan Ambrose Posted December 12 Share Posted December 12 That brings back very bad memories of steam tables and 'thermo'. Link to comment Share on other sites More sharing options...
SteamyTea Posted December 12 Share Posted December 12 3 minutes ago, Alan Ambrose said: thermo The royalty of all science. Link to comment Share on other sites More sharing options...
MikeSharp01 Posted December 12 Share Posted December 12 2 hours ago, Alan Ambrose said: That brings back very bad memories of steam tables and 'thermo'. Yeh - me too, and I build steam locomotives - don't know why, although I am currently working on a flash steam plant for a car - right pain, so I guess I will have to get back into thermo to get it working reliably - how much energy have I got in 3 litres of steam at 600PSI contained in 20m of steel tube at an average of 400degC I wonder. Link to comment Share on other sites More sharing options...
JohnMo Posted December 12 Share Posted December 12 41 minutes ago, MikeSharp01 said: Yeh - me too, and I build steam locomotives - don't know why, although I am currently working on a flash steam plant for a car - right pain, so I guess I will have to get back into thermo to get it working reliably - how much energy have I got in 3 litres of steam at 600PSI contained in 20m of steel tube at an average of 400degC I wonder. More than enough to kill you and few others, if you have the hoop stress calculation wrong. Hope you did hydrotest before allow steam in the system. Link to comment Share on other sites More sharing options...
MikeSharp01 Posted December 12 Share Posted December 12 1 hour ago, JohnMo said: More than enough to kill you and few others, if you have the hoop stress calculation wrong. Hope you did hydrotest before allow steam in the system. Yes - we did a hydro test to 1200psi, the steam generator is not my build I am just doing the controls which is proving to be a big pain. Link to comment Share on other sites More sharing options...
Alan Ambrose Posted December 13 Share Posted December 13 >>> flash steam plant for a car There's a guy in Norfolk that my dad used to know who had a steam car with amazing accelaration. I believe he built it himself. I did see it once briefly, but don't remember much about it. Link to comment Share on other sites More sharing options...
SteamyTea Posted December 13 Share Posted December 13 (edited) 19 hours ago, MikeSharp01 said: did a hydro test to 1200psi When I was a lad and did my apprenticeship at a company that made material testing machines, many if which were hydraulic, we used to pulse the pressure up and down in quick succession. This was part of the rapid aging of components (odd to test the strength of a machine that is made to test strength). Static over pressure testing is a very crude test, and you have to take temperature extremes into account, and vibration. Vibration can cause havoc with drawn steel pipes. Quick thought on the calculations. There are two elements, the stored energy due to the pressure/volume curve plus the energy from the change in temperature. You can probable treat the PV as a constant flow i.e. mass times velocity, and just find the mean dT and use that as the starting point for the real thermodynamics heat capacity of the steam. And stand well back during testing. Edited December 13 by SteamyTea Link to comment Share on other sites More sharing options...
saveasteading Posted December 13 Share Posted December 13 47 minutes ago, SteamyTea said: you have to take temperature extremes into account, and vibration. Vibration can cause havoc On which general subject of materials science, may I recommend this week's "Infinite Monkey Cage" on Radio 4, on the subject of elasticity, ductility etc. Some proper nerdy information from top experts, and entertaining with it. Link to comment Share on other sites More sharing options...
SteamyTea Posted December 13 Share Posted December 13 30 minutes ago, saveasteading said: Infinite Monkey Cage Things can only get better. One of my favourite programmes. Link to comment Share on other sites More sharing options...
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