Insulating roof space

Qyburn

Does anyone have any facts about the permeability of plaster ceilings? That's really the missing bit of info. If they're not significantly permeable there's no risk, but if they are then there needs to be a vapour barrier if any insulation is added.

My feeling is that they're not, probably not even when lime washed, and even less likely painted with emulsion. But that's not based on and study that I've seen, just really guesswork and supported because I've not seen a guide recommending a vapour barrier under vapour barrier under loft insulation.

Reed_Richards

You also need to consider the light fittings in your ceiling.  Many of my ceilings have LED downlighters and those look to me to be extremely permeable.  Indeed any overhead light fitting requires a hole in the plasterboard.  

mmmmikey

Qyburn said:

Does anyone have any facts about the permeability of plaster ceilings? That's really the missing bit of info. If they're not significantly permeable there's no risk, but if they are then there needs to be a vapour barrier if any insulation is added.

My feeling is that they're not, probably not even when lime washed, and even less likely painted with emulsion. But that's not based on and study that I've seen, just really guesswork and supported because I've not seen a guide recommending a vapour barrier under vapour barrier under loft insulation.

If a pipe bursts in the loft you get a wet patch on the ceiling then a drip as the water soaks through - and/or it runs to the nearest light fitting and tickles through that. Water vapour in humid air gets into the loft via the same route but in reverse. If you're interested and Google "interstitial condensation" you'll see there's lots of information about the problems associated with condensation between different layers of materials in building construction. This was a hot issue in the late 80's and I was working for Wimpey Homes at the time which is where my (probably patchy and maybe out of date ) knowledge comes from. It was a particular issue at the time in relation to the walls of timber frame houses, but the principles are the same.

Grandad2b

Reed_Richards said:

You also need to consider the light fittings in your ceiling.  Many of my ceilings have LED downlighters and those look to me to be extremely permeable.  Indeed any overhead light fitting requires a hole in the plasterboard.  

In the bathroom!!! 20x 15mm diameter decking lights and 5x intumescent downlights. All LED. Been like that for over a decade but we don't spend a lot of time in the loft. I've not seen any evidence of fruiting bodies or mould but I've not been specifically looking.

EcoScruples

Insulate between the joists, put some 2x4 running in the opposite direction to the joists and fill with more insulation, then board over.

Mine have been like it for 15 years, no issues, house is 1972.

Qyburn

mmmmikey said

Google "interstitial condensation" .

Thanks. But knowing the technical term (which I did as it happens) doesnt shed a lot of light on the actual scenario being considered, which is whether the water vapour permeability of plaster ceilings is sufficient to require a vapour barrier. 

silverwhistle

mmmmikey said:

silverwhistle said:

I had a similar situation, and when I wanted to upgrade I laid celotex over the boarded areas and then boarded on top of that again.

I've also put reflective insulating foil over the rafters which still allows ventilation at the eaves.

I was advised against the Celotex approach because it can lead to condensation issues between the different layers and problems with damp in the longer term. Not sure how serious that is in practice but certainly there's lots of advice about not laying insulation on top of the boards 

Thanks. No evidence of damp up there, and perhaps that may partly reflect not heating upstairs and only one person in a 3 bedroom terrace. I think one of my next upgrades might be a MHRV unit in the bathroom which might also help. I only use about 1200-1500 kWh of gas a year for heating, so down the list might be an A2A heatpump for downstairs, which I wouldn't have thought would exacerbate any potential problem?