Questions regarding ceiling vapour barrier and loft insulation

paperclap

Hello all,

I’ve a few questions concerning our ceiling, loft space and the insulation.

In our bungalow, we have polystyrene insulation and plasterboard between our rafters. Between our joists, we have fibreglass insulation.

As it is an old-ish bungalow (1955), we don’t have a vapour barrier between our plasterboard and joists (to stop moisture travelling from the living area into the loft). 

Question 1 — Do we need a vapour barrier? I’ve not noticed any condensation in the loft space. And, I can’t believe half of Britain has retrofitted vapour barriers in their houses! Not keen on having to fit a vapour barrier on the warm side (“living space”), as just had our ceilings plastered. And, don’t want to go through the hassle of doing it in the loft either!

Question 2 — I know polystyrene and plasterboard insulation isn’t the best. In fact, I’ve been told from users on here to take it down to let the loft space breathe. I’ve also been told to remove it by a HomeBuyer’s Report, as it is a fire hazard. However, if removed, this would make the loft space a “cold loft”. Removing the insulation would therefore create a larger thermal bridge between the living area and loft space, wouldn’t it? And therefore increase the potential for condensation issues? I don’t mind taking it all down, but don’t want to shoot myself in the foot in doing so!

As always, thank you in advance.

FreeBear

A vapour barrier is not needed (in my opinion) between the ceiling & loft space. As long as the loft is well ventilated, any moisture that does make its way through the ceiling will dissipate. The polystyrene isn't going to do much in the way of insulating if the loft is well ventilated - There should also be an air gap between the polystyrene and the felt..

Assuming the loft isn't boarded, you should be aiming for a minimum of 270mm of glass fibre or rockwool insulation above the ceiling making sure it goes right up to the edges where roof meets walls.

tacpot12

I would agree that you don't need a vapour barrier, as the house and the loft will be leaky enough that moisture will not have time to build up to levels where it will cause rot in the joists or rafters. Installing a vapour barrier could create a situation where water vapour is trapped in an area where it is harder for it to leave the property.

Removing the polystyrene insulation and plasterboard, you will change the point at where the dew point is reached in the loft. This could mean that instead of dew forming inside the rafters (as interstitial condensation), it will form in the air and drop out as visible water droplets on to the fibreglass insulation, which is a better place for it that inside the rafters where it cannot be see and where it will cause the rafters to go rotten IF it is a continuous situation, but as per my opnion above (and FreeBear's), it is not going to be a continuous situation in a relatively leaky old bungalow.   

Regarding the polystyrene insulation being a fire risk, I would agree that it is very dangerous IF it catches fire, but why would it catch fire? If you have taken proper precautions to avoid sources of ignition in the loft and home and have smoke detectors upstairs, then I think you can manage the risk down to an acceptable level. Obvious areas of risk to look for are electrical cables running under the fibreglass insulation, loose connections, and cables to electric showers (any joints in such cables are a potential source of a high resistance and therefore of heating). It woudl be worthwhile replacing any halogen spot lights fitted in the ceiling with LED replacement as these run much cooler and will also save you a good deal of electricity.   

Not storing any fuels or substances that can catch fire at lower temperatures inthe loft would be sensible. Ideally, the only thing in the loft should be the layers of insulation.