Insulating roof space

Qyburn

paul991 said:

moist air from the upstairs rooms needs to be able to escape either though the plasterboard and insulation or a trickle fan in a upstairs room

Don't you have vents in the upstairs windows? The last place that warm, moist air should go is into a cooler space inside the building, as it cools down the RH increases. There should be a vapour barrier on the warm side of the insulation if the ceiling is that porous.

mmmmikey

Qyburn said:

paul991 said:

moist air from the upstairs rooms needs to be able to escape either though the plasterboard and insulation or a trickle fan in a upstairs room

Don't you have vents in the upstairs windows? The last place that warm, moist air should go is into a cooler space inside the building, as it cools down the RH increases. There should be a vapour barrier on the warm side of the insulation if the ceiling is that porous.

I think the different thinking here reflects the differences in building practices & regulations over the years. In the 1970's when Grandad's house was built the common practice was to allow the warm, moist air from upstairs rooms to escape into a loft which was well ventilated effectively treating the loft as part of outside the house. It wasn't common to have trickle vents or vapour barriers at that time. As you point out that would be an effective way of managing the issue of condensation (and is more energy efficient) - it just wasn't done that way in the old days.

Qyburn

I've lived in older houses, built in 1960, 1955 and currently 1850. Ive not seen any specific provision for ventilation through the ceilings. Is plasterboard (or lath and plaster) really permeable enough for significant air movement?

Raxiel

We went the stilt route. Was a long tedious process because we needed far more stilts than I'd estimated and the old, very thin layer of insulation (where present) was disintegrating, so I cleaned it all out along with multiple birds nests (and skeletons) around the eaves. I had to re-board it once to make sure it all fit, and then take them down a second time for the insulation installers to chuck their 300mm in, then do the final fitting.

Also had to build an extension to the loft hatch so it met the new floor level. Not just because it stopped insulation from falling out the gap, but because I needed somewhere secure to mount the ladder at the new level.

And then I went and did it all again at my parents, although in their case the old boarding had been taken up when the insulation was topped up, and just left stacked up at one end.

If your loft is anything like the two I did with black dust from the felt everywhere, a disposable paper suit and a good dust mask are highly recommended for a job like this.

Oh, and Knee pads too!

mmmmikey

Qyburn said:

I've lived in older houses, built in 1960, 1955 and currently 1850. Ive not seen any specific provision for ventilation through the ceilings. Is plasterboard (or lath and plaster) really permeable enough for significant air movement?

No, sorry I wasn't clear. There is no specific provision for ventilation through the ceilings into the loft, just a recognition that in the absence of a vapour barrier on the warm side of the ceiling insulation there needs to be good ventilation on the cold side (i.e. in the loft) to prevent condensation / damp issues. Hope that makes more sense.

I absolutely agree that a vapour barrier and trickle vents or other designed in ventilation is a better way to go both from the point of view of condensation control and energy efficiency. I'm not suggesting the 1970's solution was a good one, just pointing out that at that time building regs allowed some poorly ventilated and insuated houses to be built, and where the house needs a ventilated loft for the reason discussed it's best not to obstruct that ventilation.

Qyburn

mmmmikey said:

No, sorry I wasn't clear. There is no specific provision for ventilation through the ceilings into the loft, just a recognition that in the absence of a vapour barrier on the warm side of the ceiling insulation there needs to be good ventilation on the cold side (i.e. in the loft) to prevent condensation / damp issues. Hope that makes more sense.

It does make more sense, but depending on temperatures, humidity and permeability there could be condensation within the insulation itself. In that case no amount of ventilation above the insulation will help.

mmmmikey

Qyburn said:

mmmmikey said:

No, sorry I wasn't clear. There is no specific provision for ventilation through the ceilings into the loft, just a recognition that in the absence of a vapour barrier on the warm side of the ceiling insulation there needs to be good ventilation on the cold side (i.e. in the loft) to prevent condensation / damp issues. Hope that makes more sense.

It does make more sense, but depending on temperatures, humidity and permeability there could be condensation within the insulation itself. In that case no amount of ventilation above the insulation will help.

Yes indeed - which is exactly why the building regulations have progressively changed. Pre about 1970 many houses were quite drafty with fire bricks, suspended floors, poorly fitting windows and doors etc. So there was an amount of natural ventilation and the issue was less severe although I'm sure I'm not the only forumite here who remembers condensation on the inside of the windows in the morning, sometimes freezing in the winter. In the 1970's houses became "sealed boxes" with a nod towards ventilation of loft spaces and so on. Since then regulations have evolved to the much better approach you describe - vapour barriers where needed, specific requirements around ventilation of the house space and so on.

Grandad2b

mmmmikey said:

Qyburn said:

mmmmikey said:

No, sorry I wasn't clear. There is no specific provision for ventilation through the ceilings into the loft, just a recognition that in the absence of a vapour barrier on the warm side of the ceiling insulation there needs to be good ventilation on the cold side (i.e. in the loft) to prevent condensation / damp issues. Hope that makes more sense.

It does make more sense, but depending on temperatures, humidity and permeability there could be condensation within the insulation itself. In that case no amount of ventilation above the insulation will help.

Yes indeed - which is exactly why the building regulations have progressively changed. Pre about 1970 many houses were quite drafty with fire bricks, suspended floors, poorly fitting windows and doors etc. So there was an amount of natural ventilation and the issue was less severe although I'm sure I'm not the only forumite here who remembers condensation on the inside of the windows in the morning, sometimes freezing in the winter. In the 1970's houses became "sealed boxes" with a nod towards ventilation of loft spaces and so on. Since then regulations have evolved to the much better approach you describe - vapour barriers where needed, specific requirements around ventilation of the house space and so on.

Now I should perhaps be worried: I'm told the builders went bust shortly after completing the house and there's certainly evidence of shortcuts being taken on second fix, particularly the joinery. The doors and windows have of course all been replaced and we know from other work we've had done wouldn't meet current building regs but I think they probably close the apertures far more effectively than the originals would have done.

In the case that we add insulation to that in contact with the ceiling would that increase the likelihood of condensation between the existing and new insulation? Would it be necessary (or merely advisable) to install a vapour barrier directly above the ceiling?

mmmmikey

Hi, I wouldn't worry about it, there are hundreds of thousands of houses from that era without issues. This is more of a technical discussion about why regs have changed than a practical discussion about immediate problems.
I'd just take the boards up and make sure there's no obvious damp before adding the extra insulation and making a new storage platform on stilts.
And if you have windows with trickle vents it's best to leave the vents open all year. If not then just open the windows a crack occasionally. Also use a cooker hood and bathroom extractor and/or open the window after cooking/showering.
No need to obsess or worry about it and probably things you do anyway to keep the air in the house fresh.

Reed_Richards

You get condensation when air with a high relative humidity comes into contact with something cold and cools the air to the point when it cannot sustain that quantity of water vapour.  Warm air will tend to rise, albeit very slowly when passing through mineral fibre insulation but I would have thought that condensation within the insulation is unlikely.  You might get condensation on top of the insulation but if the loft is well ventilated then any briefly condensed water will evaporate.

I don't think any vapour barrier is advisable unless you have other means of getting water vapour from the room below to outside the house.