Loft insulation size and joist spacing

[markwm]

I'm lowering a ceiling. Had to get 1800x900 plasterboard as 2400x1200 wouldn't fit up the stairs.

Anyhow, I'm planing out the timber spacing in the false ceiling. Was planning to do 450mm centre's for the joists.

Problem is, all the loft insulation available seems to be either 370mm wide or 600mm wide. I assume the 370mm stuff is for 400mm joist spacing.

370mm isn't going to fit snug in my 450mm centres(400mm gap).

 

[ajrobb]

I'm curious why you want to insulate the false ceiling at all? It is probably not a vented cavity and could cause condensation problems which could get nasty without being visible.

If you can insulate the original ceiling instead then you can more easily use the warm plenum above the false ceiling for recessed lights etc.

I'd be interested in comments on CasoLine MF as an alternative to joists.

 

[markwm]

If I don't insulate the false ceiling and insulate the loft above, surely all the warm air is going to get trapped in the void in the false ceiling?

I always thought you insulate the lowest part. Hence the reason why you don't insulate the roof instead of the loft floor.

 

[freddiemercurystwin]

I always thought you insulate the lowest part. Hence the reason why you don't insulate the roof instead of the loft floor.

Immediately above the exist ceiling is the normal position to insert your insulation. Besides, the cost of materials, labour for a skim coat and rigid insulation will take about 10 years to be recovered. A loft can be insulated with mineral wool in an afternoon and get a payback in a year!

 

[ajrobb]

Mineral wool loft insulation doesn't get damp because it is ventilated otherwise mineral wool retains condensation, ceases to insulate and causes rot. If you want to insulate a false ceiling you probably have to make sure you have a VPL on the warm side of ALL internal surfaces bounding the cold plenum (e.g. adjacent walls), not just the false ceiling. IF you can maintain the integrity of all the VPLs, you might get away with it. You'll never know if you've caused problems in the plenum until its too late. If you decide on recessed lights or other openings then you will be storing up loads of trouble.

Perhaps a safer alternative would be to install air bricks to ventilate the cold plenum. Then you will lose even MORE heat into the cold plenum through uninsulated internal walls.

Alternatively, don't insulate the false ceiling and give yourself a trouble-free warm plenum.

 

[markwm]

I always thought you insulate the lowest part. Hence the reason why you don't insulate the roof instead of the loft floor.

Immediately above the exist ceiling is the normal position to insert your insulation. Besides, the cost of materials, labour for a skim coat and rigid insulation will take about 10 years to be recovered. A loft can be insulated with mineral wool in an afternoon and get a payback in a year!

true but I have to drop the ceiling. No point doing the room and having the existing ceiling ruin it. I'm not lying when I say it's 3 inches out from one side of the room to the other.

 

[markwm]

Mineral wool loft insulation doesn't get damp because it is ventilated otherwise mineral wool retains condensation, ceases to insulate and causes rot. If you want to insulate a false ceiling you probably have to make sure you have a VPL on the warm side of ALL internal surfaces bounding the cold plenum (e.g. adjacent walls), not just the false ceiling. IF you can maintain the integrity of all the VPLs, you might get away with it. You'll never know if you've caused problems in the plenum until its too late. If you decide on recessed lights or other openings then you will be storing up loads of trouble.

Perhaps a safer alternative would be to install air bricks to ventilate the cold plenum. Then you will lose even MORE heat into the cold plenum through uninsulated internal walls.

Alternatively, don't insulate the false ceiling and give yourself a trouble-free warm plenum.

I was going to install a vapour barrier on the ceiling and external walls. No spot lights and I planning on installing a insulated loft hatch so I can keep an eye on the void.

 

[freddiemercurystwin]

I'm not lying when I say it's 3 inches out from one side of the room to the other.

My apologies, my crystal ball's not working today!

 

[ajrobb]

I was going to install a vapour barrier on the ceiling and external walls. No spot lights and I planning on installing a insulated loft hatch so I can keep an eye on the void.

I would not put a vapour barrier between a cold void and an external wall rather I would put one on the warm side of all interior walls that meet the cold void. You could punch a few holes through your existing ceiling to vent the cold void into the loft.

I still think you are making a rod for your own back though.

 

[markwm]

I'm not lying when I say it's 3 inches out from one side of the room to the other.

My apologies, my crystal ball's not working today!

No need for the rolleyes....

My post was about loft insulation sizing, not why I'm dropping a ceiling. My apologies if I didn't post enough information concerning the later.

 

[markwm]

I was going to install a vapour barrier on the ceiling and external walls. No spot lights and I planning on installing a insulated loft hatch so I can keep an eye on the void.

I would not put a vapour barrier between a cold void and an external wall rather I would put one on the warm side of all interior walls that meet the cold void. You could punch a few holes through your existing ceiling to vent the cold void into the loft.

I still think you are making a rod for your own back though.

thanks for your replies.

I would put the vapour barrier between the plasterboard and the studs, I assume this is where you mean as well. Same idea as foil backed boards..

Punching holes in the existing ceiling seems like a good idea. So much soot and muck in the loft though, not a nice job, especially when you have to sleep in the room.

 

[ajrobb]

I generally find my LA Building Control are very helpful and willing to discuss changes. I'd go and have a chat with them.

With regards to the holes, you might make them from above after you have sealed the void from below. (I'd still go for a warm plenum instead though.)

 

[ajrobb]

I generally find my LA Building Control are very helpful and willing to discuss changes. I'd go and have a chat with them.

I would put the vapour barrier between the plasterboard and the studs, I assume this is where you mean as well.

That still allows vapour to enter the cavity through the internal walls.

With regards to the holes, you might make them from above after you have sealed the void from below. (I'd still go for a warm plenum instead though.)

 

[markwm]

warm void and no membrane?

 

[ajrobb]

warm void and no membrane?

Yes, that would be my choice. Normal loft insulation.

While you might find this site helpful, if you check with local Building Control, you can be reassured you alterations won't affect any subsequent house sale.