How to insulate loft

[jarrick]

Hi all, I've got to insulate our loft and want to use PIR between the joists to minimise the amount of space lost in the loft.
The joists are 70mm deep but it's a lath &plaster ceiling and the plaster is bulging through and reducing the depth of what I can fit in.

It's not like modern plaster - it's pretty hard and gritty.
So my question is, can try and sand it down to the lath or is that recipe for disaster?

 

[Cubist]

That would not be wise in my view. Old lath and plaster can be extremely delicate and friable. The vibration of a sander may result in some serious damage below. Do you really, really need that depth of insulation?

 

[malcolm]

It's the plaster squeezed between the lath that binds the plaster to the lath. Without it the plaster will fall off.

Careful too about using PIR without a ventilated air gap on the outside. The ceiling won't be 100% air tight and the last thing you want is condensation forming around the PIR or making the joists wet.

 

[jarrick]

Thanks for the advice - yes I really need as much insulation as I can squeeze in, as there's nothing at the moment. Maybe I could lay a thinner bit or PIR board and then inject some expanding foam underneath? Does that sound like a good option?

 

[Cubist]

May be a dumb question but the photo suggests that loft space is fairly cramped, so I'm assuming you intend boarding it over - is that correct?

 

[jarrick]

The photo is just at the edge - it's just about high enough to stand up in the centre so I'll have boards over the top. I could raise the height of the joists but I'm also planning to insulate over the rafters (once I figure out ventilation) so don't want to lose too much space as it's needed for storage.

 

[Kearn]

Thanks for the advice - yes I really need as much insulation as I can squeeze in, as there's nothing at the moment. Maybe I could lay a thinner bit or PIR board and then inject some expanding foam underneath? Does that sound like a good option?

It sounds terrible.

You’ve an old house, you need to understand how it functions in terms of materials and moisture management.

Pause, research and get your head around it all, then make the correct changes, once. Your various suggestions are the wrong approach for a period building and will cause issues.

As others have said, a lime plaster ceiling on lath is a lovely thing. The nibs are the only bit that keeps an entire very heavy, historic ceiling of lime from falling on you in big pieces. Use either woodfibre batts (I like PavaFlex) or sheepswool (https://www.sheepwoolinsulation.com) between the joists. If you really need more isjuaktuon, then you’ll need to raise the height by firing the infidel joists to add more depth, at the cost of floor height.

 

[paulc]

Careful too about using PIR without a ventilated air gap on the outside. The ceiling won't be 100% air tight and the last thing you want is condensation forming around the PIR or making the joists wet.

The plaster nibs will create quite a large gap under any PIR boards. This will allow for a fair bit of cold air to circulate between the insulation and ceiling. That would severely reduce the effectiveness of any insulation. On the other hand, it will limit the amount of condensation on the underside of the PIR boards. Spraying the area with expanding foam before putting insulation down will stop the cold air from circulating but if you were to use expanding foam, might as well use the stuff to the full depth of the joists - A very bad idea on a number of counts. 1) It isn't breathable (but neither is rigid PIR board) despite what the manufactures/installers tell you. 2) it is nasty stuff to remove and sticks to everything. 3) It gives off toxic fumes if it ever catches fire and can also release unpleasant smells as it cures. 4) it could cause issues when selling the property.

Normal loft insulation like fibreglass and mineral wool are non-combustible. Easy to lay and can be packed in to stop cold draughts circulating underneath. Is breathable and will allow moisture to pass through. Can be easily removed if/when the need arises without causing damage (and can potentially be recycled) - All but the non-combustible point also applies to sheep's wool. OK, you need twice the thickness compared to PIR/PUR, but loft legs supporting any boarding avoids compressing the stuff. For storage, the loss of head height isn't really an issue in all but the tiniest of lofts.

 

[alexander]

your current requirements for maximum efficiency is 100mm between the joists 170mm over the joist running in the opposite direction, you can then get legs to raise any boarding above the insulation as long as you have not sealed the eaves then you should not have any condensation issues