Loft Insulation - I know it’s been done to death!

[Nathan90]

Hi All,

I know this topic has been done to death but I’m looking for a definitive answer there is so much conflicting advice.

I want to insulate my loft there is already some insulation in there which I plan to remove but I don’t want to cause myself problems down the line.

(Old Victorian property - solid walls with no insulation - slate roof)

My thoughts…

Add 25mm batons to the joists flush with the plasterboard.

Fill the remaining space with PIR board - I think it will be 50mm this will bring the boards flush with the joists taping the edges with gappo tape to ensure a tight fit between the joists.

Then add 100mm PIR boards laid across the joists (may use thicker if funds allow).

Then tape the gaps on the top layer.

I appreciate this may be a more costly approach to take than just using insulation rolls but I’m trying to do belt and braces - bedrooms can be freezing.

Can anyone definitively tell me that this is the right approach to take?

 

[Murdochcat]

Make sure the loft has ventilation ................

 

[Nathan90]

Make sure the loft has ventilation ................

Plenty of that…. It’s just slate on batons.

 

[freddiemercurystwin]

I appreciate this may be a more costly approach to take than just using insulation rolls but I’m trying to do belt and braces - bedrooms can be freezing.

There's no advantage to using PIR over rockwool, the 270mm rockwool will do the same thing as the PIR.

 

[cdbe]

Unless you're boarding the loft it's a massive waste of effort and money. Mineral wool is just as good as good at a fraction of the price and far easier to install. What I would do is focus on the eaves - this is where most heat is lost and yet is generally the worst insulated area - get your celotex in there - between the joists, no battens or air gaps - get as much on top as you can, moving out from the eaves till you get to the point where you can fit 300mm of mineral wool.

 

[Nathan90]

There's no advantage to using PIR over rockwool, the 270mm rockwool will do the same thing as the PIR.

What I’m thinking is it will give me a more stable platform to get around the loft space when needed without falling through the floor. Theres only about 3-4ft of clearance and its complex in its shape with a number of valleys ect

 

[RandomGrinch]

What I would do is focus on the eaves - this is where most heat is lost and yet is generally the worst insulated area - get your celotex in there - between the joists, no battens or air gaps - get as much on top as you can, moving out from the eaves till you get to the point where you can fit 300mm of mineral wool.

Sorry, this may sound a little misleading to some (I know we're dealing with an unfelted loft in this instance) - insulation can be pushed into the eaves, but the detailing is important to retain ventilation above the insulation.
With rockwool, this can be achieved with something like rafter trays..

Or with PIR, correctly cutting it to size, to maintain an air gap.

 

[cdbe]

True, but my interpretation was that he was planning to create a void between ceiling and PIR for some reason.

 

[Nathan90]

True, but my interpretation was that he was planning to create a void between ceiling and PIR for some reason.

That’s right I was / am what I’ve read is that an air gap should be left between the PIR and plasterboard?