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

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Nathan90

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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?
 
Nathan90 said:
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.
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There's no advantage to using PIR over rockwool, the 270mm rockwool will do the same thing as the PIR.
 
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.
 
freddiemercurystwin said:
There's no advantage to using PIR over rockwool, the 270mm rockwool will do the same thing as the PIR.
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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
 
cdbe said:
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.
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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..
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Or with PIR, correctly cutting it to size, to maintain an air gap.
 
True, but my interpretation was that he was planning to create a void between ceiling and PIR for some reason.
 
cdbe said:
True, but my interpretation was that he was planning to create a void between ceiling and PIR for some reason.
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That’s right I was / am what I’ve read is that an air gap should be left between the PIR and plasterboard?
 
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