Loft insulation, squished or not squished?

ohdarn

Hi everyone. I've googled this question and had conflicting results for it.
A lot of "anti-squishers" end up being people selling loft legs but then they may also be correct.

We're taking all the loft insulation out of our roof so we can put it down properly. There were three layers added over time but the first layer wasn't very good.

We then intend to board over it as we really need the storage.

The final layer looks to have been added recently so I've carefully rolled it back up with the intention to put it back as the first layer.

The thing is though, it's 200mm thick and the joists are the standard 100mm thick.

So do I reuse the 200mm stuff and then squish it down to 100mm with the boarding or should we get all new 100mm stuff?

I understand it's the air bubbles within it that trap the heat and if it's squished then the air bubbles are smaller but then there'll be more of them.

Also, if it's a little squished then it's more likely to fill up any gaps that might be present, getting a better seal?

So what would you do?

casper_gutman

I'm not sure how much difference there would be between 100mm insulation and 200mm insulation squashed down to 100mm, but I'm absolutely certain that neither option is enough.

Ideally, I'd increase the depth further, but as a minimum I'd use either loft legs or counter-battens so I could use the full 200mm insulation I already had.

rob7475

If using the soft insulation, you need it to be un-squashed to work effectively. If you're wanting to board it out (obviously checking that the timbers can take the added weight first ;-) ), you could use something like 100mm kingspan between the rafters.

ohdarn

I'd rather not use the loft legs as their weight ratings seem quite poor, 50kg per square metre which means I wouldn't be able to stand on any of it.

There's the option to build it up with timber I guess, but very expensive.

And either option will mean having to extend the electrics so they're not buried under the insulation.

The other thing to note is the the roof is split into two halves, this is one side and the other is smaller that we're not storing things in.

Maybe we put a thicker layer in the other side, although we haven't looked up there yet to see what's already installed (probably a horror going by what we've already found).

Chickereeeee

Loftleg.com says
"Strong enough to walk on AND store up to 25Kg per square metre"
And another manufacturer says:
"Although loft legs can support high weights up to 500kg we would recommend no more than 50kg per m² due to load bearing characteristics of a loft"

They assume people walk on the boards, and are trying to stop people overloading their lofts, due to concerns of house structure, NOT the legs. You would have similar restrictions with battens.

ohdarn

rob7475 said:

If using the soft insulation, you need it to be un-squashed to work effectively. If you're wanting to board it out (obviously checking that the timbers can take the added weight first ;-) ), you could use something like 100mm kingspan between the rafters.

Had a look at the kingspan but two problems.

One is that it would press on the lats and plaster of the ceiling below as the plaster coming through the lats make it less than 100mm in places.

The other is that it's incredibly expensive.

rob7475

There's usually local places about that sell damaged insulation - try and find one local to you - it'll be much cheaper.

You could screw 50mm timber on top of your current rafters to give you the space to install kingspan?

FreeBear

Put 100mm between the joists - It is what that particular thickness is sold for. The 200mm can then be laid at 90° to the base layer. The standard length of loft legs is 175mm, so any boards on top will compress the insulation slightly, the effect will be minimal.

As for load capacity, you will find the ceiling joists will be the limiting factor, and the legs will take a surprising amount of weight. Just don't go putting a hot tub up there

Eldi_Dos

If you do decide to board your loft I would use loft legs to avoid compressing your new insulation and putting pressure on your ceilings below, could lead to cracking or nailheads moving under the taping especially if any open plan areas with long spans.

sk2402005

The way the the insulation actually does its job, is by trapping the air, so squashing 200mm down to 100mm pretty much gives the insulation properties of 100mm.

you need to use some loft legs or equivalent to give the depth required.

m0t

ohdarn said:

I'd rather not use the loft legs as their weight ratings seem quite poor, 50kg per square metre which means I wouldn't be able to stand on any of it.

There's the option to build it up with timber I guess, but very expensive.

And either option will mean having to extend the electrics so they're not buried under the insulation.

The other thing to note is the the roof is split into two halves, this is one side and the other is smaller that we're not storing things in.

Maybe we put a thicker layer in the other side, although we haven't looked up there yet to see what's already installed (probably a horror going by what we've already found).

Your loft joists are designed to carry the weight of the ceiling below, the 50kg m/2 is to prevent bending of the joists/cracking of the ceilings and not a limit of the legs themselves.