How to insulate an attic with 120mm joists?

JonathonS

What approach should I take to insulate an attic with 300mm of fibreglass if the joists are 120mm high?

1) Use 100mm roll and leave a 20mm high gap between joists, then lay 200mm crossways?

2) Use 150mm roll and leave a 30mm gap over each joist before laying more 150mm??

3) Bridge the 20mm gap so that 100mm roll reaches the top of the joist? But how, and at a cost similar to fibgeglass roll?

4) Other?

JonathonS

Bump.

Isover list a 130mm Metac roll which would only be compacted by 10mm which looks like the way to go.

Anyone got any left-field suggestions on how I could get 20mm of insulation, thus allowing me to choose from the much wider selection of 100mm materials available.

BarneyMc

I guess the Metac is quite a bit more expensive than fibreglass?

sydthebeat

option 1

Metric Tensor

Working with fibre glass (particularly for someone who is not a full time insulation installer) is a nightmare.

If I was doing it myself I'd be happy to pay the extra for metac. My understanding is that there is not a huge difference in price.

stickybookmark

Are you putting plywood over the insulation to use the attic for storage is that why you're worried about the insulation being compressed?

Isn't Metac is also a fibre glass type insulation though? It's just a higher grade than normal earthwool/rockwool?

Metric Tensor

If it is I am mistaken. I know it's a mineral wool but I didn't think the fibres were glass fibres and hence not as rough on the hands and lungs during installation.

If this is the same then it's not worth the extra imo!

Edit: I just googled and it appears to be glass fibre - so ignore my previous post!

sydthebeat

knauf earthwool everytime

much better material to work with

Metric Tensor

sydthebeat wrote: »

knauf earthwool everytime

much better material to work with

That's what I was thinking of! Thanks syd.

Murphy14

Metric Tensor wrote: »

That's what I was thinking of! Thanks syd.

Metac has a superior thermal conductivity and higher density than earthwool , earthwool might be more user friendly but Metac is the better product

Metric Tensor

Murphy14 wrote: »

Metac has a superior thermal conductivity and higher density than earthwool , earthwool might be more user friendly but Metac is the better product

I was purely talking about the ease of installation for a non professional. Can't stand fibreglass.

Stone mineral wool would be the way I'd prefer but it's expensive stuff!

JonathonS

BarneyMc wrote: »

I guess the Metac is quite a bit more expensive than fibreglass?

Hmmm.....'fraid so. So expensive as to be excluded almost as quickly as it was included.

Prices of Isover G3 100mm is typically around €2.00 per m2, Metac is c €7.50 (incl vat).

JonathonS

stickybookmark wrote: »

Are you putting plywood over the insulation to use the attic for storage is that why you're worried about the insulation being compressed?

My plan is to fill to the top of the joists with 120mm of insulation, and put 200mm over that, excluding walkways. For those I will use sheet insulation+plywood, or one of the pre-prepared insulated walkway products.

The reason for wanting to avoid compressing the insulation is that it reduces its effectiveness, which I suspect is the reason Syd recommends option 1.

stickybookmark wrote: »

Isn't Metac is also a fibre glass type insulation though? It's just a higher grade than normal earthwool/rockwool?

On further reading its designed for friction-fitting between rafters for warm attic insulation. Obviously they must be using Viagra to stiffen it, that's the only possible explanation for the price difference.

stickybookmark

JonathonS wrote: »

The reason for wanting to avoid compressing the insulation is that it reduces its effectiveness, which I suspect is the reason Syd recommends option 1.

.

No but what I asked was why are you worried about it being compressed if you're not putting plywood over it, what's going to compress it?

JonathonS

stickybookmark wrote: »

No but what I asked was why are you worried about it being compressed if you're not putting plywood over it, what's going to compress it?

The 200mm on top of it......