Insulating attic query

BullBauld

Just a quick query....

The joists are 6". But I can get a good deal on 100mm insulation. If I put down the 100mm and then and then cross with another layer of insulation there will be approx 50mm gap between the 2 layers i.e. the 100mm will be bout 50mm shy of the top of the joists. Is this a bad idea.

Or if I go for 170mm as the bottom layer it will be bout 20mm higher than joists and will be slightly compressed by top layer of insulation. Another bad ides?

I know I can get 150mm for it to be perfect but as I said there is a deal for 100mm on at the moment.

Any thoughts?

Thanks
Bull

99nsr125

BullBauld wrote: »

Just a quick query....

The joists are 6". But I can get a good deal on 100mm insulation. If I put down the 100mm and then and then cross with another layer of insulation there will be approx 50mm gap between the 2 layers i.e. the 100mm will be bout 50mm shy of the top of the joists. Is this a bad idea.

Or if I go for 170mm as the bottom layer it will be bout 20mm higher than joists and will be slightly compressed by top layer of insulation. Another bad ides?

I know I can get 150mm for it to be perfect but as I said there is a deal for 100mm on at the moment.

Any thoughts?

Thanks
Bull

If your using Rock Wool / Fiberglass you don't want to compress it,
tack 50mm battens on top of the joists to bring the height up to 200mm
and lay two layers between the joist and then another at right angles
above the joist.

If however you're able to get the £3 subsidised rolls from across the border
there is polystyrene boards that are subsidised too and come in 50mm
combing these two together would be easy, cheap and provide the same
performance as 200mm of Rock Wool. If you do get insulation board, try and
get foil backed board and lay the foil side down.

Don't block the eves and if you're not sure where they are, where the
joist joins the rafters is where they start.

paddy147

99nsr125 wrote: »

If your using Rock Wool / Fiberglass you don't want to compress it,
tack 50mm battens on top of the joists to bring the height up to 200mm
and lay two layers between the joist and then another at right angles
above the joist.

If however you're able to get the £3 subsidised rolls from across the border
there is polystyrene boards that are subsidised too and come in 50mm
combing these two together would be easy, cheap and provide the same
performance as 200mm of Rock Wool. If you do get insulation board, try and
get foil backed board and lay the foil side down.

Don't block the eves and if you're not sure where they are, where the
joist joins the rafters is where they start.

You need to be ablle to provide a fully legit UK or NI address and postcode to get the subsidized rolls of Insulation.

Its a British Goverment energy effiecency scheme that is in place and you need to give address and postcode of where you are living to get it.


Been there,done it and got the t-shirt at this stage.

heinbloed

The OP asks:

The joists are 6". But I can get a good deal on 100mm insulation. If I put down the 100mm and then and then cross with another layer of insulation there will be approx 50mm gap between the 2 layers i.e. the 100mm will be bout 50mm shy of the top of the joists. Is this a bad idea.

?


Yes. It is a very bad idea to lay two or more layers of mineral wool

cross with another layer

.

You would be creating hundreds of gaps. Where the layers/sheets meet.If you can push a pencil or stick through a cross/gap and it touches the ceiling/plaster board below you have serious gap.


This method of cross-laying was published by some people in the past who obviously did not understand the laws of physics.

The correct method is to lay the additional layers paralel to the existing ones. The upper layer overlapping the lower layer at it's center.
By this method there will be no gaps created.

BullBauld

Thanks for replies

Sorry I'm confused by this. I thought it was standard to lay 2 layers of insulation in attic.

heinbloed wrote: »

The OP asks:

?


Yes. It is a very bad idea to lay two or more layers of mineral wool .



You would be creating hundreds of gaps. Where the layers/sheets meet.If you can push a pencil or stick through a cross/gap and it touches the ceiling/plaster board below you have serious gap.


This method of cross-laying was published by some people in the past who obviously did not understand the laws of physics.

The correct method is to lay the additional layers paralel to the existing ones. The upper layer overlapping the lower layer at it's center.
By this method there will be no gaps created.

So your saying the info on the SEAI website on how to lay the insulation is wrong?

heinbloed

@ BullBauld:

You can lay even more than 2 layers, yes. But this should never be done crosswise but paralel, each new layer running in the same direction as the previous one, overlapping the previous one ideally at it's center.

The SEAI hasn't got much logic in most of their technical/financial advise.
Employ logic, always.

BullBauld

Thanks for reply heinbloed

Ok so I run both layers in same direction but not directly over each other if that makes sense.

Just another thing. All/most insulation is pre-cut to 400mm or 600mm witdths. My joists are bout 360mm wide in most places and narrower again in others.

Is there a problem in fitting the 400mm here(into 360mm space) as in losing any of its effectiveness or is this just a concern when compressing downwards?

Thanks for help

heinbloed

Is there a problem in fitting the 400mm here(into 360mm space) as in losing any of its effectiveness or is this just a concern when compressing downwards?

Rolls come in width of up to 2m, these can bu cut to demand.
Read the installation advice from the manufacturer. 1% compression is allowed for(must be to get a snug fitting) most go for 2-3%.
The stiffer the material due to it's resin/glue content the less compaction is allowed, the fleeces would buckle.