Advice on Loft Insulation

[FozzieBear]

As the title says I am looking for some advice on loft insulation. I have a bungalow which I rebuilt in 2000 so external walls are now cavity and have cavity insulation. The roof spaces are part open and part boarded for storage. The ceiling joists are 125mm and the fiberglass comes up just below the tops of the joists so I suspect I used 100mm insulation at the time. The same thickness insulation is under the boarded areas, in the load bearing zones which has 18mm chipboard flooring.
I want to increase the insulation and I can fairly easily add to the unboarded areas with another 100mm or more but its the boarded areas i'm unsure of.

Is the boarded area more effectively insulated because it has boards on top which keep an air pocket between the cold space and the insulation below or are they only effectively insulated to the same degree as the open areas with the same thickness of insulation?

Also to be effective does the insulation have to be lightly packed in the space so there is air between the strands or can you compact it down and achieve a higher insulation effect e.g can I put another 100mm under the boards by squashing it down? This seems sort of counter intuitive as I think the insulation is achieved from the captured air cells. Certainly if you think of expanded polystyrene or celotex then they are full of air bubbles.

I could replace all the 100mm insulation under the boards with 150mm or put a 25mm layer of celotex on top of the 100mm insulation but that would be expensive.

I could also plant another 50mm on top of the joists to lift the boards but again thats labour intensive not cheap and would further reduce the already low head height
Any thoughts would be appreciated

Fozzie

 

[phatboy]

Yes, the insulation needs the air space within it to work.

Ideally, buy a LoftZone kit (Or similar), then you can add 200mm of insulation across the existing and re-fit your loft boards on top. Knauf Earthwool is usually cheap in the autumn at B&Q..... £15 a roll when you buy 3 or more I think usually.

 

[FozzieBear]

Yes, the insulation needs the air space within it to work.

Ideally, buy a LoftZone kit (Or similar), then you can add 200mm of insulation across the existing and re-fit your loft boards on top. Knauf Earthwool is usually cheap in the autumn at B&Q..... £15 a roll when you buy 3 or more I think usually.

Many thanks Phatboy, unfortunately a loftzone will make the space unusable. At a pinch I could lift the boards by 50mm but any more would make the space unusable.
As it happens Travis Perkins have just sent me a deal on Knauf Earthwool thats £15 per roll. I'm wondering if I could lay 50mm celotex on the joists and put the boards back down on top of that for the limited boarded area? Its load bearing if laid on a flat surface but not sure how it would fair supported by the joists?
Fozzie

 

[alan333]

Personally I wouldn't bother adding more insulation, see my post #6 of THIS THREAD.

 

[JohnD]

if the joists are adequate for boarding, you can lift the boards, put down the PU insultaion boards, relay the flooring on top and screw it down to the joists (using long screws).

Knauf used to recommend this method and had a youtube video on it.

the recommendation and video disappeared shortly after the Grenfell Tower disaster.

i would not have PU boards inside my house. I made this decision shortly after the Grenfell Tower disaster.

mineral wool is non-combustible and does not emit poisonous fumes in a fire, nor does it drip flaming molten plastic in a fire. I use that inside my house.

It is possible to cross-batten over the joists to add enough depth for another layer of mineral wool if you have the space and feel the need.