Loft insulation, squished or not squished?

FreeBear

Chickereeeee said: TL;DR - compression INCREASES the R-value of a material such as fiberglass or rockwall insulation.

But the heat loss through insulation is a function of the R-value and the thickness. So whilst the R-value may increase a little, because the thickness is reduced, so is its overall effectiveness. The page referred to also includes this little quote -

When you compress fiber glass batt insulation, the R-value per inch goes up, but the overall R-value goes down because you have less inches or thickness of insulation.

Section62

Chickereeeee said:

For those interested, OP is probably referring to something  like this page:

https://www.energyvanguard.com/blog/compressed-fiberglass-insulation-really-so-bad/

TL;DR - compression INCREASES the R-value of a material such as fiberglass or rockwall insulation.

Although air is removed, there are still plenty of air pockets, and being smaller, convection is reduced. 200mm of uncompressed insulation is better than 100mm of uncompressed insulation, but (apparently) 200mm of insulation compressed to 100mm is a lot better than 100mm uncompressed. 

( And I imagine covering that compressed 100mm with 18mm of board would reduce convection and heat transfer still further. What it does to moisture transfer I have no idea).

That article is about cavity wall insulation.

Does anyone say the same applies to loft insulation?

Chickereeeee

Section62 said:

Chickereeeee said:

For those interested, OP is probably referring to something  like this page:

https://www.energyvanguard.com/blog/compressed-fiberglass-insulation-really-so-bad/

TL;DR - compression INCREASES the R-value of a material such as fiberglass or rockwall insulation.

Although air is removed, there are still plenty of air pockets, and being smaller, convection is reduced. 200mm of uncompressed insulation is better than 100mm of uncompressed insulation, but (apparently) 200mm of insulation compressed to 100mm is a lot better than 100mm uncompressed. 

( And I imagine covering that compressed 100mm with 18mm of board would reduce convection and heat transfer still further. What it does to moisture transfer I have no idea).

That article is about cavity wall insulation.

Does anyone say the same applies to loft insulation?

Vertical versus horizontal orientation would not matter.

Different materials probably behave a bit differently, but I imagine the same arguments apply to both loft and wall insulation compression.

Anyhow, i now realise the levels of compression mentioned in the article are not close to the 2:1 suggested by OP,  except for the final coulimn of the chart, where the R-value goes from 8 to 3.3, so received wisdom is probably correct!

Section62

Chickereeeee said:

Section62 said:

Chickereeeee said:

For those interested, OP is probably referring to something  like this page:

https://www.energyvanguard.com/blog/compressed-fiberglass-insulation-really-so-bad/

TL;DR - compression INCREASES the R-value of a material such as fiberglass or rockwall insulation.

Although air is removed, there are still plenty of air pockets, and being smaller, convection is reduced. 200mm of uncompressed insulation is better than 100mm of uncompressed insulation, but (apparently) 200mm of insulation compressed to 100mm is a lot better than 100mm uncompressed. 

( And I imagine covering that compressed 100mm with 18mm of board would reduce convection and heat transfer still further. What it does to moisture transfer I have no idea).

That article is about cavity wall insulation.

Does anyone say the same applies to loft insulation?

Vertical versus horizontal orientation would not matter.

Different materials probably behave a bit differently, but I imagine the same arguments apply to both loft and wall insulation compression.

Anyhow, i now realise the levels of compression mentioned in the article are not close to the 2:1 suggested by OP,  except for the final coulimn of the chart, where the R-value goes from 8 to 3.3, so received wisdom is probably correct!

Really?  On what basis?

A skim read of the link and associated information suggests the key issue they are concerned about is convection.  A vertical air-space in a cavity wall represents a totally different proposition when it comes to convection than insulation laid horizontally over the top of a warm space with a cold space above.

The TL;DR I'd take away from that source is the suggestion that cavities should be fully filled and consequently compressed insulation which fully fills the cavity has benefits over 'non-compressed' insulation which may leave vertical air-space.  [on the other hand, completely filling a cavity can have disbenefits in a damp climate we have in the UK]

Essentially this demonstrates the danger of reading information which applies to one situation (in one country) and assuming it applies equally in completely different circumstances... regardless of what our local building regulations actually require, and what good practice suggests.