Attic insulation earthwool

betterbuilt

I've bought enough rock wool insulation for my attic to create a 300mm layer of insulation but have since realised my ceiling joists are only roughly 200mm deep.

I'm aiming to put the Earth wool in between the ceiling joists and then put flooring over the top, for attic storage. (I would lay the 100 across the joists perpendicular to the 200 mm layer only for the fact that I want to lay the osb board flooring on the joists).

Does anyone know which gives the better insulation overall - to pack/squash the 200 mm layer plus the 100mm layer together in between the joists, or to just use the 200mm layer and allow it to expand fully into the available c 200mm space?

(I'm aiming in the long term to convert the attic into living space, so eventually the sloping roof joists will be insulated anyway, but if having both layers packed into the 200mm deep space between the ceiling joists would make a difference in the short/medium term I'll definitely use both layers since I have them bought anyway.)

While I'm at it I might as well also ask if anyone knows whether or not the rough 3x1 timbers running diagonally across the sloping joists the length of the house are superfluous and removable at this stage of the build and likewise similar timbers running the length of the house
directly on and perpendicular to the ceiling joists. (I have the feeling they were possibly only intended as temporary timbers while the roof was being installed. If I remember correctly I think it's called a 'truss roof' (?) - as in, it has been built to accommodate easy future conversion to living space)

Many thanks if anyone knows the answer to either of these queries.

GreeBo

Don't compress insulation, that removes the air content which renders it useless.
Also don't start removing pieces of your roof if you don't know what you are doing!
You can raise the joists either with legs/stilts or timber and then lay the extra layer perpendicular and floor above

Lumen

GreeBo wrote: »

Don't compress insulation, that removes the air content which renders it useless.

I do not believe this is true. Rockwool is around 99% air, so the difference in air volume for a given thickness of compressed or uncompressed insulation is negligible.

If you were correct, higher density insulation like Metac or acoustic rockwool would be useless.

listermint

Lumen wrote: »

I do not believe this is true. Rockwool is around 99% air, so the difference in air volume for a given thickness of compressed or uncompressed insulation is negligible.

If you were correct, higher density insulation like Metac or acoustic rockwool would be useless.

I've never heard anyone say compressing Rockwool was a good idea.

The other high density ones are constructed to do that job it's in the design. False equivalence.

Why bother extending the ceiling joists if we could just mush it all into 100 mm instead

Still waters

GreeBo wrote: »

Don't compress insulation, that removes the air content which renders it useless.

Not being picky or anything but is there any research on it to back up your claim as to render it useless, surely compressing may inhibit its characteristic slightly but to render it completely useless seems implausible

Lumen

listermint wrote: »

I've never heard anyone say compressing Rockwool was a good idea.

The other high density ones are constructed to do that job it's in the design. False equivalence.

Why bother extending the ceiling joists if we could just mush it all into 100 mm instead

I'm not saying compressing rockwool is a good idea, but the effect of compression is usually expressed as a loss of thickness, not an increase in density. In this specific case the thickness is the same 200mm.

The best thing to do is raise the floor. It's better to do the best thing.

Lumen

Also, in my limited experience Rockwool never quite recovers its rated thickness, and there is a possibility that increasing the density reduces unwanted air flow across the insulated void.

So I think it's possible that actual 200mm of compressed rockwool outperforms nominal 200mm of uncompressed rockwool in the real world.

Doctors room ghost

Be careful if you’re packing the insulation in tight between the joists.you could pop the ceiling pins and damage the ceiling below if you press it too tight.
Also wear good mask for dust

Roger Mellie Man on the Telly

betterbuilt wrote: »

While I'm at it I might as well also ask if anyone knows whether or not the rough 3x1 timbers running diagonally across the sloping joists the length of the house are superfluous and removable

This is diagonal bracing. It's important.

GreeBo

Lumen wrote: »

I do not believe this is true. Rockwool is around 99% air, so the difference in air volume for a given thickness of compressed or uncompressed insulation is negligible.

If you were correct, higher density insulation like Metac or acoustic rockwool would be useless.

If it's 99% air and you compress it to half its designed thickness then you've just lost at least 50% of the air and probsbky more.
What are you compressing if not the air? It had to go somewhere if you reduce the thickness and the air is the only thing making it insulating, hence it being 99% air.

You'd have the exact same issue if you compressed the metac.

Calahonda52

Roger Mellie Man on the Telly wrote: »

This is diagonal bracing. It's important.

Yes at construction time, especially with remade trusses.
.
with the roof finished and assuming either a hip or a full concrete block gable, they don't do a lot.
If the house is TF with no block work then yes.

OP the other ones sound like purlins which cannot be removed, bst post some pictures

Calahonda52

Lads and lassies here: just on the commentary about entrained air in insulation and compressing the ...wools?
a few ideas to think about when using the ...wool type?
.

1. why is airtightness more important than insulation?
2. why do we use wind tight and air tight membranes?
3. why does a wind breaker make such a difference when wearing a wooly jumper on a windily day?
4. . and for the rigid
5. Why does performance of the XPS's reduce as they de-gas
   .
   what doe we need to consider when using the high density wood fibres etc?

betterbuilt

Calahonda52 wrote: »

Lads and lassies here: just on the commentary about entrained air in insulation and compressing the ...wools?
a few ideas to think about when using the ...wool type?
.

1. why is airtightness more important than insulation?
2. why do we use wind tight and air tight membranes?
3. why does a wind breaker make such a difference when wearing a wooly jumper on a windily day?
4. . and for the rigid
5. Why does performance of the XPS's reduce as they de-gas
   .
   what doe we need to consider when using the high density wood fibres etc?

Seriously thanks to all of you who have taken time to respond i hugely appreciate it. I will try posting some pics of the timbers when I next get a chance to get up there

Lumen

GreeBo wrote: »

If it's 99% air and you compress it to half its designed thickness then you've just lost at least 50% of the air and probsbky more.
What are you compressing if not the air? It had to go somewhere if you reduce the thickness and the air is the only thing making it insulating, hence it being 99% air.

You'd have the exact same issue if you compressed the metac.

No, because the volume is the same in the two cases under consideration.

Calahonda52

Lumen wrote: »

No, because the volume is the same in the two cases under consideration.

The volume of the material is the same but that is not the insulant here

Lumen

Calahonda52 wrote: »

The volume of the material is the same but that is not the insulant here

What is the insulant?

dathi

https://dcpd6wotaa0mb.cloudfront.net/mdms/dms/Residential%20Insulation/10017857/10017857-Building-Insul-Compressed-R-Value-Chart-Tech-Bulletin.pdf?v=1428909128000


chart showing the reduction in r values when fiberglass is compressed

listermint

Lumen wrote: »

What is the insulant?

Combination of the two, the material and its ability to trap and retain air

Lumen

listermint wrote: »

Combination of the two, the material and its ability to trap and retain air

Actually, I think the flaw in my logic is that the air is not important, it's the glass.

Glass has ~25x more conductivity than air, so when the insulation is compressed, there's more glass fibres touching each other and conducting the heat through the void.

Interestingly though, it's not linear - according to that chart posted above, the drop off in performance is about half the rate of compression.

Anyway, I withdraw my suggestion that compressing the insulation is relatively harmless, but I think I'm even keener on buying insulation that retains its shape better (and therefore achieves the nominal thickness). This wasn't my experience with Earthwool loft insulation, but the acoustic rockwool was much better.

cruizer101

That table seems to imply you are just as well of if not better compressing it.
e.g. take the 14" r-49 if compressed to 9.25 it has R= 37 where as the 9.5 r-30 compressed to 9.25 has R= 30
37 better than 30

take the 9.5 r-30 if compressed to 5.5 it has R= 21 which is the same as what is listed for the 5.5 r-21 and better that the 6.75 compressed to that.

So compressing the insulation reduces it R value but it seems only to a similar level as if you used the thinner stuff anyway.

GreeBo

Lumen wrote: »

No, because the volume is the same in the two cases under consideration.

The volume of air isnt the same. Its the air that is insulating, not the material itself. (Hence why rock wool is 99% air, if the wool was the important bit it would make up more than 1%!)

GreeBo

cruizer101 wrote: »

So compressing the insulation reduces it R value but it seems only to a similar level as if you used the thinner stuff anyway.

Which makes total sense.
Air is whats insulating, so if you compress 200mm down to 50mm you get the performance of 50mm since you basically just made it 50mm.
The fact that it still has the amount of wool content at 200mm is effectively irrelevant, that wool doesnt do anything other than "hold" the air.

betterbuilt

Thanks so much to everyone who has responded here. This is my 4th attempt to successfully post this thanks apologies, struggling with technology argggh! So helpful to hear everyone's response. I've gone with the 200 in the 200 space!

Re timbers I'm going to try posting pics now. Kinda hoping they see removable to be honest as it will make laying flooring and future plasterboard on the sloping interior roof walls so much easier i think?!

betterbuilt

Calahonda52 wrote: »

Yes at construction time, especially with remade trusses.
.
with the roof finished and assuming either a hip or a full concrete block gable, they don't do a lot.
If the house is TF with no block work then yes.

OP the other ones sound like purlins which cannot be removed, bst post some pictures

Hi pics attached here i hope...

One shows some horizontal timbers 3x1 rough, which i also am hoping were just for during construction...

betterbuilt

betterbuilt wrote: »

Hi pics attached here i hope...

One shows some horizontal timbers 3x1 rough, which i also am hoping were just for during construction...

Here are photos this time i hope...

betterbuilt

betterbuilt wrote: »

Here are photos this time i hope...

And a few more