Roof Insulation

hargo

Quick Question , what is the recommended thickness of fibreglass wool for roof insulation. I have about 2" at the moment of loose stuff?

Reyman

8" ---- 2" is a waste of time.

7 times more heat loss

hargo

thanks reyman 8" was what i was thinking of doing.

I've gone for 400mm
A bit OTT by todays standards but then again a few years ago, 100mm was considered OTT:rolleyes:

Anything that reduces fuel bills has to be investigated, assuming resonable payback of course.

Mellor

Deleted User wrote:

I've gone for 400mm
A bit OTT by todays standards but then again a few years ago, 100mm was considered OTT:rolleyes:

Anything that reduces fuel bills has to be investigated, assuming resonable payback of course.

how did you insulate, along the slope or on flat. Are you able to walk in your attic. just curious

transitman

Yes,about 8 inches will do a good job.:D

muffler

150mm laid between the joists with another layer of 100mm square across the top of that is more than enough

Mellor

Mellor wrote:

how did you insulate, along the slope or on flat. Are you able to walk in your attic. just curious

Just on the flat, difficult to walk up there, I still have to fit an elevated "service walkway" to where the MHRV system is. The loft is not intended for stowage, Thats what the garage is for:D

Viking House

We put 450mm on the flat in the attic and 350mm in the sloping part of the roof.

bowsie casey

I was planning to lay down some OSB boards in the attic of my parents house to give them some storage space. However, taking a look at the insulation (old raggy yellow stuff, looks about 2" thick), I think I need to re-do the insulation before putting down any boards.

Now.....reading plenty of insulation posts here, I see suggestions of minimum 300 mm, or 150mm between joists and 100mm at right angles.

Obviously, if I do this, I will need to raise the flooring boards up by 6". This will make this a bigger job than I want to undertake.

Is there another way to do this? No super-duper insulation where I can lay 100mm in the section which I will floor ? The floored area will make up about 20% of the attic space, the rest I will do the full insulation job.

Reyman

I'm not sure that you get too much value from going beyond 200mm insulation.
Glass fibre:
At 100mm the R value is 2.5 and U value 0.4W/m2/oC
At 200mm the Rvalue is 5.0 and U value 0.2W/m2/oC
At 300mm the Rvalue is 7.5 and U value 0.13W/m2/oC.
(ignoring the plasterboard insulation value)

There's diminishing returns at greater thicknesses of insulation and by insulating too thickly you're risking burst pipe problems up in the attic due to the cold.

A burst pipe in the attic is very serious news (usually occurs when you're on your holidays and get no warning). Every ceiling will collapse.

200mm is fine

Reyman

I should have mentioned.

Do not cover the electrical wires in the attic with 200mm of insulation under any circumstances

ardara1

Reyman wrote:

I'm not sure that you get too much value from going beyond 200mm insulation.
Glass fibre:
At 100mm the R value is 2.5 and U value 0.4W/m2/oC
At 200mm the Rvalue is 5.0 and U value 0.2W/m2/oC
At 300mm the Rvalue is 7.5 and U value 0.13W/m2/oC.
(ignoring the plasterboard insulation value)
Reyman - you're not taking the ceiling joist into account - they are 12.5% of the total ceiling area - so vital
100m between joists at 400mm centres & 170mm over is the MINIMUM to meet the current building regs at 0.16 U-value

If you're goint to reduce insulation just to use as storage place - BUY A SHED - you'll end up paying for the storage every month.

Recommend 2 layers 6" as a starting point

Reyman

Reyman wrote:

I should have mentioned.

Do not cover the electrical wires in the attic with 200mm of insulation under any circumstances

Doesn't this depend on whether the cables have not been derated assuming they will be buried in insulation
If correctly rated there should be no problem?

Mellor

Reyman wrote:

I'm not sure that you get too much value from going beyond 200mm insulation.
Glass fibre:
At 100mm the R value is 2.5 and U value 0.4W/m2/oC
At 200mm the Rvalue is 5.0 and U value 0.2W/m2/oC
At 300mm the Rvalue is 7.5 and U value 0.13W/m2/oC.
(ignoring the plasterboard insulation value)

There's diminishing returns at greater thicknesses of insulation and by insulating too thickly you're risking burst pipe problems up in the attic due to the cold.

A burst pipe in the attic is very serious news (usually occurs when you're on your holidays and get no warning). Every ceiling will collapse.

200mm is fine

Well insulation shouldn't be under the pipes/water tank in the first place.

To the op, how big is this storage space. If you insulate the rest to very high level, and just fill inbetween the joist and OSB it it will be a HUGE improvement on what you have. Other option is 4" battens at right angles to the joists to raise the floor.

Reyman

ardara1 wrote:

Reyman wrote:

I'm not sure that you get too much value from going beyond 200mm insulation.
Glass fibre:
At 100mm the R value is 2.5 and U value 0.4W/m2/oC
At 200mm the Rvalue is 5.0 and U value 0.2W/m2/oC
At 300mm the Rvalue is 7.5 and U value 0.13W/m2/oC.
(ignoring the plasterboard insulation value)
Reyman - you're not taking the ceiling joist into account - they are 12.5% of the total ceiling area - so vital
100m between joists at 400mm centres & 170mm over is the MINIMUM to meet the current building regs at 0.16 U-value

I'm not sure of the thickness of the Op's joists but assuming timber has an R value of 1 per 25mm (to the best of my memory). Then a 100mm joist will have an R value of 4, close enough to 200mm of insulation, so maybe not too important.

I'm just appreciating the Op's problem of trying to floor the attic. If there's a lot of insulation this is an almighty task.

Reyman

Deleted User wrote:

Doesn't this depend on whether the cables have not been derated assuming they will be buried in insulation
If correctly rated there should be no problem?

I'm not sure what derated means ?

Maybe you'd enlighten me !

muffler

Aeroboard were making an insulated flooring system which consisted of ply with insulation fixed to underside. The idea was to screw it into the joists and in theory you had the benefit of having your attic insulated and floored with one single product.

I cant recall the name of it and indeed I cant recall ever seeing it being used in practice but I would imagine it would be a bit expensive

muffler

Reyman wrote:

I'm not sure what derated means ?

Maybe you'd enlighten me !

derate

v : lower the rated electrical capability of electrical apparatus
WordNet ® 2.0, © 2003 Princeton University

bro'

Possibly a bit off topic but in relation to roof insulation can people advise what the requirements are or what would be recommended for preventing electrical fire hazard in the attic especially with downlighters. My electician has just ripped away the insulation surrounding the downlights and i would like to know what can be done to re-instate the attic insulation and yet not create a danger for a fire hazard.

Thanks

Reyman

You should keep the insulation a few inches clear of the light.

Somebody else here suggested an upside down flower pot with the bottom taken out to keep the insulation off the light fitting

bro'

thanks Reyman, I was checking out fire hoods for the downlighers in the attic but these are very expensive_ the idea you mention is very good and a considerably cheaper alternative to what I want to do i.e keep the insulation away from lights.:)

Reyman

muffler wrote:

derate

v : lower the rated electrical capability of electrical apparatus
WordNet ® 2.0, © 2003 Princeton University

Ok , the cable is probably designed away above actual current load, which is normal safety practice.

But if you have an immersion heater cable (2.5mm2) up there and it's carrying 15 amps you're looking for trouble if you cover it with insulation.
Even the lights cable (1,5mm2) might be carrying a few amps and again I wouldn't cover them.

Have a chat with the ESB lads in Distribution if you need an official view on this.

Viking House

Here are some details on how they insulate roofs in Scandinavia. The outside layer on the top photo is windstop insulation which allows a good airflow in the roof and stops the insulated air being blown out of the insulation. You can see the service cavity where you can fit the lights without breaking through the vapour barrier.



In this photo you can see the cold bridge which is the wood being broken by the insulation on top of it. You also see the vapour barrier at the warm side of the insulation which prevents heated air leaking out of your house and keeps the insulation dry from moist air internally.

Thats just how I am insulating my house (second image) special attention to the wallplate area.

malpas

just came across this thread..by coincidence I got a quote tonight to insulate my attic ..company has recommended (as the current recommended industry standard) 12inches or 300mm insulation split into two rolls of 6inches each..one laid between joists and the other crossways to that on top. I asked where they will put electrical cables and he said that their standard practice in all houses is to put the cables between both insulation layers. I have some doubts about fire hazard aspects and I would appreciate any advice/comments anyone may have. Thanks