# attic insulation advice needed



## prf (4 Mar 2011)

I am thinking of getting my attic insulated,my house was built in 1975,at that time 2 inches of fibreglas was put between joists.Later on I added extra insulation that came in 4 x2 foot slabs.this is also 2 inches thick.Recently I have noticed that there are black marks on my ceilings where the joists are positioned.this is very noticable above the room where I have a solid fuel stove for heating.I have checked the insulation in attic and it seems to have "shrunk" in thickness ,I am thinking of taking it all out and redoing the entire thing again.Has anybody got any ideas about the marks on ceilings?.Also some advice on what product to use and depth needed.All help is appreciated.Thanks


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## Shane007 (6 Mar 2011)

It's referred to as "pattern staining" caused by differential thermal properties of the timber joists and the insulation adjacent to them. 

When insulating again, try to obtain a continuous insulation covering. Fibreglass insulation will always lose it's thickness over time and therefore it's thermal properties. If you use a material such as polyiso, this will not occur.

With regard to thickness, it will depend on the material that you will be using; see this thread: http://www.askaboutmoney.com/showthread.php?t=149436


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## prf (7 Mar 2011)

Shane,thanks for quick reply.I see on here a lot of people have a foil "attached" to the plaster ceiling slabs.My slabs have no foil or anything "attached".Is this polyiso available in rolls or is it just slabs?.Can you clarify that if the polyiso is laid between the ceiling joists first and then at right angles to the joists that the staining will not appear again.If using polyiso, do I install 300mm in total thickness.Thanks again for the great info on here.


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## Shane007 (7 Mar 2011)

The foil is a vapour membrane. It will come with polyiso. Polyiso is a slab and generally comes in 8 x 4 sheets, however, Xtratherm will cut them to whatever size you wish before delivery, say 8 x 2 or 4 x 2 so you can get them through a loft hatch. Speak to their sales rep prior to ordering. I am sure most manufacturers will do the same.

Leave the insulation you have in situ, it will do no harm and will add a small bit of thermal property to the roof. If you are using polyiso with U value of 0.023, then you will only require a total thickness of 150mm. If you are putting in between joists, you will need to allow a further 15% in thickness to compensate for the loss in thermal conductivity from the joists. 

Presuming your ceiling joists are 5 x 2's, I would put 125mm polyiso between joists and then a further 50mm perpendicularly across the top. 

Don't forget to allow for air circulation around any recessed light fittings & electrical cables. Try to fit insulation below electrical cables, lying cables on top of insulation if you have enough slack.

You can also build a cw storage tank surround with the insulation, removing any insulation from below it and this will bring the tank into the heated space below. Insulate pipework also.

Fit a boarded walkway on top of the polyiso to tank and valves and anyother place you require access to.


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## Lex Foutish (7 Mar 2011)

Shane007 said:


> The foil is a vapour membrane. It will come with polyiso. Polyiso is a slab and generally comes in 8 x 4 sheets, however, Xtratherm will cut them to whatever size you wish before delivery, say 8 x 2 or 4 x 2 so you can get them through a loft hatch. Speak to their sales rep prior to ordering. I am sure most manufacturers will do the same.
> 
> Leave the insulation you have in situ, it will do no harm and will add a small bit of thermal property to the roof. If you are using polyiso with U value of 0.023, then you will only require a total thickness of 150mm. If you are putting in between joists, you will need to allow a further 15% in thickness to compensate for the loss in thermal conductivity from the joists.
> 
> ...


 
Hi Shane. Is there any danger of dry rot, etc. if you insulate between the joists with fibreglass insulation and then cover the entire attic space with foil backed rigid insulation like Xtratherm?


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## Shane007 (7 Mar 2011)

No, as long as the vapour barrier is on the warm side of the insulation. Fibreglass is not a barrier for moisture, therefore it will not trap any.


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## Lex Foutish (7 Mar 2011)

Shane007 said:


> No, as long as the vapour barrier is on the warm side of the insulation. Fibreglass is not a barrier for moisture, therefore it will not trap any.


 
Thanks, Shane. What if there's a vapour barrier on both sides, like on Xtratherm?


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## Shane007 (7 Mar 2011)

All polyiso insulation (& phenolic), including Quinn & Kingspan have a vapour barrier on both sides. The saturation point is on the cold side of the insulation or within a few mm's of it and it will disapate into the air surrounding it, causing no problem whatsoever as long as adequate ventilation is provided for within that space. 

What I meant was if you are physically putting in a vapour barrier, it should be located on the warm side of the insulation, so in theory to be officially correct, you could say that another vapour barrier could be placed on the warm side of the existing fibreglass insulation or remove it, but really there is no need. As indeed when you have multiple layers of insulation, you will also have multiple layers of vapour barrier with no effect of damaging surrounding timbers.


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## Lex Foutish (7 Mar 2011)

Thanks, Shane. Good to know.


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## Ciara2011 (4 Apr 2011)

I got my attic insulated using rockwool, made a huge difference to heating our house.  We no longer have to have the temperature on the Gas bolier up as high as we used to.  Over the cold spell none of our pipes froze  unlike neighbours whos pipes froze.

Previous Cold spell our pipes froze and when you openned the trap door to attic, it used to be like a hurricane  of cold wind was coming from the attic.  This is no longer the case with the rockwool insulation

Next on the list is to insulate the walls of the house


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## Leo (4 Apr 2011)

Ciara2011 said:


> ...when you openned the trap door to attic, it used to be like a hurricane of cold wind was coming from the attic. This is no longer the case with the rockwool insulation


 
You still need to have the attic space above the insullation well ventilated or you will end up with condensation problems. Were the gaps at the eaves blocked by the insulation?
Leo


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## Ciara2011 (4 Apr 2011)

Leo said:


> You still need to have the attic space above the insullation well ventilated or you will end up with condensation problems. Were the gaps at the eaves blocked by the insulation?
> Leo



No, we asked installers could they completely cover all holes in attic, because of draughts, they said they could but it would not then qualify for grants and would not then be certifiable and attic area has to breath via the vents (which cannot be blocked) you see under the guttering outside when looking up. 

We also had to get high level storage fitted so that there is nothing lying on top of this insulation.

Hope i am explaining things right and am understanding your question as I would not be technical.

All i know is that the Rockwool insulation for us, has already fully paid for itself in reduced heating bills.  And it spared the inconvenience of the cold weather of frozen pipes etc.


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## monagt (6 Jul 2013)

Ciara2011 - who did you use for the insulation?


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