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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Building Questions >> Insulation in 40 degree roof rafter using cellotex https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1373189229 Message started by saaya2020 on Jul 7^th, 2013, 10:27am

Title: Insulation in 40 degree roof rafter using cellotex
Post by saaya2020 on Jul 7^th, 2013, 10:27am

Background: last year, we converted our Bungalow into a 2 storey house.
The Bungalow had a attic so the Roof rafters had 50mm cellotex which I was able to salvage.

When we converted to a house, the loft space was not converted into an attic. Put simply, British Gas laid 120m2 of yellow fibreglass loft insulation blanket over the horizontal floor space in my loft. The insulation is 100mm thick

My upstanding roof rafters are at 40 degrees & the dimensions are 200mm by 40mm (8 by 2's) and they are at 400mm centres. I have breathable roof felt.

My solution: In order to have proper ventilation air flow in the roof space, I know that I will need a 50mm airgap from the felt to the 50mm cellotex. Then Iam thinking to put the 100m fibreglass on top of the cellotex. Then I put foil backed plasterboard over the rafter & skim.

Is my solution ok or will I run into condensation issues? I am simply trying to re-use the existing cellotex and 120m2 fibreglss insulation.

I'd be grateful for any solution or advise to re-use this insulation rather than skip it!

Summary : 200mm rafters. So, 50mm airgap + 50mm cellotex + 100mm loft blanket. Then, foil backed plasterboard + skim.

Thanks

PS - regs not needed for this project.

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by woodsmith on Jul 7^th, 2013, 10:05pm

If you look at the Tyvek website they show various insulation arrangements for pitched roofs (google Tyvek roof specifications). The way you are planning to do this looks oK to me as you plan to use foil backed plasterboard and this will act as a vapour barrier. As far as I know the gap between the felt and insulation is required so that the felt can sag in between the raters which will encourage any water that gets past the tiles to run off easily rather than as a ventilation space.

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by saaya2020 on Jul 8^th, 2013, 12:42am

thanks for the 52 page technical guide on the tyrek site. My biggest concern was the impact of having the 100mm loft blanket over the cellotex ... but the 2 key points to me seem to be 1. having an air gap between the felt and the cellotex and then 2. having foil backed plasterboard.

Started fitting the 50mm cellotex - like a jigsaw puzzle - as all the lengths are already cut. Just need to slot them inbetween the rafters. Will get some metal tape to seal the cellotex joints. It leaves a 90-100mm gap where I will put the loft blanket at the end before covering with the foil backed plasterboard.

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by woodsmith on Jul 8^th, 2013, 7:15am

Yes sealing the joints in the Celotex is crucial to its performance. Getting the loft blanket to stay in place whilst you fix the plasterboard is the other challenge. I usually add a thin plastic vapour barrier which I staple to the rafters and that keeps everything in place. That way I can just use standard plasterboard. Otherwise you are probably going to need something like, thin string in a matrix or paper stapled between the rafters, to hold it in place.

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by saaya2020 on Jul 8^th, 2013, 9:41am

does it work out cheaper to fix a thin vapour barrier + normal plasterboard vs foil backed plasterboard in your opinion?

normal plasterboard 2400 x 1200 = £5
foiled plasterboard 2400 x 1200 = £10

As I've not bought anything yet, i have an opportunity to try your method.

Also, can you recommend a good vapour barrier to attach and how mucg does it cost?

thanks

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by woodsmith on Jul 8^th, 2013, 11:03am

If you go to your local builders merchant they should stock it as vapour barrier. It comes on a very big roll but I think it cost me about £15 last time I bought some. Don't let them sell you visquene damp proof membrane it is a bit too thick if you have to fold it over anywhere and its far more expensive.

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by TIMBA-WOLF on Jul 8^th, 2013, 9:33pm

Is there going to be a bathroom/shower room, that will have the foil backed plasterboard you intend using, as it's ceiling??

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by saaya2020 on Jul 9^th, 2013, 8:36am

No - it will only be used for storage - e.g. suitcases, tools, surplus packed foods - baked beans tins, biscuits, cereal boxes etc (i intend to make cupboards either side of the triangular space.

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by Lectrician on Jul 9^th, 2013, 9:54pm

wrote on Jul 8^th, 2013, 9:41am:

does it work out cheaper to fix a thin vapour barrier + normal plasterboard vs foil backed plasterboard in your opinion?

Also, can you recommend a good vapour barrier to attach and how much does it cost?

I used this stuff and for belts and braces, also foil backed board.

http://www.screwfix.com/p/vapour-barrier-2-5m-x-20m/12869

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by saaya2020 on Jul 10^th, 2013, 1:38am

thanks for the link - now i know what a vapour barrier looks like! Just plastic sheeting! How do you recommend attaching this to the rafters - maybe staples ?

thanks again

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by Lectrician on Jul 10^th, 2013, 6:21am

Yep, staples. Not many, just enough to hold it until boarding.

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by saaya2020 on Jul 10^th, 2013, 8:52am

thanks.

I have fully covered the cellotex to one side of the roof. BUT, I have realised that this new roof is a much steeper & taller roof - so I have already used up the majority the cellotex!

Ironicaly, I have 16 opened rolls the 200mm loft insulation left. Yes 16 !

Question :

Before i purchase more cellotex, i just wanted to confirm with the experts on this forum that there is no way to adapt to use the loft insulation in the roof space on its own?

Thanks

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by woodsmith on Jul 10^th, 2013, 10:50am

Before Cellotex fibreglass was just stuffed between the rafters on a job like this. My own house is done that way, but Cellotex is a much better insulant in that it is more insulating, it doesn't slump, as long as you seal round the edges it is unaffected by wind blowing through it and it doesn't gradually expand filling the gap under the tiles. Plus I have bats living in my insulation, they have eaten through the Tyvek and created homes in the fibreglass I don't suppose they would have found Cellotex so inviting ::)

So you could use it,it's just a case of weighing the cost V benefits and how long you expect to live there maybe.

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by saaya2020 on Jul 10^th, 2013, 11:45am

thanks. Good Point. Yes, having converted it from a bungalow, i am definitely here for the long run! I'll need to think about this and see if i can sell on the fibreglass to make the cellotex more affordable. The 50mm sheets are £22 each. I probably will need 15 sheets as a ball park.

Bats - yikes!

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by CWatters on Jul 11^th, 2013, 9:29am

I noticed you said..

Quote:

I have breathable roof felt

In which case you may not need a 50mm ventilated cavity.

In addition some makes of membrane and insulation allow the insulation to be installed in contact with the membrane. However you will probably need to allow the membrane to drape slightly between the rafters. That's to allow any water that gets past the tiles to run down the membrane under the battens. If the membrane is pushed against the battens by the insulation dirt can collect on the uphill side of the batten and eventually water can pool there as well. .

So in short you might get away with a 25mm unventilated cavity allowing another 25mm of insulation if you wanted.

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by saaya2020 on Jul 11^th, 2013, 3:43pm

Hi. My metal tape has arrived to seal the joints of the cellotex. Before i start it, i just want to clarify with this community what somebody mentioned to me:

"i should not put the loft blanket on top of the cellotex as it will create air pockets leading to condensation which in turn will make the rafters damp."

From my limited knowledge, i thought that the 50mm air gap between felt and cellotex would give enough air circulation. Plus, if I put foil backed plasterboard plus a vapour barrier as mentioned above, then that would "double" prevent the condensation - that's how i understood it!

Is my understanding correct or am i safer to not put this loft blanket over the cellotex? what do you think?

(The reason i am a little nervous is because I have seen condensation in my dad's old house. It felt like the roof was leaking in rain but it was condensation!)

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by woodsmith on Jul 12^th, 2013, 7:31am

I don't think that you will have a problem as long as you have a vapour barrier (you don't need to use both foiled backed plasterboard and a plastic barrier, I would just use the plastic sheet) and you have a breathable felt.

The insulation will be more effective if you put the fibreglass between the Cellotex and the plasterboard. If you put the fibreglass on top, under the tiles, wind will be able to blow through it reducing its effectiveness.

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by saaya2020 on Jul 13^th, 2013, 11:33am

Thanks for your reassurance. Very grateful for your comments. I confirm that the fibreglass will be between the Cellotex and the plasterboard. I will purchase the screwfix vapour barrier as recommended above.

Cheers

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by saaya2020 on Aug 15^th, 2013, 8:55pm

Quick update on my attic roof insulation project ... i have finally put in all the celotex between the rafters & taped all the joints - big job!

Quick reminder of my set up from the outside:

Roof Tile -> breathable felt -> 50 mm air gap -> 50mm original celotex -> **50MM GAP** -> vapour barrier -> plasterboard -> skim

The next step in my plan is to put my existing rockwool left by British Gas in the **50MM GAP** before plasterboarding

BUT, i had a builder that came today to quote me some attic partitions and said that if I am going to dry my clothes in the attic (my wife has a washing line up there & is always drying clothes up there) , then it is not advisable to put this rockwool in the **50MM GAP**.

He says that it will lead to massive mould problems. He strongly said i should not put it in. He said that i either put nothing or i put another 50mm celotex. Also, he said i should also put air bricks on both gable walls for extra ventilation.

I appreciate that in the above posts, i did not mention that my wife dries clothes in the attic .. so can somebody please guide me whether this builder is saying correctly or not?

Many thanks in advance for your comments

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by woodsmith on Aug 16^th, 2013, 8:18am

I can't see why he would think adding insulation will create a bigger problem unless he thinks that warming the roof will make the walls comparatively colder and then you would get a lot of condensation, and mould, on the walls. However I think you are likely to get that anyway if you dry clothes in there.

It is not a good idea to dry clothes in a room at all unless you have an enormous amount of ventilation or have a specialist dehumidifier which can be set for laundry. I don't think adding a couple of vents will make much difference, other than to make the room colder.

Title: Re: Insulation in 40 degree roof rafter using cellotex
Post by CWatters on Aug 29^th, 2013, 10:05am

I agree with woodsmith.

The basic idea is to ensure that any water vapour created in the room can find a way outdoors. It's important that this is via ventilation and not through the structure. A correctly installed vapour barrier will deal with one issue but you need sufficient ventilation to deal with the other.

50mm celotex +50mm rockwool isn't a lot of insulation. I have around 150mm Celotex in my pitched roof and wish I had more. If the room isn't adequately heated and ventilated it's just possible you may get condensation on the room side of the plasterboard when drying clothes in there.

Obviously the need for ventilation pushes up the cost of heating the space. One potential way out of that catch22 is some sort of heat recovery ventilation system (MHRV). Single room systems exist but are quite expensive and I'm not really up to speed on what's available.

I'd try drying clothes and see how you get on. Just be aware of the potential for condensation.