Insulation in new attic conversion

Wailin

Hi
Just had our attic converted recently and we used 150mm rockwool insulation and covered it with plastic sheeting across the entire attic then plasterboard. Some people have told us this is not a good idea due to condensation and no air can circulate leading to timber rot. The joist sizes are 95 by 35mm so you can imagine how tight the insulation is packed in. Is this a serious problem or does it take years for condensation to have affect on the timber? Should we have used kingspan insulation instead or what should we have done?
Hopefully someone can help me out here!!

PaleRider

Hello,

You should have allowed for ventilation. Jamming in 6in insulation into 4in gap is not correct. You could have used 2in Insulation between the rafters, and insulated plasterboard under, or over rafters. Insulation should not be compressed in any-case, because it will lower the performance/value. The value of this method will reduce over time due to the insulation sag, due mostly to no ventilation. You should always seek professional advice when not sure. Is this work completed.

Southern Comfort

Hi guys. I'm hoping to have a converted attic insulated shortly also. My rafters are also 100mm (4 inches) deep and I cannot afford to lose any head height inside the attic room which has a plasterboard ceiling.

One solution that the guy insulating it came up with was to remove all the tiles and old felt, put in 100mm of Rockwool Flexi and then cover it with a breathable membrane which he feels will cut out the need for space for air circulation. It would only give me a U value of 0.41 and the recommended one is 2.0.

My question PaleRider is............. I know that fibreglass insulation doesn't insulate properly if it's compressed but does the same apply to Rockwool Flexi?

I PMed a Mod today about the job generally and he gave me a few good suggestions. Since then, I've come across Xtratherm Thin-R rigid insulation board http://www.xtratherm.com/products/thinrpitchedroof.php which has a much lower U value. I could put in a bit less than 100mm of it with the breathable membrane to guarantee good air circulation but it'd be difficult to cut it for an exact fit between the rafters.

Has anyone done anything like this and, if so, how did it work out?

Wailin

PaleRider wrote: »

Hello,

You should have allowed for ventilation. Jamming in 6in insulation into 4in gap is not correct. You could have used 2in Insulation between the rafters, and insulated plasterboard under, or over rafters. Insulation should not be compressed in any-case, because it will lower the performance/value. The value of this method will reduce over time due to the insulation sag, due mostly to no ventilation. You should always seek professional advice when not sure. Is this work completed.

Well its almost completed, plasterboards are up and its skimmed so cant really go back on it now. The storage areas either side of the room can be accessed obviously so I'll probably remove the insulation in these areas and replace with rigid insulation such as kingspan and leave a ventilation gap. Never knew the insulation loses its performance value when compressed unfortunately. A 'friend' done the conversion and proclaimed this was the best way, cowboy more like:mad:

PaleRider

Wailin, If i understand correctly the Dwarf walls may be ok as there is ventilation, the insulation may be fine. If you post a few photos, may be able to advice better.

I have seen a good contractor make the same mistake years ago in regard to the way you carried out the insulation work.

The ceiling that is skimmed may give trouble. This will show on the nails/screws, because the insulation will sweat, water vapour will condense on the nails and travel down the nail and soften the plasterboard. You may notice the plasterboard sagging, moving away from the ceiling.

PaleRider

Southern Comfort wrote: »

My question PaleRider is............. I know that fibreglass insulation doesn't insulate properly if it's compressed but does the same apply to Rockwool Flexi?

I PMed a Mod today about the job generally and he gave me a few good suggestions. Since then, I've come across Xtratherm Thin-R rigid insulation board http://www.xtratherm.com/products/thinrpitchedroof.php which has a much lower U value. I could put in a bit less than 100mm of it with the breathable membrane to guarantee good air circulation but it'd be difficult to cut it for an exact fit between the rafters.

Has anyone done anything like this and, if so, how did it work out?

You are on the right track, what you are proposing sounds fine. Lift the phone and phone Xtherm they will give sound advice, but you are doing just fine. I believe the full fill with the breathable felt / Good.

Wailin

PaleRider wrote: »

Wailin, If i understand correctly the Dwarf walls may be ok as there is ventilation, the insulation may be fine. If you post a few photos, may be able to advice better.

I have seen a good contractor make the same mistake years ago in regard to the way you carried out the insulation work.

The ceiling that is skimmed may give trouble. This will show on the nails/screws, because the insulation will sweat, water vapour will condense on the nails and travel down the nail and soften the plasterboard. You may notice the plasterboard sagging, moving away from the ceiling.

Here are some photos which may give a better idea of what I'm talking about. You can see the finished room with one of the storage doors in photo 1. We put up the insulation and then put up tarpaulin below it to prevent moisture on the plasterboards. You can see the tarpaulin inside the storage bulging with insulation in photo 2 and 3. This is the same across the entire roof, except in the opposite storage area which I haven't insulated yet as you can see in photo 4. As I said in a previous post, I think I will remove what insulation I can and put in some 50mm rigid insulation, obviously I cant where the plasterboard is unless I demolish it which I dont plan on doing. Is there a better solution, and do you think this will pose serious problems in the future?

RKQ

Wailin, photo 4 shows your existing bitumen felt & space between rafters is fully insulated - no 50mm air gap?
This could lead to dry rot of the roof timbers as it is contrary to the Building Regulations.

creedp

My apologies for hijacking tis thread - my question is about insulating the attic at joist level initially and flooring out the attic for storage. I mightt subsequently add insulation at rafter level to make the storage space more comfortable.

In relation to the joist level insulation. My builder was proposing to use 150mm joists and batten them by 100mm to achieve a 250mm space for rookkwool. The attic could then be flooed out with chipboard. He then proposed to add a 42.5mm insulated plasterboard underneath. I know I will have to confirm the u value of this buildup with my engineer but could someone give me an indication of the overall U value I would achieve with this buildup?

I also asked if it would be better to install 200mm joists which could be battened out to 300mm before fullfilling with rockwool and flooring as not only would it provide more space for insulation but it wold also provide a more solid floor structure. He wasn't too keen on this but said he would give me an estimate on the additional cost. Am I silly to be specifying 200mm joists for an attic space which, in reality, I will probably not make substantial use of - would I be better off going with the 150mm joists and battenning/counter battenning to achieve the 300mm of space for the rockwool?

Finally on the insulated plasterboard - previously I read somewhere it is better to install insulation board on ceiling, then taping the joints and taping the edge board to the wall which could then be plastered over thereby achieving a much better level of airtightness. The plaster board could then be attached to the insulation and skimmed. My guy doesn't want to do this as he wants the 1 step approach of an composite plasterboard. Should I insist on the former approach?

Sorry about all the questions but I'm getting very little helpful advice from my professionals on these issue - they seem intent on confusing the hell out of me with jargon and telling me what I can't or shouldn't be doing.

All advice greatly appreciated

PaleRider

Wailin,

The work looks tidy, I like what you have done. However, make sure recessed lights insulation is not close to lamp holder, this would be dangerous. I agree you could remove Insulation, and replace with 50mm, you could also use 25mm 4ftx2ft etc across rafters. Take a look to see if you have a vent tile on roof at apex, so as at least that is vented. You could also try if to push up 50mm land drainage on the room side up slope entering the roof space at dwarf wall, into ceiling space, to help ventilation.

It may be OK,it,s easy finding fault, in the past there was very little training for tradesmen. Please keep me posted how thing pan out. It,s very hard to have all the answers, when you are not on site. Regards

Wailin

Thanks Palerider, I did leave a gap around the lights to prevent overheating, was thinking of maybe putting in LED bulbs instead as they do not get hot like halogens, not cheap though! There is no ventilation in the roof unfortunately. The work is quiet tidy, its just a pity I didnt know what I know now about insulation.....:o
Cheers for the comments.