Upcomming Attic Conversion. Insulation Query.

hopalong_ie

Hi All.

I am planning on converting my attic within the next year. Unfortunatly the insulation in my attics needs replacing now. 20 years old (100mm) covered in rat urines etc so its had its day. I was planning on fitting Polyiso insulation but am now considering rockwool. I will be using RSJs for the conversions and as such will get 200mm of Rockwoll in vs 100mm of Polyiso and 100mm of rockwoll. Is there any advantage to using polyiso.

My main reason for the change of plan is costs. I will only be able to get 50mm of Polyiso in the converted ceiling and will use thermal plasterboard for the ceiling so am thinking some rising heat would be good.

RKQ

hopalong_ie wrote: »

My main reason for the change of plan is costs. I will only be able to get 50mm of Polyiso in the converted ceiling and will use thermal plasterboard for the ceiling so am thinking some rising heat would be good.

All heat rises... so insulation at floor joist level will only slow the heat rising. Therefore the thicker the insulation the slower the heat loss (heat rise).
Personally I'd zone the heating, insulating each floor fully. Rads will heat the attic conversion suffiently for sleeping - no need to heat the conversion all day. Try to heat the floor you are occupying only.

Mixing both types of insulation isn't a problem, thought there are a few schools of thought! Rockwool has great Fire proofing qualities, so it might be better regarding fire spread.

Why only 50mm in new ceiling?
Any chance of a sketch cross section of your proposed design?

hopalong_ie

Hi,

Going for only 50mm as the raters are only 100mm deep. Will consider closer the time battening an additional 25 or 50mm as it will never be anything other than storage space officially. the reason i was thinking of rockwool was that i planned on using 565mm wide 100mm xtratherm in the ceiling but some raters are 57cm wide leaving a space that would be uninsulated. To fill this would be a huge job so i am considering the alternative of just install 200mm of rockwool between the ceiling and the rsjs. Also the use of rockwool would mean i would not need to move plumbing etc but the trade off is the amount of insulation. Would be nice to have 400mm seperating the floors for thermal reasons but also to reduce noise.

RKQ

hopalong_ie wrote: »

Going for only 50mm as the raters are only 100mm deep. Will consider closer the time battening an additional 25 or 50mm as it will never be anything other than storage space officially.

50mm board is approx equivalent to 100mm fibre glass ( check manufacturers u-values to compare) IMO you should really consider battening an additional 25 or 50mm.

50mm will be ok to store christmas decorations etc but if its to be used as a "Games room" or "Study", then increase ceiling insulation.:)

Lex Luthor

Fair play to ya RKQ. You are busy replying to all the insulation queries at the moment....Welll done

hopalong_ie

Yes, i agree fairplay, i understand a 100mm is desirable in the ceiling. Do you feel though and this is what i'm most intrested in, within the insulation envelope of the new room is 200mm of rockwool enough, will use 100mm polyiso outside of the envelope but was thinking rockwool within to save costs and avoid having to move plumbing etc.

RKQ

hopalong_ie wrote: »

Yes, i agree fairplay, i understand a 100mm is desirable in the ceiling. Do you feel though and this is what i'm most intrested in, within the insulation envelope of the new room is 200mm of rockwool enough, will use 100mm polyiso outside of the envelope but was thinking rockwool within to save costs and avoid having to move plumbing etc.

Not sure what you mean by the insulation envelope. If by envelope you mean 200mm fibre glass in the floor with 100 - 150mm in vertical walls and sloped ceiling with 250- 300mm in attic ceiling (depending on u-values of insulation) would be good. Insulation boards are thinner but can give a better u-value, so check manufacturers IAB Cert and u-values.

http://www.insulation.kingspan.com/ireland/pdf/futureproof_solutions_guide.pdf

DIY refurbishment would have a different requirement to new build but ideally both would be the same. The more insulation the better.

RKQ

Here's a copy of page 11 - Document F of the Building Regulations.

Roof ventilation Regulations.pdf

It will help you see where the 50mm air gap must be maintained, to ventilate a pitched or flat roof.
Regards RKQ

bibibobo

RKQ wrote: »

Here's a copy of page 11 - Document F of the Building Regulations.

Roof ventilation Regulations.pdf

It will help you see where the 50mm air gap must be maintained, to ventilate a pitched or flat roof.
Regards RKQ

Sorry to hijack the thread but my pitched roof does not have any air gap at the eaves. Not sure if they were ever there since the fascia soffits have been covered over with newer PVC types which have vents but this is not brought through the old timber to the attic. Pitch is not insulated neither is the floor joists (work in progress). There is an old asbestos vent in the roof from the previous heating system. There is practically no air movement in the attic at all. Timbers are in good nick. If I insulate between the floor joists will i get problems with condensation etc?

RKQ

bibibobo wrote: »

my pitched roof does not have any air gap at the eaves. Not sure if they were ever there since the fascia soffits have been covered over with newer PVC types which have vents but this is not brought through the old timber to the attic.

Why not remove vent covers in pvc, drill new holes in old timber soffit and put back vent covers. A slight continous 5mm gap is all thats required, so it might exist in your old timber soffit.

Ventiltion is very important - it prevents dry rot!

bibibobo wrote: »

If I insulate between the floor joists will i get problems with condensation etc?

No, if you don't have a problem now you won't if you install insulation. Consider putting down a plastic vapour barrier. Recommend 300mm overall thickness of insulation. Layer between joist and then a layer perpendicular to joists.

bibibobo

my attic joists are ~ 110mm - 120mm depth and spaced irregularly (1970's house) but with a typical interval of 32cm. would i be best to install the first layer between the joists at a thickness of 100mm or 150mm fiberglass? Obviously the 150mm would be compressed slightly by the overlaid fiberglass at 90 degrees to the joists. if i installed the 100mm then there might be a gap of 2cm from top of joist insulation to 90 degree insulation (assuming the fiberglass when unrolled pops up to the specified 100mm).

RKQ

bibibobo wrote: »

my attic joists are ~ 110mm - 120mm depth and spaced irregularly (1970's house) but with a typical interval of 32cm. .

120mm might be hard to get, so I'd go 100mm between joist and 200mm perpendicular.