Roof Insulation: Kingspan vs Wool???

Matty0682

Woohoo :T100th post

Anyway.........I am in the process of renovating my house and am now ready to insulate the roof. As the loft area is exposed (no ceiling) we are going to put the insulation between the rafters and then insulated plaster board on top.
Unfortunately the rafter depth is only 60mm, therefore after leaving a small air gap we are looking at 45-50mm depth insulation (ideally this would be thicker I know)

I do not know what type of insulation to go for.

Choice 1
Kingspan Thermapitch 2400 x 1200 x 45mm boards (K value=0.023 w/mk, therefore R value = 2.1739)

Choice 2
Wool Insulation 50mm x 400mm *10m (K value = 0.038 w/mk, or R value = 1.316)
Please can someone advise me whether this difference in insulation K and R values are significant. The Kingspan provides better insulation based on these scientific values

However I like the idea of wool because:
1) its £200 cheaper (based on 100sq metres area to insulate)
2) it's easier to work with and fits between the rafters (easier to ensure no gaps etc) and it's in a 10m roll. With Kingspan a lot of cutting required
3) Natural Material

Is the difference is K value a big difference?

Thanks

latecomer

We had to have a minimum of 50mm clearance behind our insulation to allow airflow - this was required by building regulations. We had to have 140mm of insulation to meet the requirements - 100mm Celotex tuffR and then 40mm insulated plasterboard on top.

If its going to be inspected by building control it might be worth checking up on the requirements first.

iamcornholio

Fibre/wool insulation will be a waste of time/money at that thickness.

Full fill between the rafters with 60mm Celotex/Kingspan and then tape the joints with foil tape before boarding

Or full fill with fibre insulation, an then fix 25mm or better still 50mm of Celotex/Kingspan across the underside of the rafters, and again, tape all joints before boarding

Or better still, as above but Celotex/Kingspan between and under the rafters

aa4466

Before you do anything check what the structure of the roof is on top and find out what the ventilation requirements are. As latecomer said, you may have to leave a 50mm gap below the boards (if you're Scottish) / membrane (if you're English) so that air can travel from the eaves to the ridge to remove moisture. If you don't get it right then you risk damp, mould and rot. Your local council building control department may be a good source of help. Our local building control department has a duty officer on every day whose job is to answer questions from the public.

Your house will be full of warm moist air that will be trying to work its way out. If you use something like a foil-faced Kingspan / Celotex / Xtratherm board then the foil surface and greater density will act as vapour barrier to prevent the moisture travelling across the barrier. If you use something like wool without a vapour barrier then the risk is that as the moist air gets closer to the cold roof then you will get condensation within the insulation and end up with a soggy mess.

Think hard about how to achieve the vapour barrier. If you are using insulated plasterboard as the vapour barrier what will you do about holes for light fittings etc.?

None of these things are hard to overcome but it is important to get them right.

If you want to get an idea of what the U-value of the completed roof will be then Celotex have a good tool on their site to help.

http://www.celotex.co.uk/Technical-Services/Further-Information/U-value-Calculator

iamcornholio

An air gap is not required if you design out the chances of condensation occuring within the roof void

Matty0682

iamcornholio wrote: »

Fibre/wool insulation will be a waste of time/money at that thickness.

Full fill between the rafters with 60mm Celotex/Kingspan and then tape the joints with foil tape before boarding

Or full fill with fibre insulation, an then fix 25mm or better still 50mm of Celotex/Kingspan across the underside of the rafters, and again, tape all joints before boarding

Or better still, as above but Celotex/Kingspan between and under the rafters

Thanks for your helpful advise. We don't want to add an too much of an additional layer beyond the 60mm rafter depth as it will start to hide the original oak "A" frames (the cottage is 400 yr old).
If we were to full fill with fibre insulation, what would you use?
All the rafters will like a jig saw, so isn't it easier to fill all the gaps with fibre insulation rather than cutting kinspan?

Matty0682

aa4466 wrote: »

Your house will be full of warm moist air that will be trying to work its way out. If you use something like a foil-faced Kingspan / Celotex / Xtratherm board then the foil surface and greater density will act as vapour barrier to prevent the moisture travelling across the barrier. If you use something like wool without a vapour barrier then the risk is that as the moist air gets closer to the cold roof then you will get condensation within the insulation and end up with a soggy mess.

Think hard about how to achieve the vapour barrier. If you are using insulated plasterboard as the vapour barrier what will you do about holes for light fittings etc.?

Luckily no light fitting will be required on the roof, so the first vapour barrier will be insulated plaster board. I like Iamcornholi's idea of using fibre insulation to full fill between the rafters, then using kingpan on top of the rafterss, then plaster board