Insulation of 130mm between rafters...is that right?

[ipoppy]

Hi There,
I wonder if someone can tell me what regulations are for insulation between rafters set on peaked roof. I have read this forum througout and it seems that insulation thickness are vary from place to place. Architect told me that insulation should be at least 130 mm with gap of 25 mm based on size of rafters:
Depth - 190 mm
Thick - 45 mm
Distance between rafters are 560 mm

130 mm seems a bit too much for me to be honest but thats why I am here to ask if that is right.
Also I wonder if I am entitle to receive grant from government for that type of insulation. Be specific I am not over 70, live in the top floor apartment and between loft and my flat there is already insulation placed.
Thanks in advance.

 

[freddiemercurystwin]

You'd need about that in a new build assuming its rigid insulation, normally you'd stick 80mm between the rafters and 50mm below (on the face of) the rafters. Depends what insulation you've got there already really but sounds like whats there already is pretty useless. Why don't you just stick some more mineral wool in at ceiling level? I presume its a non-converted loft you're talking about?

 

[ipoppy]

Hi,
Thanks for quick reply. As a matter of fact I am converting that loft into a liveable space. These apartments are not relatively new; they are about 10 years old. As I said there is already insulation but inside floor (glass mineral wool), which I think wouldn't change much in terms of insulation thickness going onto roof.
I will attach picture if that any help.
Thanks again.

 

[freddiemercurystwin]

U values are a measure of the amount of heat transfer you get through a particular element ie wall, roof, floor etc. Building Regulations determine what U values are required. The lower the figure required by the Building Regulations the more heat is retained inside the building.

Your house was built 10 years ago when the required U values were higher. The insulation above the ceiling would not achieve todays required U value's. You need about 250mm in a new build today.

Rigid insulation has a lower thermal conductivity (than mineral wool insulation) which means it resists the heat transfer more which in turn means you need less thickness than mineral wool. If he spec'd mineral wool you'd need (as mentioned) about 250 and it wouldn't really work as you'd need to make the rafters deeper and would give you a lower ceiling level. Your architect is correct.

I can't see how you would get a grant as you are doing a conversion not just making your house more energy efficient.