Insulating loft room

[CLASSMAZ]

Hello All
I have just moved into a timber framed 4 bed detached house built in 1999, it has a room in loft which was completed at the time of building. I am shocked at the annual estimated gas usage 22,500kwh which is double what we used to use in our old house (3 bed detached built in 2002).

I have looked in the eaves and noticed that the dwarf walls have not been insulated. When purchasing the house, I did ask my solicitor to find out if permission for the loft room had been sought and was assured it was part of the initial house construction. I would have thought the building regulations at the time would have insisted that it was insulated?

We can't really afford to rip the room apart and start again, I am hoping to do a temporary fix and insulate the dwarf walls and the two eaves hatches on the eaves side.
I am hoping to use Foiled both sides PIR unless you advise differently. The wall length is 11m long and 1.2m tall and looking at the framing board the widths vary from 450-590 with a 90mm depth which reduces to 45mm at the horizontal supports. I am hoping to use 100mm thickness with gapotape, would I have issues with this, or should I try and obtain 90mm? Also do you think I need to use a multifoil insulation on top of the PIR?

Any advice/tips appreciated as the house is leaking so much heat.

Thanks
Mike

 

[flameport]

I am shocked at the annual estimated gas usage 22,500kwh which is 6 times more than we used to use in our old house (3 bed detached built in 2002).

6x less is highly implausible.

If you really used only 3750kWh of gas in your previous house, it would be about 10kWh per day, which is equivalent to a typical house hot water demand only with no heating at all.
At 5p/kWh which was towards the higher end before the recent price increases, your annual gas bill would be less than £200.

Average annual gas use for a UK home is 13600kWh/year, and a 4 bed detached will typically be more than that.

Insulation will certainly help to reduce costs, but you will not be paying ~£200 per year for gas under any circumstances.

 

[CLASSMAZ]

Apologies flameport - I have Just double checked my bill and I got the electricity and gas annual usage mixed up. I have now amended original post to say double rather than 6 times.