DIY conversion of loft space into 'bonus room'

foolishboy

Morning all!

..thinking of sorting the loft to make a very occasional office space / storage room.

It's a 1930s Semi, approx loft floor area of 40sqm. Floor joists are 4x2" with a couple of extra 4x2" at right angles near the front and rear of the loft space. Roof rafters are also 4x2" with boards then tiles .. no felt. Joist and rafter spacing is around 14". I have a ton of rock wool insulation available so would like to utilise that somehow.

In my head, and after quick chat with builder the plan of action is:

- Fit velux window
- Lay 55mm space board throughout loft
- Lay 18mm chipboard panels on top of space board
- Pin rockwool to roof slats between rafters
- Put insulation boards (tbd) at 90 degrees to roof rafters
- Put foil backed plasterboard (is foil backed required?) on top of this

On reading various forums I've also seen people say that laying more 4x2" at right angles to existing joists then flooring on top is best course of action.

Opinions? People done this before? When we were house hunting we saw plenty of houses with 'loft rooms' so we cannot be alone.

Cheers!
FB

Al1x

Have you spoken to a few builders? I would get a few quotes from different companys.

We don't use foil backed plasterboard but the insulation board we use (between rafters and on the outside walls) is foil backed - we often use 90mm between rafters etc then inch stuff over the top, foil tape the joins then plasterboard over the top. afaik we haven't used space board, just use the rockwall insulation in wall joining to neighbours property and in the floor between joists and put the chipboard flooring over that.

foolishboy

Al1x wrote: »

Have you spoken to a few builders? I would get a few quotes from different companys.

We don't use foil backed plasterboard but the insulation board we use (between rafters and on the outside walls) is foil backed - we often use 90mm between rafters etc then 10mm over the top, foil tape the joins then plasterboard over the top. afaik we haven't used space board, just use the rockwall insulation in wall joining to neighbours property and in the floor between joists and put the chipboard flooring over that.

This is mostly DIY (apart from Velux) with a builder's guidance - we've just spent £20k with him so on quite good terms :-)

After some more googling I'm thinking (for the rafters) of stuffing some rock wool in between rafters (do I need a breathing gap seeing as it's not solid insulation?) then either foil roll / celotex (would rather do foil due to cost obviously but whatever is suitable, then plasterboard battened to it all.

Cheers,
FB

System

you would never get something near building regs with rockwool between the rafters, usually you have a layer of rigid insualtion between them and then another (continuous) layer on the inside... if you have a vented roof space at the moment, there must be a clear air gap over the insulation

foolishboy

the_r_sole wrote: »

you would never get something near building regs with rockwool between the rafters, usually you have a layer of rigid insualtion between them and then another (continuous) layer on the inside... if you have a vented roof space at the moment, there must be a clear air gap over the insulation

Makes sense. So, as my rafters are 100mm deep, I'm thinking:

75mm Celotex between rafters (leaving 25mm gap)
25mm Celotex sheets over this
Foil tape the joins
Battens across and plasterboard (foil backed required?) to the battens?

..not fussed about regs, it'll never be anything more than a 'comfortable storage space' but I don't want it to be a bodge or ice cold.

Cheers!
FB

Al1x

foolishboy wrote: »

Battens across and plasterboard (foil backed required?) to the battens?

You shouldn't need to use battens (is that what you are planning on screwing the plasterboard to?) You just have to make sure you mark where your rafters are and then use long enough screws to go through the inch insulation and into the rafters.

I don't think you'd need the foil backed insulation if you are using celotex and foil taping the joints.

foolishboy

Al1x wrote: »

You shouldn't need to use battens (is that what you are planning on screwing the plasterboard to?) You just have to make sure you mark where your rafters are and then use long enough screws to go through the inch insulation and into the rafters.

I don't think you'd need the foil backed insulation if you are using celotex and foil taping the joints.

Awesome .. another few ££ saved :-)

Only real dilemma is on the flooring now.. will the extra 4x2s at 90 degrees really strengthen much? Or just add additional weight?

Cheers,
FB

latecomer

Just to give you some idea of what we had to do for our loft conversion

Existing Floor
6"x2" joists at 16" spacing

We were iniitally required to double up to 8"x2" C16 but managed to get this down to 7"x2" C24 with extra support around the openings i.e. stairs.

The roof had to be 50mm clear air gap, 100mm of Celotex tuffR between rafters and then 40mm Celotex across this. there was then a vapour barrier over the top of the insulation underneath the insulation.

foolishboy

latecomer wrote: »

Just to give you some idea of what we had to do for our loft conversion

Existing Floor
6"x2" joists at 16" spacing

We were iniitally required to double up to 8"x2" C16 but managed to get this down to 7"x2" C24 with extra support around the openings i.e. stairs.

The roof had to be 50mm clear air gap, 100mm of Celotex tuffR between rafters and then 40mm Celotex across this. there was then a vapour barrier over the top of the insulation underneath the insulation.

Thanks! That was for a full conversion though yes? This is only intended to be a 'comfortable storage area'. Doing it to regs (within reason) is not our aim.

Cheers,
FB

latecomer

Yup was for a fully spec'd and approved conversion. I believe the 50mm air gap is to allow sufficient airflow to prevent moisture build up. although I'm guessing if its not heated then this might not be so much of an issue

foolishboy

latecomer wrote: »

Yup was for a fully spec'd and approved conversion. I believe the 50mm air gap is to allow sufficient airflow to prevent moisture build up. although I'm guessing if its not heated then this might not be so much of an issue

That's what I read too .. unless you use a material that's suitable for a 'full fill' application - not that I've found any yet!

..I'd be leaving 25mm - I was told I'd need less of a gap as my tiles don't have any felt underneath them - just slats - it's quite breezy up there! lol

Cheers,
FB