Insulating room above undercroft garage (but don't want to lose head height in garage)

[JBM2015]

Hi All,

Have received some brilliant advice on here before so hoping to get some again.

Scenario
Living room/hallway above undercroft garage currently has 100mm Rockwool or similar between joists. Garage is completely uninsulated so you can imagine how cold living area is, especially with hardwood flooring as well. Obviously every joist a cold bridge too. We didn't install any of this, it was here when we moved in.

Want to upgrade the insulation and insulate garage too but crucially the head height of the garage is 2.1m and I don't want to lose anything further if I can help it. Architect who did some preliminary drawings a few years ago suggested taking up 6" slab on garage floor and digging down further 6" to achieve a) insulated floor and b) more head room which makes sense but cost is prohibitive - the area is 35m2.

Was thinking upgrading to 100mm Celotex and 20mm insulated board for garage ceiling and 100mm celotex to all the walls...and then maybe a floating floor down there - perhaps the expensive vacuum PIR board (not used for parking).

Anyone have any experience with the vacuum PIR boards, or thoughts on how else I could do this without a) massive costs in digging up/re-laying floor slab and b) further reducing head height.

Thanks

 

[freddiemercurystwin]

What size are the joists? In anycase fill the voids between the joists completely with insulation then board underneath with your 20mm continuous insulation. Do that before you even consider doing anything else to the walls or floor of the garage.

 

[JBM2015]

Thanks for the reply. They're 6x2s.

By chance, do you have any experience with the vacuum PIR board? Had a quote from Kingspan which I didn't think was too bad. £85 per m2 for 20mm with a 0.29 target u-value. Be looking to use this on a floating floor system down there.

 

[JohnD]

Insulate between the joists. Most heat loss through a floor is by cold draughts, especially round the edges at the gap under the skirting, and through any holes for pipes and cables.

Mineral wool is very suitable for this as it can be stuffed into irregular gaps to block draughts without needing precision cutting. It is also cheaper than foam.

Have a fire foam gun with you for the edges, gaps, holes and joints.

Here's one I did earlier.



Do any wiring for sockets, lights, LAN, alarms, phones, cameras etc while you have access.