Garage conversion floor design, u values and meeting building regs

[Timberframeguy]

Good evening,

 

I'm about to embark on a garage conversion with a small extension to the rear of the garage to match an existing rear single storey extension across the back of my house.

 

My current conundrum is around the garage floor and meeting the required U values for BC, the garage has an existing concrete slab floor which falls to the front of the property by around 100mm over the 8m. 

 

To meet a u value of 0.18 kingspan advise a DPM then a 90mm PIR with a T&G floating floor. This would give a total thickness of around 108mm above the slab, unfortunately the step down into the garage from the kitchen is only 90mm and  there's even less at the very back of the garage (~70mm). 

 

I really would like to avoid digging up the whole slab if possible, any insights would be much appreciated.

 

Thanks

[ETC]

You will only need a U-Value of 0.25W/m2K and in any case if it causes problems with adjoining floor levels you can use less.

[Timberframeguy]

9 hours ago, ETC said:

You will only need a U-Value of 0.25W/m2K and in any case if it causes problems with adjoining floor levels you can use less.

Thank you for this information, that is absolutely huge news for my project. Please could you link me to the specific regs on this? Thank you!

[Timberframeguy]

Sorry I see now that image below your post was the regs I require, I’ll have a delve into them when I can. At first glance it would be this table for a garage conversion? 
 

This has some pretty huge implications for my build as the current architect design is to meet 0.18 in the walls and floor. 

[Redbeard]

8 minutes ago, Timberframeguy said:

This has some pretty huge implications for my build as the current architect design is to meet 0.18 in the walls and floor. 

May we ask how the archo suggests you achieve 0.18 on retrofitted walls? If it's via EWI that is probably OK. If it is via 'hard-to-the-wall IWI that sounds like interstitial condensation waiting to happen. What is the plan so far? Note however that if you can get to 0.18 rather than 0.3 (in actuality rather than on paper) it will/should be better for the occupants in the long term.

[Timberframeguy]

Constructing a full fill 150 cavity

off the existing single skin brick work.
 

There is the requirement to allow for a future second storey above. I’m hoping we can now go to a 100 cavity and deal with the u value requirements above in the future. 
 

Would there really be a noticeable difference for the occupants (us!) by getting down to 0.18? 

Edited May 12 by Timberframeguy

[Redbeard]

2 hours ago, Timberframeguy said:

 

Would there really be a noticeable difference for the occupants (us!) by getting down to 0.18? 

 

Noticeable? While you are stood in it with the heating on? Probably not. To your heating bills? Well, the 'through-the-wall' losses will be just over half compared to 0.3. That won't look like a big sum in an intermittently-heated space in a year, but if this is your lifetime home and more of your later lifetime may be spent on the ground floor, perhaps wanting to be a bit warmer as the years go by...

[Timberframeguy]

On 12/05/2025 at 11:43, Redbeard said:

 

Noticeable? While you are stood in it with the heating on? Probably not. To your heating bills? Well, the 'through-the-wall' losses will be just over half compared to 0.3. That won't look like a big sum in an intermittently-heated space in a year, but if this is your lifetime home and more of your later lifetime may be spent on the ground floor, perhaps wanting to be a bit warmer as the years go by...

Thank you for your reply. I take your point, it's a 'cost now' versus future energy savings versus (in the case of cavities) a loss of internal area. A 100 cavity using a 32 type cavity batt is coming in at 0.24 which I'm leaning quite heavily towards as it's saves on floor area in what is already a reasonably narrow garage and does not interfere with the garage door opening (a 150 cavity will). 

 

This brings me back round to my initial conundrum; the floor. I've established the concrete slab can remain intact with a floating floor meeting a u-value of 0.25. Does anyone who's read this far have any insight into how to overcome the fall in slab when building directly off it? Mine falls by around 100mm over the length - scribing or packing I suppose, at first thought although this would introduce a floor cavity below the insulation.

 

Thanks again.

[Redbeard]

Make a timber shutter which gives you a level top surface. Perhaps a coat of PVA on the existing sloped concrete, then dump concrete  (perhaps just cement and grit-sand; no gravel) in. Wait for it to go off. Remove shuttering, point in the gaps left by its removal, leave to dry then build off it.

[Timberframeguy]

Thank you, if you are able could you clear up a couple of additional queries:

 

 - Can I dig a soil pipe channel through the existing garage floor then make good and screed as you've outlined above? 

 

- For an extension to the rear of the garage would BC be happy with insulating over a newly poured slab to match the level in the garage conversion and carry the floating floor across the entire area? The new floor would of course require more insulation to meet the u value for new elements (0.18). 

 

Much appreciated!

 

 

Edited May 15 by Timberframeguy