Detached garage foundation insulation - worth doing?

[Kelvin]

I'll soon be starting our garage build ahead of the house construction. I considered all the various options to build it before deciding on a insulated steel building. It's 40mm PIR in roof and walls with an insulated sectional door. I have an uninsulated foundation design but wondered if it's worth insulating under the slab and how best to do it especially at the perimeter. While it's a garage it'll never be used to store a car and will be used as a workshop so I want it to be useable all year round. 

 

 

[Redbeard]

If you want it to be useable I would bump up the 40mm PIR straight away and yes, insulate your floor. If you are looking for quick warm-up, however, you may be better with a floating floor on top of the slab rather than insulating the slab.

[Temp]

40mm isn't great. If you are likely to heat it I definitely look at perhaps doubling that or more. Would it be easy to add insulated plasterboard later?

 

100mm+ in the floor. 

 

Consult the supplier before you do the slab but Ideally arrange for it slab to be slightly smaller than the building and a bit above ground level. That way water can't pool on the top of the slab and possibly run under the walls. Instead it drips off onto the ground surrounding the slab. Alternatively make it bigger and raise the building on a brick course.

 

Either way plan for how water will be prevented from running under the door. Perhaps look at how they recommend doing a Part M level access front door on a house.

[Kelvin]

Boarding it out  with insulated plasterboard was something I am intending doing afterwards for a few reasons. It’s not exactly easy to do but it is doable. 
 

The size of the slab and how to detail for rain runoff is well in hand and very clear in all their drawings. The slab should match the dimensions of the kit so that the cladding overhangs the edge of the slab to let rain runoff. It’s more the build up and perimeter I’m trying to understand. 

Edited January 11, 2023 by Kelvin

[Kelvin]

Just finishing this off. Decision made. 80mm for walls and roof. Added £1500 to the overall cost. Plus an insulated sectional door. At some point I'll create a separate office that I can further insulate. 

 

In terms of the floor insulation. There are two ways they have done it. They've only ever done option 1 once before for a classic car showroom. All the others are option 2 with the obvious thermal bridge. 

 

 

Edited January 25, 2023 by Kelvin

[Roger440]

I did option 1.

 

As pic pic, allows an unbroken unsulating layer.

 

Edit: I used composite steel panels, so no internal works required as nice white painted surface.

 

For the pennies it cost, wack some underfloor heating pipes in while you do it. But dont let the BCO know.

Edited January 25, 2023 by Roger440

[Kelvin]

Looks good. 
 

I ruled UFH. A2A heat pump instead.