SIP Garden Room Base

[EdSta]

Hi all, I’m shortly going to be building a garden room using a SIPS system. I’m getting conflicting advice about the foundations. I’ve laid a reinforced concrete slab. The floor will be a SIPs panel (in hindsight perhaps not the wisest choice!). In some places it looks like the floor panels can be laid directly on the concrete (above a DPM), others suggest it needs to be vented and sit on bearers (say 50mm above the slab). I’m not keen on the bearers idea because it gives a place for vermin to reside and I don’t understand why you would need ventilation if you’ve got a VCL on the inside and a DPM below (and indeed that would be no different from how a SIPs roof would be in terms of ventilation). I’m waiting to hear from my supplier but have sketched out a proposal to put it on 50mm insulation on top of the slab with a DPM above the insulation but below the SIPs floor. Very interested in the views of others whilst I wait to hear. Many thanks, Ed

[Russell griffiths]

Replace the pir with XPS , make sure you put some form of rodent barrier over the ends and sides. 

[EdSta]

Thanks for the reply Russell. Seems like a good call on the XPS. Interestingly my provider said no problem on the breathability side of my proposed build up but expressed a bit of concern about point loading on the walls so I might use a timber perimeter with the XPS inside. 

[Russell griffiths]

20 hours ago, EdSta said:

Thanks for the reply Russell. Seems like a good call on the XPS. Interestingly my provider said no problem on the breathability side of my proposed build up but expressed a bit of concern about point loading on the walls so I might use a timber perimeter with the XPS inside. 

Get a higher loading xps. 500 or more. They build car parks on it. 

[Onoff]

On 27/05/2025 at 20:26, Russell griffiths said:

make sure you put some form of rodent barrier over the ends and sides. 

 

A layer of broken glass used to be put down around here before they concreted the slabs...

[saveasteading]

If there was local ponding, then  it will reach above the concrete slab.  Never rely on dpm providing tanking. I would lift the slab higher.

XPS is half the price and half the insulation of PIR. Which is your priority?

 

48 minutes ago, Russell griffiths said:

They build car parks on it. 

Yes but...   if you walk on it, it dents. A car wheel spread over a wide area is less pressure than the wall sitting on the edge, as shown.  I would sit the wall on the concrete or a kerb.  insulation lost there is easily regained elsewhere.