Partial garage conversion

[lwalton]

I'm looking at having a partial garage conversion done and have spoken to my local council regarding building regs. My understanding of building methods is quite limited so wondered if anyone could clarify if the requested construction makes sense.

One wall is an external facing single skin brick wall. For this wall the council have requested the following:
- A stud wall be built to provide a cavity from the brick wall.
- The stud wall should be built on top of the raised floor.
- It should have 100mm rigid insulation within the frame.
- It should have a breather membrane installed on the back of the stud frame (on the cold side in the cavity).

My main concern is whether there is a risk of moisture (from either inside or outside the building) causing problems within this cavity since the bottom of the cavity will be the floor boards of the raised floor.

1) Would it be a good idea to also install a vapour control layer on the warm side to prevent condensation within the cavity?

2) Should the breather membrane connect to the DPM in the floor?

3) Is there any way that water could form within the cavity? The cavity will have no ventilation and no where for water to run off, so is this an issue or is my understanding of how a cavity works wrong?

 

[noseall]

The only way I see it working is, when you fit the floor membrane (+insulation?), turn this up the inside of the garage wall.

I don't see how you can staple a breather membrane to the back side of a stud wall with a garage wall blocking you? No doubt you could construct the wall remotely, then staple it to the back before you erect it maybe? However, it won't nbe continuous if the stud wall changes direction.

 

[lwalton]

Yes it seems the only options would be either attach it to the garage wall instead, or construct the stud wall independently and move it into place with the membrane already attached.

 

[noseall]

Yes it seems the only options would be either attach it to the garage wall instead, or construct the stud wall independently and move it into place with the membrane already attached.

Me personally - I'd drape polythene up the wall - fit some insulation boards directly against the poly, then trap the lot back with a stud wall. I'd then PIR fill between studs and foil tape the lot - then fix foil back boards to the studs.

No air gap - no air.

 

[lwalton]

Thanks for the suggestion, I would prefer this approach, might have to see if the council would accept it.

 

[noseall]

Thanks for the suggestion, I would prefer this approach, might have to see if the council would accept it.

You need to present it in a way that the floor works with the walls and the walls work with the roof etc. A continuation of both the floor - wall insulation and the same for the DPM. The DPM for instance, need not continue from the floor all the way to the top, from the outset. You could say leave 600mm of poly, draped up the wall, ready for when the wall poly is draped down and laps over it.

The stud wall can be fixed top and bottom and need not puncture the poly.