Floor joists for garage conversion

[ktp73]

I have BC sign-off on specification to convert my integral garage. There is already an insulated concrete floor, the specification is to put down DPM (sheet) and then to install 50 x 50 timbers resting on top (shimmed to deal with slope of floor), 50mm PIR between battens and finish with 22mm chipboard.

On further inspection the height difference between the floor of the existing house and the garage floor is greater than 50mm (more like 100mm to 150mm). My question is can I just use larger joists (with shimming as required) and then still use 50mm PIR (with battens to support)?

 

[tony1851]

Why not use thicker PIR if you've got the depth?

 

[ktp73]

Why not use thicker PIR if you've got the depth?

Mainly due to cost - Kingspan K103 @ 100mm is pretty much double the price of 50mm (I expect similar for other PIR manufacturers).

The target U-value is met by the 50mm PIR.

 

[freddiemercurystwin]

Polystyrene or its posh name Jablite.

 

[ktp73]

Polystyrene or its posh name Jablite.

Under the 50mm PIR?

 

[freddiemercurystwin]

What about a levelling screed then polystyrene with a floating timber deck?

 

[ktp73]

What about a levelling screed then polystyrene with a floating timber deck?

The spec says 50mm battens (joists) with 50mm PIR in between - all sitting on DPM. Shimming will be a pain as will be trying to get the PIR to sit flush with the top of the battens where they are shimmed but that's the spec. My only question is if the 50mm battens are more like 100mm joists (still shimmed) and I use 50mm PIR (supported by short battens attached to the joists to keep it flush with the top of the joists) then does creating a void have a negative impact on the thermal properties of the floor? I've asked the architectural tech. who did the drawings/spec but I thought I'd ask here to see if anyone would know.

I'm not looking to change the overall construction method here as that will involve going back to BC.

 

[mrrusty]

Shimming. Yuk. It's a PITA to do nicely. If it's just a few, you could use folding wedges screwed on to the bottom of your joists. Alternatively you could use a joist made with a sister either side coach screwed through so the sisters are on the concrete and the top of the joist in the middle is level, or level the floor first with screed, either with joists or a a floating floor over XPS. Anything to avoid trying to cut loads of different thickness shims and still end up with a solid floor.....

 

[Loofah]

+1 for sistering the joists.
Unsure on leaving a void but perhaps PIR on top of the DPM and fill the rest with wool?

 

[ktp73]

I've got an answer from the technician now - the solution is to use wool instead as there can't be any void under the floor. If the depth is 150mm then wool gives the equivalent u-value of 50mm PIR. Where it is 100mm it is slightly worse than 50mm PIR but is acceptable.

Good news is that wool is way cheaper than PIR (comparing with Kingspan at least).