Detailing insulation / VCL below internal stud walls

[andyscotland]

I've realised there's a detail on my garage conversion that I'd not quite thought through.

 

There are a couple of internal (non-loadbearing) stud walls to divide off rooms within the space.

 

At the moment I have the soleplates for these sitting directly on the concrete slab (bridging the floor insulation). But I hadn't quite thought about how the VCL would detail round these (and hadn't drawn it on the building control plans). I've drawn what I think would be easiest on my section below - the VCL passing below the soleplate so it's continuous across the whole area. However this will allow vapour to get down to the cold slab/soleplate junction and I'm wondering if that's a condensation risk.

 

Alternatives I can think of are:

• Make the PIR and VCL continuous across the whole slab, build the wall on top of the insulation. But not sure if that would be OK structurally in terms of loading on the PIR?
• Keep the soleplate directly on the slab, but take the VCL over the top, sealing it around all the vertical studs. So the bottom timbers are cold but dry (though moisture could still penetrate through / around the vertical stud junctions?)
• Take the VCL between the soleplate and the bottom member of the wall - so the VCL is continuous, and the coldest timber is dry - the bottom member should be less likely to hit dew point?
• Sit the wall on something insulating with more compressive strength (compacfoam or marmox thermoblock) to reduce the thermal bridge and keep the timber warm - but looks relatively pricey / and/or hard to source in small quantities.

 

Or am I worrying too much? Only one of the walls runs to the outside wall of the property, the others are all in the centre of the slab or adjacent to the house - I know most slab heat loss is around the exposed perimeter so potentially where my walls are sitting won't get too cold?

 

What do you all think?

 

Cheers

 

 

 

[Oz07]

What about sole  plate ontop of chipboard how much weight is in your partitions 

Edited December 15, 2019 by Oz07

[andyscotland]

47 minutes ago, Oz07 said:

What about sole  plate ontop of chipboard how much weight is in your partitions 

 

Good question, not certain - I'll try to work it out. Not enormous I think (they're only about 2.4m tall), though the heaviest will have 18mm ply and shower panels on one side, potentially fermacell on the other. So a bit heavier than a standard plasterboard stud.

 

I'd not got them sitting on the chipboard as that's just floating, I'd read somewhere some concerns about whether the wall would move as the flooring expands/contracts. though perhaps could fix the soleplate down through the chipboard and insulation to eliminate that.

[Russell griffiths]

Why not get some eps300 and substitute the insulation under the sole plate with this, just a 300mm wide strip with the pir betting up to the sides of it. 

[A_L]

PIR has a 10% deformation compressive strength of 120kPa (or higher). This is too high to use but tells us that the 1% value is 40kPa or better. This corresponds to about 4000kg/m2 . If your sole plate is 50mm wide then they could load the floor to 200kg/m run.  

[Oz07]

I don't think the chipboard will expand too much. Like when you leave 10mm expansion on things that you also have to nail. I'd be suprised if most chipboard would expand any more than 3 or 4 mm round the whole perimeter of a room

[andyscotland]

18 hours ago, Russell griffiths said:

Why not get some eps300 and substitute the insulation under the sole plate with this, just a 300mm wide strip with the pir betting up to the sides of it. 

 

Should EPS300 be easy to source for a couple of sheets? I made a couple of calls this morning to my usual merchants but they didn't carry it. A quick google is only turning it up online in quite big quantities.

 

18 hours ago, A_L said:

PIR has a 10% deformation compressive strength of 120kPa (or higher). This is too high to use but tells us that the 1% value is 40kPa or better. This corresponds to about 4000kg/m2 . If your sole plate is 50mm wide then they could load the floor to 200kg/m run.  

 

Thanks. Looked at the eco-versal datasheet as that's what I have : they say a continuous static load of 30kN/m² (possibly due to compressive creep?). I am assuming that's a working limit, rather than a failure load that needs an additional safety margin applied.

 

The narrowest studs I have are 38 x 63 CLS. If I have it right, that works out to a load limit of 1.89kN/metre of wall. Which would be 192kg/m.

 

By my reckoning, the walls weigh about 100kg/m as below (plus a little bit for fixings, filler, paint, etc I guess):

 

item	qty	width	depth	height	volume	density	mass
		m	m	m	m3	kg/m3	kg
soleplate	1	1.000	0.063	0.038	0.0024	370	0.89
base member	1	1.000	0.063	0.038	0.0024	370	0.89
noggin	1	1.000	0.063	0.038	0.0024	370	0.89
top member	1	1.000	0.063	0.038	0.0024	370	0.89
head	1	1.000	0.063	0.038	0.0024	370	0.89
stud @ 600 ctr	1.67	0.038	0.063	2.500	0.0060	370	2.21
insulation	1	1.000	0.063	2.500	0.1575	10	1.58
fermacell	1	1.000	0.013	2.500	0.0313	1200	37.50
ply	1	1.000	0.018	2.500	0.0450	680	30.60
ply battens vert	2	0.020	0.020	2.500	0.0080	370	2.96
ply battens horz	4	1.000	0.020	0.020	0.0004	370	0.15
wall panel	1	1.000	0.010	2.500	0.0250	800	20.00
total mass per metre of wall (kg/m)							99.43

Does that seem plausible? If so then it would be well within the bearing capacity of the PIR, I think? One slight complication is that a couple of the walls have doorways, so the weight will be a bit different and the bearing area will be smaller, but if the above calc is correct there's a fair bit of headroom and the calc doesn't allow for any weight being taken where the end studs fix into the adjacent masonry walls.

 

17 hours ago, Oz07 said:

I don't think the chipboard will expand too much. Like when you leave 10mm expansion on things that you also have to nail. I'd be suprised if most chipboard would expand any more than 3 or 4 mm round the whole perimeter of a room

 

3 or 4mm could be enough to make a difference at the bottoms of the walls though (especially in terms of the tile/skirting/whatever junction in the bathroom)? And maybe a risk of forces on the wall (e.g. from opening/closing doors) pushing the chipboard towards the perimeter expansion gaps - though I guess there'd be quite a lot of friction to overcome. Just feel a little nervous about having a "floating wall". I know NHBC standards rule out building a wall off a floating floor - I don't have to comply with them, but presumably that rule is due to defects they've encountered in the past...

 

[PeterW]

Rip some 400mm strips from a sheet of 18mm OSB and set that below your standard flooring on top of a slightly reduced PIR if you can. It will spread the load and then glue and screw your chipboard to the edge of it.  It will spread the load of the wall and it won’t move and your floating floor will also stay put ... 

[andyscotland]

On 16/12/2019 at 10:56, PeterW said:

Rip some 400mm strips from a sheet of 18mm OSB and set that below your standard flooring on top of a slightly reduced PIR if you can. It will spread the load and then glue and screw your chipboard to the edge of it.  It will spread the load of the wall and it won’t move and your floating floor will also stay put ... 

 

@PeterW thanks - sounds like a good solution. I guess I'd need to reduce the PIR by 20mm down to the next standard thickness, so would then need to pack a few mm between the OSB and the chipboard to keep the levels? That should be doable.

 

I asked Ecotherm technical and they've come back and quoted 28kN/m² for a permanent loading figure at 1-2% yield, so does sound like increasing the bearing area as you've suggested is a good plan.

Edited December 17, 2019 by andyscotland