Garage conversion - wall/floor detail, soleplate and DPM...

[tony1851]

What your doing is likened to a perpendicular floor structure but instead of timber board cover, yours is a plasterboard. If the studs are free standing from the block work wall, there should be little chance of condensation and if you create the ventilation between walls, what little could be caused, will be allowed to vented away. However these are my thoughts on the matter, it's up to you to put your thoughts into practice...pinenot

Pinenot; there should be a vapour barrier on the WARM side of the insulation, to stop interstitial condensation. The cavity between the blockwork and studwork is not there to vent off any condensation 'which might get through', but to allow any water penetration from outside to drain off. (If any internally-generated vapour did get through the insulation, it could condense on the outer face of the studs and cause problems). All basic stuff really.

 

[walty666]

Pinenot; there should be a vapour barrier on the WARM side of the insulation, to stop interstitial condensation. The cavity between the blockwork and studwork is not there to vent off any condensation 'which might get through', but to allow any water penetration from outside to drain off. (If any internally-generated vapour did get through the insulation, it could condense on the outer face of the studs and cause problems). All basic stuff really.

Yeah, that's what I thought, hence the requirement for a breather membrane on the outside of the studwork.

Trouble is how do I fit a breather membrane on the back of the stud if it needs to be built in situ? I assumed the breather needs to be run horizontally with the higher courses overlapping on the outside of the lower ones, which is going to be near impossible with the studwork in place and only a 2" gap to work behind. However the guy at Buildbase reckons I can run it in vertical strips which would be easier but probably not as effective. Unfortunately, I can't build all the studwork panels on the floor first due to some of the wall angles being non-perpendicular and the sloping ceiling, preventing them from being lifted into place. I was going to install the top plates, drop plumb lines down to get the sole plates in the right place then shoot vertical studs up between...

You sure the breather membrane can't go on the inside of the blockwork wall?

 

[pinenot]

Unless your planning on having a sauna or doing a lot of un-vented tumble drying in this converted garage/office, there's really little chance of producing so much moisture internally to merit putting something in, that bear in mind controls moisture in such a way as to allow for a gravitational fall, which would collect where??
Assuming a breather membrane between the inside of the framing/insulation and the plasterboard then any moisture would have to fall, stopping/ponding at the nearest obstacle, where it would receive no venting to dry it...not a good idea when you think of it. I know the information available/proffered is in the best of intention, but unless your prepared to vent from the room created through the plasterboard to the breather membrane, then there's little point in this, is there...pinenot

 

[^woody^]

Pinenot, you don't seen to realise the mechanics of how timber rots.

Add that to the list of things you don't seen to know about

 

[tony1851]

It gets worse.

 

[walty666]

I'm not proposing to put a breather on the inside of the stud?! It would go on the outside of the stud (or inside of the blockwork wall) and drain to the bottom of the cavity, ideally on the outside of the DPM, but that's the detail that I was hoping one of you lot might confirm...

 

[tony1851]

Walty; your problems here are twofold;

1. stopping interstitial condensation in your insulation and studwork and
2. stopping leakage into the cavity from the blockwork.

Number 1 is fairly staightforward, as you just apply a vapour barrier across the studs before fixing the plasterboard. Some argue that the foil on the PIR boards themselves is sufficient as a v.b.

Number 2 is much harder to solve because ideally the studwork is fixed first, and then the blockwork built up. In that way, a proper cavity tray can be incorporated at the bottom of the wall, and any leakage from rain will be directed away from the studwork and discharge through weepholes.

In your case, I don't see an effective way of stopping water ingress from getting at the sole plate. One way of keeping the studwork dry would be to build it on a couple of courses of brick (see sketch). This would preclude any insulated floor, and would perhaps be a bit 'shed-like' rather than home office. I suspect your initial design may be too sophisticated for the structure you already have, what with additional flooring/skirting boards etc.
If you don't want smelly damp/rot problems futher down the road, you may need to compromise and make your structure and finish a little more robust and utilitarian.

Untitled

• tony1851
• 13 May 2013
• 1

 

[walty666]

Why would putting the bricks in preclude insulating the floor? I could just run DPM over the brick before fixing the sole plate.

Alternatively, what's the problem with:

1. Suspending the breather membrane from the cavity side of the top plate and tucking the bottom between the DPM and wall where the DPM laps up the wall. I could fix it tight by clout nailing it to the bottom of the wall. Any moisture's going to run to the bottom of the wall albeit above the DPC, exactly where it came from in the first place - a wet wall. If it seeps under the DPM, so what. Isn't that what the DPM's for? If I make sure there's a good strip of DPC (possibly even sealed with some bitumen paint to the floor) beneath the DPM where the sole plate is, plus if I stick some silicon down the fixing holes surely that'll stop moisture wicking up to the sole plate. There's no such thing as rising damp right?

Or

2. Don't run the DPM to the wall but stop it at the cavity side of the sole plate and lap it up the back of the stud a bit. Then fix breather membrane from the top plate and let if fall to the bottom of the cavity between the DPM and wall. That way any moisture ends up in the 2" gap between the stud and wall which isn't protected by either the existing DPC or DPM beneath the floor slab. It's damp already so any moisture off the breather shouldn't make a lot of difference.

All the outside walls are rendered. How much moisture is there realistically going to be coming in from the outside?

I'm also using tanalised 4x2, not CLS, so should be resistant to a degree if moisture anyway.

If I add some low level cavity ventilation protected by weather/insect cowls as suggested by pinenot surely this will serve to keep the cavity relatively dry?

 

[pinenot]

Pinenot, you don't seen to realise the mechanics of how timber rots.

Add that to the list of things you don't seen to know about

I encountered blowhards like you on many of my sites, most left with tails between their legs, unable to suffer the indignity of being wrong.
In your instance before I ignore you I'll say this...your no often right, but yer wrang again

 

[walty666]

<<YAWN>>

Your comments are all very welcome (with varying degrees of helpfulness) but can we keep things constructive please. My materials are being delivered today so would like to make a start this evening. I understand the mechanisms of interstitial condensation from inside and the possibility of moisture being driven through the blockwork from outside, and the importance of the VCL, DPM and breather membranes.

I just want to know where best to terminate the DPM and breather membrane at the bottom of the cavity. I also don't understand what Tony means by:

This would preclude any insulated floor, and would perhaps be a bit 'shed-like' rather than home office. I suspect your initial design may be too sophisticated for the structure you already have, what with additional flooring/skirting boards etc.
If you don't want smelly damp/rot problems futher down the road, you may need to compromise and make your structure and finish a little more robust and utilitarian.

Why does including a layer or two of bricks preclude any insulated floor?
Why's my initial design too sophisticated for the structure I have?
How/why do I need to make my structure and finish a little more robust and utilitarian?

To all intents and purposes it's just another f***ing garage conversion. I just want a bit of help with the minute detail...

 

[geraldthehamster]

To all intents and purposes it's just another f***ing garage conversion. I just want a bit of help with the minute detail...

If you have a vapour barrier on the warm side of the studwork, under the plasterboard, you won't get condensation in the studwork or the cavity. The vapour barrier could be either plastic sheet, or the foil on the outside of your Kingspan, with joints taped.

If you have any sort of DPM at all under the soleplate, you won't get damp rising into it from the floor.

If the outside of your garage wall is rendered, painted and well maintained, you shouldn't get rain penetration through the blockwork.

In these circumstances I think breather membrane on the inside of the studwork serves no real purpose, especially with tanalised timber.

That's how I did my utility room anyway. It used to be a coal shed. I'd just do it and not over-think.

Cheers
Richard

 

[walty666]

Thanks Richard!