Bear with me on this as I have no pictures!

I am waterproofing my basement with 'bubble' drainage membrane.

The manufacturer has identified a risk of interstitial condensation on what should be the 'dry' side of the 'bubble' membrane when insulation is installed to the walls.

Their recommendation to address this is in 3 parts:

1. Do not join the wall and floor membranes, so that if interstitial condensation *does* occur it can drain harmlessly into the perimeter drainage channel. (This is fine for me, but would not be fine if you were also using the membrane as a Radon-resistant layer).

2. Install humidistat-operated ventilation.

3. Install a good vapour control layer (VCL). Here is where I get the issue.

On the walls and floor I have used extruded polystyrene (XPS) as both insulation and (because it is vapour-closed) VCL, with all joints taped.

The basement is not intended to be habitable. It is to be used as cool dry storage, and will (nearly) always be kept (unheated) at a lower temperature than the habitable rooms above.

The logical completion of the VCL is to lay a plastic sheet over the basement ceiling joists before fixing plasterboard, with this sheet taped to the wall's XPS. In the void between the basement ceiling joists (the ground floor joists) is quilt insulation, to reduce heat-loss from the habitable rooms to the cooler basement.

Normal 'insulation law' says that there should be a VCL on the warm (habitable ground floor) side of any insulation, but for vapour control in the basement, I will have fitted a membrane on the 'wrong' side (the potentially moisture-affected side of the basement rooms, but the cold side of the insulation below the ground floor.

Worst-case scenario would appear to be a pool of water in which the quilt sits, on top of the basement ceiling. I have considered an (expensive) 'intelligent' membrane, which would be vapour-closed to the basement in most circumstances, but allow breathing to the basement 'in extremis'.

Any thoughts gratefully received.

Nick

Posted By: Nick ParsonsI am waterproofing my basement ... Do not join the wall and floor membranes ... This is fine for me

Are you sure you're 'waterproofing' only against rain-damped ground, not 'tanking' against standing hydrostatic pressure? Sure no spring-line, at any time of year?

FT asked: ''Are you sure you're 'waterproofing' only against rain-damped ground, not 'tanking' against standing hydrostatic pressure? Sure no spring-line, at any time of year?''

No, I don't think I am only waterproofing against damp ground. I *do* believe that there is ground water sometimes.Although the membranes are not connected, they both 'dump' into perimeter drains with a cross-section of over 250mm2, draining (in my case) to an external drain. (Others may use a sump and pumps). The floor membrane is a 20mm formed 'egg box' section, which can also act as a reservoir if required. The manufacturer recommends this set-up as acceptable.

Ed Davies asked: ''Presumably you're not wanting to take the floor above up to put the VCL above the joists, where it ought to be, because of the work and disruption? Is there also a “building science” reason not to do that?''

In the case of the current room, which is below the kitchen, I really cannot de-commission at this stage.

As to ''Is there also a “building science” reason not to do that?'', I don't *think* so, but it feels a little odd when a cold zone meets a warm one, and the top of one zone is the bottom of the next.

djh, I agree it does not feel like a good idea. You may be right about the Intello. As regards any w.v. finding its way to the perimeter drain, it might, but I just don't like, in principle, the fact that it may do so down the 'dry' side of the membrane - it doesn't feel right!

Posted By: Nick ParsonsAlthough the membranes are not connected, they both 'dump' into perimeter drains with a cross-section of over 250mm2, draining (in my case) to an external drain. (Others may use a sump and pumps). The floor membrane is a 20mm formed 'egg box' section, which can also act as a reservoir if required. The manufacturer recommends this set-up as acceptable.

OK, I like that - more robust than simply trusting a pinprick-free membrane!

That being the case, who cares if there's condensation on the egg-crate, when it can drain freely? Why not leave it at that, omit so-called vapour barriers which we all know can't be trusted as better than a mere vapour-check (somewhat resisting/slowing the inexorable vapour flow) but which completely preclude re-drying inward?

Plentiful re-drying potential is 1000% more valuable, long-term robust and realistic than false hopes of sealing-off against the powerful pressure-engines of vapour diffusion.

Posted By: Nick ParsonsThe basement is not intended to be habitable. It is to be used as cool dry storage, and will (nearly) always be kept (unheated) at a lower temperature than the habitable rooms above.

For interest's sake, how much insulation did you apply ? for example, "just token", or the same (or more) as your above-ground wall insulation ? Also, was the Basement ventilated (to outside...) and were such vents / intakes covered up by new insulation ?

As I understand it, from web-gleaned stuff, insulating the basement by definition means it is "heated" even if it does not have a heater... and it will likely be *further* heated by heat-loss from floor-above despite your ceiling insulation... (Notwithstanding, this should not necessarily counter your intent for "cool, dry storage").

I suspect that the real problem will actually be air-management, and the need for ventilation, and the potential weak spot could be the timber joists. I'd suggest that the floor needs to be left open to air : also, as stated above, if ever you had a kitchen leak, the joists would need to dry out etc...

It sounds like you have actually constructed something analogous to a "conditioned crawlspace" (albeit habitable): in which case it could be supplied with *house* air, via floor vents... allowing circulation meaning that the CS is part of the heated volume. If you are storing books etc down there, this could be advantageous...

Alternatively, the CS could be ventilated on a lost-air principle, requiring an air vent and a fan, shoving house air downstairs : the airflow only needs to be quite small - from memory, around 10 CFM for 50 sq. foot of CS floor area.

(In addition, in some circumstances, you will likley get upward air movement through house grilles due to stack effect, pulling external air into the Basement then into the house).

There are other ventilation strategies also...

The important thing, I suspect, is to install an RH meter and keep an eye on it...

gg