Foil in or out?

[88v8]

Being unbothered about insulation I must admit an unfamiliarity with its installation. Google generates conflicting information.

A lean-to built in the 90s, 6" timber frame on dwarf brick wall with dpc, internally plasterboard, then rockwooly stuff, then external ply, then Kraft paper with a foil face. Then 1" batten, then feather edge which laps 1" down over the brickwork.
All good one might think.

And yet, removing some of the feather edge I find that the Kraft paper is disintegrating at the bottom, the bottom few inches of the ply is damp-stained, the frame in places is rotted and the lower nails holding the battens are very rusted.

The Kraft has the paper facing out and the foil facing in. The room is somewhat damp, and has been unheated. I suspect condensation behind the foil.

Gradually I shall remove and renew all the feather edge.
The room will remain somewhat damp - one wall has no dpc and I know it's damp as the Snowcemy stuff is peeling off the bottom few inches.
The room will in future be heated, but to avoid a recurrence of the condensation, if that's what it is, I'm tempted just to remove the Kraft paper.

Google say the foil face could be inwards, or outwards. Would that make a difference?

What do we think?

Ivor

 

[MikeG]

Makes not a jot of difference, Ivor. It's in the wrong place in the wall anyway. That is a classic mis-built wall, with all the vapour barriers on the outside, which is where you actually need vapour permeable materials. Insides of heated houses are damper than outside, so the passage of damp is from the inside to out. Stop the damp entering with the wall with a vapour barrier on the inside face, and allow it to leave the outside with a breather membrane. Ply does not readily allow the passage of vapour (due to all the glue), so ideally it should be on the inside face of the timber framing, not the outside.

If I were you I would haul all the feather edge board off, take off the foil/ Kraft paper, and drill umpteen one inch holes in the ply (choose your bit carefully as this will snag the insulation). Make sure you have holes into each "compartment" formed by the studs. Cover the ply with some breather paper, then counterbattens.

I'll come back to this later.

 

[MikeG]

You need a free flow of air behind the external boarding, so vertical counterbattens, and no horizontal batten at the bottom and top. Here, though, you'll need insect mesh, which can take a bit of working out.

Ideally on the inside of the wall you would have a high resistance to the passage of vapour. This is where the ply or OSB really should be, straight behind the palsterboard. If you do ever take the plasterboard off the inside, then you have a number of choices including replacing it with foil-backed plasterboard, or using some 9mm OSB. This can be really handy in some rooms for the fixing of cupboards, wall units or sanitaryware, but its really purpose it as a vapour control layer.

I suspect the damage you have discovered is as a result of moisture penetrating the wall from inside the house, then condensing on the inside of the ply, running down the inside face and sitting on the sole plate. For a couple of decades now I have been advocating a completely different approach to modern timber framing, involving putting the sheathing board on the inside of the frame, and I've been predicting an international scandal as situations like the one you have found are discovered everywhere. Yet still we plod on doing it wrong. We even see the faintly ridiculous sight of breather membranes overlaying OSB sheathing on the outside of a frame! To what end? What on earth does anyone think that can achieve?

Oh, and don't be too anti-insulation. My heating bill in an unfinished 300 year old house is less than £100 per annum, and we're warmer in winter than anyone who is reading this.

 

[88v8]

Thankyou for that.

Having taken the ply off the other end of the structure I find there is in fact an internal vapour barrier, as there is dpm behind the plasterboard.
Then rockwool, then polystyrene slab, then ply, then Kraft paper/foil.
The ply was a bit degraded, a smattering of worm, but nothing compared to the other end.

Not sure how clear this pic is



but here at the rotten end we have an empire of ants. They've tunnelled into the polystyrene, eaten the soggy 6x2" frame from top to bottom, the only part that has survived is the treated batten.

In the ceiling space is more foil. Plasterboard, I hate that modern stuff, infilled with closed-cell foam slab lined both sides with foil. Whether this has caused condensation runoff I don't know.

I'll remove the rest of the external feather edge and see how far I have to remake the frame. And as you suggest, I'll drill some holes in the ply.

It's not insulation I'm anti.... it's the damage people do to install it. And the plasterboard they put on afterwards. Flat, straight, destructive of detail and patina. I suppose what I really detest is plasterboard.

Ivor

 

[MikeG]

Goodness, Ivor, you've a job on there. If that is softwood, I can't imagine what state it's in.

Oh, and polystyrene: rip it out. The old stuff is lethal in a fire.

 

[plasticpigeon]

What a pain, something built only 20 years ago is now ruined, all becuase of bad design and materials. So much for environmentally friendly insulation!

 

[jsmac]

I don't wish to hijack this thread but it is timely as I am in the process of rebuilding a small timber frame extension of our house. The room is only about 2m x 1.2m but I want to ensure that it is built correctly and I have all but given up taking advice from builders at face value. I have discovered to my detriment and additional expense that it is often better to double check.

I understand from Mike's comments that it is unadvisable to put ply or OSB board on the outside facing wall of a timber frame (this is of course is exactly what our builder was proposing) and instead put it on the inside. Is my understanding correct? If this is the case then what does one use in place of ply or OSB on the outside or is it simply not necessary?

We are planning on covering the external walls with rendered corkboard. I didn't think this buildup would have the structural integrity on its own and so something like OSB or ply would be necessary but it sounds like this is not the case?

Scott

 

[MikeG]

Yes, your understanding is correct. You'll find lots of people who should know better haven't yet thought this through, so my insistence on this is something of a minority view still.

The outside just needs a breather membrane (Tyvek "Housewrap" is the most common, I think). You may be able to buy off cuts on Ebay for a job that small. Don't forget to counterbatten over it on the outside to form a cavity/ air-space behind the cladding. It is this more than anything which protects the structure.

 

[88v8]

... If that is softwood, I can't imagine what state it's in.

Yes, it is/was.
State... something like an crumbly old bath sponge, although some parts survive intact.
And talking of sponges, the rockwool insulation was soggy as well.

Yesterday I went and ordered timber to remake that part of the frame. About a ten foot run from window to window. I wanted proper Imperial timber sizes so it will be sawn to suit, then I won't have to faff around padding out the modern foreign stuff.

Must say I was very impressed with the treated batten. It survived the immersion unscathed and I have removed the rusty nails and will be reusing most of it. Wonder what the treatment was and whether the H&S crackpots have banned it............ I hear they're planning to ban Glyphosate. We're doooomed.......

Anyway, I'll remove all the Kraft paper, and drill some holes in the ply. And replace the insulation with... something.

Ants... I associate then with dry, not damp. Wrongly, it seems. Have to feel sorry for the ants. Their home perhaps for a decade. Scuttling to carry their eggs to safety while I hoovered them up in their thousands. An ant Pompei. Don't suppose they've read Robert Harris' excellent book, and now they'll never have the chance.

Ivor