A problem with my timber framed extension! Help/Advice will be appreciated.

Hi all.

I've signed up on the hope that somebody can help me out.

I built a two storey oak framed extension which was erected end sept last year. The frame and panels were exposed for two months of weather before the roof going on (Mid Nov). It was plaster boarded and plastered in February this year.
The extension compromises of an oak frame skeleton with a timber frame encapsulation for my walls.
The panel compromises of (Inside to outside);

12mm Plasterboard
VCL
38x89 studs, soleplates and wall plates.
50mm Kingspan within the studs top to bottom.
12mm OSB
75mm Kingspan
Breather membrane
47x72 studs
Larch feather edge cladding

The panel itself is sat on block work on top of DPM which is running underneath into cavity.

There is then a canted brick on exterior wall 3 bricks high where I have lead flashing running back to meet breather membrane of the wall panel. This is effectively capping cavity, should this return to 47x72 studs only??

The problem I'm getting is a smell on one wall only of damp/wet wood/dirt. From the soleplates the smell is worse. There is no water getting into the extension or no sign of any leaks. The smell is only on the ground floor, its become worse since we've had more rain.

I have inspected the wall internally by removing the skirting board and cutting away some plasterboard and there is no sign of water ingress. But I cant see a great deal to be honest.

On the exterior I have removed the bottom featheredge board and lead flashing to look into cavity etc. I have peeled up the membrane and cut away a small amount of the 75mm Kingspan. Behind this is where I have found the OSB to be dampish and it smells just like the described smell above, it looks slightly black as well. I can only see approx 30mm high as I don't want to go cutting away more insulation. The Kingspan is dry also.

The elevation concerned takes a fair amount of stick when it rains as its an open area with no protection from the existing house or and surrounding buildings.

Building regs have signed all of my work of as well and were happy with everything they saw.

I am completely unsure of the problem here as I keep reading different things on the internet and also thinking the worse case scenario. Condensation? Water leak? Not enough ventilation within cavity as lead returns to breather paper capping cavity possibly or maybe the osb has got wet from before cladding and roof went on.

Can anyone shed any light on my problem please. Sorry if it isn't explained well enough. I can add a few photos if needed.

Many thanks

My initial thought is that you have insulation on both sides of the OSB boarding.

That means on each side of the osb there is a vapour proof foil face. The inner insulation is between studs so the vapour barrier is not complete.

The inner vpl may be breached at electrical sockets etc, so a bit of potential for vapour travelling through.

How was the bottom edge of the osb treated? -could moisture get in and travel up the board.

Did the osb get wet before it was covered? -with the foil on both sides maybe there is a risk of the moisture getting trapped.

Thankyou for replying.

There is an electrical socket on the wall directly opposite the problem. The vcl was broken here and has been since it was plastered and painted. Obviously i had no idea until i started to smell something and tracked it to that wall. Popped off the socket and vcl was broken. (Electricians).

How much of a break in this layer would problems arrise??

The osb sits over the cavity. Along with the 75mm insulation. I have attached two photos, they are spaced 300mm apart. Not the best im afraid.

Should the cavity be ventilated??

Hopefully it will give you an idea.

Many thanks

As Notch7 said you have insulation both sides of the OSB.

I am no expert on this method of construction but i am quite sure the Breather Membrane should be over the OSB and the 75mm of Kingspan shouldn't be there at all.

Also you have used 89mm studs and filled with 50mm Kingspan.. Does the 50mm Kingspan sit up against the OSB or the VCL? Because if the 50mm Kingspan sits against the VCL then you have created an AirGap/Cold Spot between the Kingspan and OSB where condensation will form and could cause your problem.

I am not going into sizes & thicknesses of materials but working from outside to inside this is a twin wall timber frame construction which I think should be more like :- horizontal timber board cladding, tanalised vertical timber battens/ drained ventilation gap, Housewrap or similar breather membrane, wbp ply sheathing, timber stud outer frame with insulation between studs, OSB sheathing on inner timber frame with insulation between studs, continuous vapour barrier , plasterboard and skim finish. Obviously correct type and thickness of insulation ,detailing at base and top of wall , heads ,cills & jambs of openings etc. all required.

What is the ground floor slab make up? Beam & Block or Ground bearing? If there is know risk of high water tables / or raised external works, its best practice for the timber frame sole-plate should be raised and sat on a single, or in some cases a double course or block. I'd guess you have a beam and block suspended floor and that the sole-plate was fixed directly to this structure. Your floor maybe insulation and screed therefore you can view the sole-plate when the skirting is removed.... Otherwise you would see the face of a block. A few photos would be really helpful... PS - There is nothing wrong with the make up of the structure as described within your post. Perfectly normal to comply with Building Regs U-Value requirements. The lead tray should be behind the counter battens and tucked up behind the breather membrane, this way any water that may penetrate the cladding would discharge down the face of the breather membrane and out via the lead flashing. The last point is to make sure there is an air gap behind the cladding both top and bottom with insect mesh installed for obvious reasons.

cAtLeYx - so you don't see anything wrong with having the outer stud wall on the outside of the breather membrane, or having a vapour barrier each side of the osb due the insulation being foil faced as others have pointed out?
ps where do counterbattens come into it , if the cladding is horizontal boarding shouldn't it just fix to vertical battens ?

Regardless of the internal VCL, you would still have an unventilated cavity in a timber structure, which to me sounds wrong. There's a reason that housewrap is breathable.

If detailed wrong lead will also produce a lot of condensation.

ivixor - what are you referring to ? I suggested Housewrap breather membrane behind the ventilated and drained gap behind the timber boarding
cladding !

Leofric said:

cAtLeYx - so you don't see anything wrong with having the outer stud wall on the outside of the breather membrane, or having a vapour barrier each side of the osb due the insulation being foil faced as others have pointed out?
ps where do counterbattens come into it , if the cladding is horizontal boarding shouldn't it just fix to vertical battens ?

Click to expand...

If it was a true "outer stud wall" then of course its no use being outside the protective envelope of the structure. Without knowing what sits at the top of this stud it could be load bearing or simply a "counter batten" to create a void behind the cladding. There shouldn't be any moisture in the cavity to worry about the foil faced insulation acting as a VCL.

There are battens and counterbattens. Counterbattens run at right angles to the battens , as in a situation where battens are horizontal then counterbattens are required vertically to allow a drained cavity. I see battens as approx. 50 x 25mm not 72 x 47mm.

Leofric said:

ivixor - what are you referring to ? I suggested Housewrap breather membrane behind the ventilated and drained gap behind the timber boarding
cladding !

Click to expand...

I just mean that in OP's design, even if the internal VCL was perfect, you would still have a problem by having an unventilated and un-breathable cavity.