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Radical retrofit for timber frame houses

Question: We live in an early 1980s timber framed bungalow in Perthshire in Scotland. The walls are of standard construction with 90mm of mineral fibre inside the timber frame. Outside the timber frame there’s a cavity created by the externally rendered concrete block walls.

Can you please advise how to make a significant improvement to the thermal insulation properties of the walls without creating any interstitial condensation problems that might lead to wood rot.
My current thinking is that a good but perhaps difficult solution would involve removing the outer block walls to expose the original timber frame in order to fit wood / hemp fibre wall insulation panels (which have good water vapour permeability) then rebuild the cavity and outer wall cladding (possible as timber).

Answer: An interesting project. I like your radical approach as it could potentially solve at least three problems. External wall insulation (EWI) obviously improves the u value by adding to the existing insulation but also reduces the thermal bridging of the timber studs and has the potential of substantially reduces the edge losses at wall/floor and wall roof junctions, and at window reveals.

If installed continuously and the joints are well taped the EWI will cut down on draughts through the building and wind movement around and through the existing wall structure, called thermal bypass, which wicks heat out of the building. I also agree with you using a vapour open material such as wood fibre board as this reduces the risk of interstitial condensation forming within the stud wall particularly if the existing internal vapour control barrier is not consistent or installed correctly. If interstitial condensation is forming, the wood fibre board, being a natural product, has the ability to allow moisture to exit the structure or if you like breath, without losing its integrity both in structure and thermal performance.

Without knowing your house it is hard to make a judgement on how difficult this work might be. Does the existing masonry outer leaf have any structural properties ie holding the roof up, or acting as ‘racking’ for the timber frame? Consider how the new timber rain screen interacts with the dampproof course, how the windows and doors will be detailed, alterations to flues, soil and vent pipe, rain water downpipes, etc. At the very minimum I would suggest you need to produce careful design detailing or maybe employ a good practical architect and/or a thinking builder to give you assurance and support.

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