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Marsden Bungalow Radical Retrofit: The Timber Frame Roof

As described in our previous blog, the timber frame upper storey was connected to our masonry ground floor.

We worked closely with timber frame manufacturer SMS Timber Frame, a local business to us. They were great at accommodating their system to our needs. However, we were not able to use our preferred option of IBeam joist-engineered timber, instead of using solid timber. This included: 200mm X 50mm studs in the walls and 250mm X 50mm rafters in the roof.

The ‘Timber fraction’

Timber frame systems normally need extra timber for structural integrity. The photo above illustrates this, showing additional timber supporting a structural laminated timber ridge beam above a window opening. This has an effect on the ‘timber fraction’ (the proportion of timber stud compared to insulation) which means there is less insulation and reduced thermal performance. In Passivhaus Planning Package (PHPP) the thermal performance of timber frame systems is adjusted by between 15-20%  to reflect the effect of this thermal bridging. SMS worked with us to minimize this. We used PanelVent, a vapour open anti-racking board, over the timber frame walls, with 60mm wood fibreboard on top of both the walls and the roof, to help minimise the thermal bridging caused by the additional structural timber.

MVHR ducting

The MVHR ducting was concealed in the intermediate floor, using EasiJoists.

An unventilated roof

We are using windtight and vapour open membranes Solitex Plus on the roof and Solitex WA on the walls externally, with Intello Plus intelligent vapour check and airtight membrane on the inside of the roof and walls. This combination of membranes means that moisture in the roof area is handled safely and we don’t need to ventilate the roof. This minimises thermal bypass- i.e. wind blowing through the insulation.

Eaves & soffit detail

I’ve hand-drawn a diagram that shows our strategy at the eaves in more detail.

We are using vertical timber counter-battens, on top of the roof fibre and the Solitex Plus membrane, to raise the roof tile battens to stop water collecting behind the slaters’ battening. Because the roof does not need to be ventilated, we needed to create an appropriate eaves and soffit detail which did not compromise the windtightness strategy in the roof. We used the counter battens to hold a bespoke eaves and verge detail designed to stop the wind blowing around the insulation and to minimise thermal bridging.  This truncated eaves detail minimises thermal bridging because the rafters do not have to run through to form the eaves. It also allows a continuity of the windtightness membranes, without a lot of taping around the rafter ends.

Bill Butcher, Director, Green Building Store

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