Marsden bungalow radical retrofit: It’s a wrap

The building team is rattling through projects at the moment so the bungalow retrofit is now completed and our clients returned and happily settled back in. They share some photos and feedback on the retrofit below. For our part, it just remains to reflect on a few aspects of the project and consider what we might do differently another time.

A note on the IWI strategy

Internal wall insulation at Marsden radical retrofit project

As mentioned in an earlier blog, for the ground floor of the original cavity wall bungalow 65 mm PU insulated plasterboard was used as our IWI strategy, appearing to go counter to perceived good practice with regard to interstitial condensation and breathable materials.

This method is obviously potentially risky in terms of interstitial condensation between PU insulation and rendered blockwork, but we have been able to draw on WUFI modelling, which predicts that we’re ok due to the existing blown mineral fibre cavity insulation warming the inner leaf. A solid masonry wall would be a totally different story. Of course, a thicker PU board raises the risk of interstitial condensation but we always keep to the accepted practice of not going below 0.35 U value with internal wall insulation (IWI). This in theory is to protect timber in the wall such as joist ends from rotting. On this particular project we do not have that detail.

A fully filled adhesive layer between render and PU board ideally applied with a notched trowel is essential to inhibit air movement. The industry standard of a ‘dot and dab’ application would be an absolute non-starter. We have used standard Gyproc Drywall Adhesive.

The IWI strategy was very much driven by the clients’ budget, and potential loss of floor space that would have occurred with the usual 100m of wood fibre (as opposed to 50mm of PU) to achieve a U value of around 0.35 W/m2K.

Insulating the floor

PERFORMANCE triple glazed timber windows and doors at Marsden radical retrofit project

The floor is a weak spot on the retrofit as we were only able to make limited improvements to it. The cost and disruption of digging up the concrete floor were the main inhibitors to making more extensive improvements. There is a lot of embodied energy in a concrete floor and the benefits of digging it up could have been marginal to the overall performance of the house, given it did already have some insulation.  It might be interesting to model that in PHribbon, but in the end as always in real life, we had to be pragmatic, balancing performance, costs and time.

Because it was a 1980s build, the floor did already have some insulation in it (25mm of EPS polystyrene). We were able to add 25-50mm pf polyurethane on the existing concrete floor, topped with plywood. In the kitchen, there was already a floating floor anyway. By raising thresholds, we could get away with a slightly raised floor.

Heating & MVHR

We reused the bungalow’s original gas combination boiler which we didn’t have to upgrade because the new house is even better than the original bungalow at half the size.

The MVHR (mechanical ventilation with heat recovery) system for the project was designed by our in-house MVHR design team. An MVHR Zehnder 350 unit was used and Easijoists in intermediate floors were used to run the ductwork very easily.

PHPP & testing

PERFORMANCE triple glazed timber windows and doors at Marsden radical retrofit project

A PHPP model of the retrofit suggests space heating need of 44 kWh/m2/year and heating load of 19 W/m2.

The building team (and me) were really pleased by the first fix airtightness results of 0.9 ach @ 50 Pa. Well done to our building team (Jude, Andy, Danny, Stevie, Isaac and George) who have excelled themselves yet again.

I’d move into the house myself tomorrow!

What would we do differently?

Probably the only major thing we’d do differently is remove the trusses and ceiling joists from the original bungalow completely. The ceiling and trusses got in the way and getting rid of them would have given us more headroom.

The project developed beyond the original specification while we were building. The original plan was to leave the ceiling in place so occupancy could have in theory continued during construction. It was subsequently decided to retrofit the existing ground floor after we’d commenced building meaning vacating the space. The timber frame company has a system of not needing a scaffolding tent by using the new floor decking as a temporary flat roof allowing occupancy to continue. In retrospect, it would have made sense to remove the complete truss structure.

Client feedback

We will leave the final wrap-up comments to our clients:

“We love our new house! The retrofit has not only doubled the size of our house, but it has also created a warm, light draught-free space that feels completely different from the draughty, mould-prone house before.   It’s really comfortable and we’re especially enjoying the MVHR – the air quality is fantastic – and no more mouldy reveals or dehumidifiers! We’ve already noticed that on sunny winter days the house really gains and holds onto its heat and we don’t need the heating on in the evening at all. When we do use the heating it’s on for a much shorter time than before, and it’s a really even temperature throughout. “

 

PERFORMANCE triple glazed timber windows and doors at Marsden radical retrofit project

23rd March 2022

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