I am converting a small church into a residential home on the Isle of Skye, and need to settle on the best approach to establishing the insulated envelope. I would like to outline the problem and solutions being considered in the hope of getting some feedback.
The building is a simple rectangular building built in 1935 of solid double brick, with a cement based wet-dash rough-cast render externally and a suspended timber floor. Internally it measures 12.4m x 5.6m. It sits on a concrete strip foundation and there is no DPC in the walls. The timber joists sit across 4 sleeper walls, 1 within each side wall, and 2 others equally spaced across the width. There are sub-floor vents along each side, but none at the gable ends. There are no gaps in the internal sleeper walls. Internally the walls were lined with timber ‘V’ panels, though these have already been removed from gable ends and the remaining panels are coming away having rotten at their base and many of the supporting battens. The joists that run parallel and adjacent the gable ends have disintegrated due to rot as has about half a meter of the timber flooring adjacent the gable ends and in some places along the sides. Some of the other joist ends are a little rotten, but those close to the vents are in reasonable condition. The gable end walls are very wet to the touch internally, which is thought to be due to ingress of water from above from failing junctions around the skews plus penetrating water through the render which is cracked and blown in many places. There have been no significant floor coverings over the timber as far as one can see and within the central part of the subfloor it looks pretty dry as far as one can see peering from the rotten wet end. While much of the timber floor not adjacent a wall is sound, there are dark wet lines and some rot in the floor immediately under the purlins which is thought to be caused water dripping from the purlins into these areas where they remain wet. I think this water is tracking along from the gable ends which have suffered some rot and have been partially repaired with steel brackets.
As a conversion of an unheated, post 1919, building, the regulations specify target U-values Wall 0.22, Floor 0.18, roof 0.18. Windows 0.16. i.e. it is not granted dispensation on the grounds of being a ’traditional’ building of heritage value.
The approach recommended by the designer is to replace the timber floor with an insulated concrete slab and to construct an internal timber frame within the walls into which impervious insulation is placed, leaving a 1” gap between the frame and the brickwork, and create a 1” service void between the insulation and the dry-lining. Also clearly the ingress of water from the roof needs fixing asap and the render repairing (or replacing).
I have read widely on the topic on how moisture moves through ‘traditional' buildings and the potential perils of changing this approach by the introduction of modern internal wall insulation (trapping moisture and not letting the building ‘breath’ etc). Similarly that installing a concrete floor can force more moisture out to the perimeter walls and increase the level of moisture rising in the walls. I am therefore concerned that with the potential for an increase in moisture pushed to the perimeter by a new floor slab, and in the confined space behind the internal wall insulation that there is the potential for the the accumulation of high moisture levels between the masonry and timber studs and that this could cause damage by freezing, decay of the internal framework, a reduction in the effectiveness of the insulation, dampness and eventually a even a break-down of the internal wall.
To prevent moisture build-up behind the internal wall insulation the designer has suggested introducing weep-holes in the existing masonry and others have suggested inserting vents, or vent bricks. An alternative to a concrete slab, but which achieves a similar finished effect, is to use a suspended floor consisting of light steel joists, metal sheeting over and a thin screed in which UFH can be installed (see http://cdi.dev.indigo.ws/wp-content/uploads/2016/08/LEWIS-Fast-Slab-Ground-Floor-system.pdf) . This would enable a ventilated void to be retained under the floor, though it is estimated that this would be approximately £5,000 more expensive than a ‘simple’ concrete slab, so not something to do without being pretty sure it’s necessary. Another option for help disperse moisture is to remove entirely the cement based external render and reinstate a breathable modern render, such as modern silicon or lime based render. This would also be significant additional expense, and is not common in the area, so finding tradesmen experience in its application is a problem.
I would be interested in other’s opinion’s on which of these measures they should be adopted, if anyone has experience of using them in a similar context or any other useful tips on the subject.
cheers,
Paul
