DIY Doctor

Renovation of Small Church to Home and Insulation Conundrum

Postby paulmb » Sun Feb 17, 2019 12:14 am

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
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Postby MantisMike » Thu Feb 21, 2019 12:18 pm

Hi Paul,

As you are located in a very severe weather exposure area i would recommend an air gap between the walls and timber frame of at least 30mm and ensure that this is ventilated from top and bottom (this is the best method as weep holes would be very exposed and could cause more moisture build up).

You could also consider adding a weathertight layer (vapor open, air tight membrane with air tight sealing tape) between the walls and the timber frame & insulation to avoid any damp transfer from the brick.

The layers would be as follows:
Solid Brick > Water Tight Membrane > Structural Battens & Insulation > Vapour Membrane > Plasterboard

You can avoid the slab pushing any moisture into the perimeter by making sure the damp proof membrane is connected to a plastic cavity tray which would go from the internal walls to the external brick.

Overall you should consider connecting the floor insulation with the internal wall insulation and roof insulation where possible (thermal structural blocks) to ensure that you create a whole building insulated envelope. As long as you then have the correct ventilation in the building you should not have any issues with cold spots or moisture build up. This approach would actually work better for an area like Isle of Skye.

I understand you may just want to meet building regs however have you considered an AECB or Enerphit standard approach? This could work quite well in your case.

Mike
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Postby paulmb » Thu Feb 21, 2019 10:04 pm

Hi Mike,

Thank you very much for your detailed reply - really appreciate you taking the time.

Do you know of any guidelines for determining how much ventilation is needed?

I can't quite envisage your suggestion:

You can avoid the slab pushing any moisture into the perimeter by making sure the damp proof membrane is connected to a plastic cavity tray which would go from the internal walls to the external brick.


A DPM under the new slab would be lapped-up the existing brick walls, the narrow air-gap distance from the new internal framework base. Would the cavity tray sit at the base of this narrow gap. I'm struggling to see how this would impede rising damp within the existing brick wall that lacks an DPC.

regards,

Paul
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Postby MantisMike » Fri Feb 22, 2019 9:30 am

Hi Paul

If you ask your Architect to review the BRE guidelines on thermal insulation they should provide more detail on ventilation requirements. Also all current building regulations will provide information on ventilation and weephole spacing requirements which should provide adequate ventilation.

As for impeding rising damp. This is only a suggestion and it may not be possible as it would require creating a gap in the existing wall along the entire perimeter (this would have to be completed in stages) to ensure that the damp proof membrane went from under the floor all the way through to the external wall which would then prevent damp rising. Please see attached picture.

If this is not possible you could consider the damp proof membrane connecting to the external weathertight membrane between the solid wall and timber-frame & insulation. Which would prevent internal rising damp.
Attachments
FloorWallDetail02.jpg
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Postby paulmb » Sun Mar 24, 2019 10:14 pm

Further to our earlier exchange Mike, as you kindly provided such a detailed and knowledgeable reply before I would be very interested in any feedback on another variation of approach that has been suggested. That being to fix a tanking membrane mechanically to the wall, which in itself provides a small (7mm) air gap beneath its dimples against the wall , and fixing rigid insulation directly to that membrane using over (warm) battening and the plaster board fixed to the battens. The claim being that a continuous insulation layer is achieved and cold bridging minimised, and any moisture from the wall can be drained or vented away.
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Postby collectors » Wed Mar 27, 2019 4:45 pm

Assuming you have the hight. Have a look at block & beam floors with a 100mm celotex & possibly an external cladding for insulation. . With an external cladding, you can go thicker & you won’t lose any internal space you will also cover anything unsightly & get a fresh start. If it is possible? you could go to celotex up to 200mm, but this may be over the top.
But if 100mm CT was ok it would save you 200mm/8" of room size. That's a lot.
https://www.celotex.co.uk/products/xr4000
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Postby paulmb » Wed Mar 27, 2019 5:44 pm

While I agree that a well executed EWI has much to recommend it, the concerns that steer me away are:
- lack of DPC mean that I need to deal with rising moisture
- to do it well entails extending the roofline and reforming the soffits
- lack of specialist experience locally
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