Can I add insulation to outside walls of my old stone cottage?

Is it okay to insulate externally an old stone cottage (that is now plastered)? I’ve heard different things about it trapping the damp.

Most professionals in the construction industry would say no, it is not okay. They will say that traditionally constructed buildings were never designed or built to be air-tight; they must remain “breathable” in order to function. They would point out that traditional buildings cannot be expected to perform to the same thermal standard as that of a modern build, and to impose modern materials on the fabric of a traditional building will often create widespread dampness.

That said, the issue is a little more complicated. I would say it depends on the wall thickness, and while respecting the views of my peers, would see no reason not to. An uninsulated wall can lose up to 45 per cent of heat (if you believe the insulation manufacturer; and I do!), therefore external insulation is a good idea for an old cottage, which is unlikely to have any wall insulation.

So I believe that is the best option if the wall thickness is less than 400mm (15–16 inches) thick. To explain why it may not be effective where the walls are thicker, I’ll first describe how external insulation works.

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The old stone walls of a cottage provide a heat sink (they retain heat). Insulation panels are fitted to the wall with a mix of adhesive and mechanical fixings. The panels are then covered with a layer of mesh, finished with a render coat and painted. Thus, wrapped with a blanket of insulation, the heat sink is intensified and the comfort factor greatly increased.

External insulation is less efficient where the walls are thicker than 400mm (16 inches) and certainly above 450mm (18 inches) because heat has to permeate the wall to that depth before reaching the insulation. This is unlikely to happen because a significant portion of the heat will leak out at the top and bottom of the wall due to cold bridging (an area in a building where a gap occurs in the insulation). If the wall is more than 400mm thick, then drylining internally will be the best option. An independent ventilated drylining system should be used.

Other benefits of external wall insulation include:

• A more air-tight house (I’m not a big fan but regulations are regulations).
• Less risk of condensation and mould on the walls.
• Increased soundproofing.
• A fresh-looking home.

The downside:

• You will have to remove or extend window sills to allow for the increased wall thickness. This is often done with a pressed metal sill which is overlain on your old concrete sill. If the existing sill is a nice stone one, it will be covered over. Similar treatments are required around door openings and any wall vents.
• Rainwater and waste pipes will have to be diverted, realigned or offset to facilitate free access to the wall for the works and refitted against the new, thicker wall.
• Traditional Irish cottages often have structural timbers embedded in the walls such as timber lintels, wall plates, joists, beams, etc and reducing the permeability of the walls will often cause widespread problems with structural timbers.
• The finished floor level of many traditional buildings is often at, or even below, finished external ground level. This can result in dampness passing laterally through the wall at low level. Consider excavating externally to reduce the ground level along the wall and ensure the internal floor level is at least 150mm above the outside ground level. This may be a significant additional cost.
• The wall insulation externally has to be kept above the damp course so as not to impede its performance. There is a slight risk of cold bridging at this point with a very slight risk of condensation at floor level internally.
• If the cottage is a terrace, external insulation on one will affect the appearance of the entire terrace.
• If the cottage is a protected structure, you may need planning permission to line the outside facade with insulation.

On the risk of trapping dampness, the insulation will seal the wall externally and could trap moisture; however, if the wall is porous internally (for example, where a lime-based plaster is used), the wall can breathe on the inside and, assuming the space is adequately heated and vented, there should not be trapped moisture on the external side. This, of course, assumes that all possible sources of water ingress – insufficient surface water drainage, poorly constructed rainwater goods at roof level, etc – are removed before commencement.

Once the external wall insulation is completed and assuming the cottage wall is of a reasonable thickness, the wall will retain its level of heat even after the heating is switched off (the heat sink effect), resulting in a permanent heat source and reducing the risk of dampness in the wall.

Any historic issues of dampness, such as those mentioned above, would need to be addressed before applying the external insulation.

There is an SEAI grant of up to €8,000 for externally insulating a detached house. Most contractors would take about two weeks to do a standard cottage but allow for three weeks of disruption. It is good advice to consult your local chartered conservation building surveyor or architect to discuss your proposals before engaging a contractor.

Pat McGovern is a chartered building surveyor and a member of the Society of Chartered Surveyors Ireland

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