A cost perspective on internal vs external insulation

Financial cost is always an issue in Irish construction projects and can only increase in significance in the current straitened circumstances. This is only right, but our plea to homeowners embarking on an energy-efficient upgrade is that they would give greatest weight to long-term value, long-term costs and long-term benefits. A great example of this can be seen in the comparison of internal and external wall insulation. Most forms of internal insulation are cheaper to install on ‘day 1’, per square metre of prepared surface, than external wall insulation. While ‘Breaking the Mould II’ discusses in detail hidden long-term costs that must be considered, such as the dependability of the thermal upgrade, mould potential etc, there are also other hidden financial costs.


Internal insulation requires that occupants move out and rent or stay elsewhere, that they put furniture and paintings etc in storage (even if still on site) and re-decorate after. External wall insulation on the other hand requires that scaffolding be erected (some external wall insulation installers do this, some leave it to the general builder). For a proper installation with minimum thermal bridging, windows should be unfixed and moved to the outside line of the original wall, drainpipes should also be moved out, roof eave fascias need adapting and any concrete paving surrounding the house should be cut back to ensure the insulation clads the full height of the relevant external walls. This may sound expensive, but that view may be countered by the shorter construction time, the fact that the occupants can often remain resident, and the fact that the only re-decoration needed inside is the cladding of the widened internal window reveals with timber liner boards. However the final, and perhaps most persuasive hidden cost in this comparison, may be the remaining size of the house.

Take a typical Dublin 3-bed semi-detached house with hollow block walls, a floor area of ~100m² and overall internal dimensions of 10m x 5m. It has an external wall perimeter of 20m per floor, giving 40m over two floors. This excludes the party wall of course. Assume the external walls are built of hollow block with sand-cement internal plaster and pebbledash outside. A best practice, healthy approach to drylining these walls (to achieve a reasonable U-value of 0.45 W/m²K, see ‘Breaking the Mould V’) would be to use 60mm of ‘Warmcell 100’ or equivalent blown cellulose insulation (λ = 0.035 W/mK), continuously sealed with an intelligent vapour control layer (to surrounding structure), and a 38mm service zone with a plasterboard finish. This buildup can be 110mm thick. The floor area lost would be 40m x 0.11m = 4.4 m² of lost floor area, which converts to 47 ft². Assuming a conservative average value of €250 per ft² for residential floor space (the value could be double this in certain parts of the city) means that this ‘lost’ floor area has a value of €11,750. [1]


Where the discussion is about insulating inside or outside the deciding question could be ‘can we afford to lose further value by losing space in a market already falling?’. An alternate response to this news, wherein the thinnest or cheapest internal insulation is selected, would be a mistake if those installations have dubious credentials, don’t allow a proper airtight installation, or result in poor thermal bridging or health. As with almost anything it is better to do a little work well than a lot badly.


[1] The Author is grateful to Jay Stuart of DWME Ecoco for persuading him over a cool draught beer that the financial impact of lost space needed to be highlighted here.


Wednesday, November 10, 2010