We’re now part-way through installing external wall insulation. This has been a learning process for myself and Bespoke Builders, neither of us had previous experience of this.
The first step is to fix the insulation baords to the outside of the house. Bespoke ordered Kay-Cel expanded polystyrene (EPS) insulation boards; this wasn’t the material that I wanted although it did meet the spec that we’d given the builders so I can’t blame them for getting what they were advised would be an appropriate and economical product. EPS has about half the insulating value of urethane or phenolic insulation boards (like Kingspan for example) and it’s also far less dense. Density is important because there will be a relatively thin brittle layer of render over the insulation board, so a denser board should be more resilient to accidental damage in the future. We eventually decided to buy Kingspan Therma boards instead, which we got an highly discounted rates from Seconds&Co.
Bespoke selected a render system from Parex, which includes a base-coat (a bagged powder which is mixed into a smooth white paste) and a reinforcement mesh (a green man-made fibre net supplied in 1m wide rolls).
The Kingspan boards are stuck to the house using dabs of the Parex basecoat and then drilled and fixed with plastic fixings (similar to these).
The insulation board is covered with a layer of the Parex base-coat (applied with a trowl like render), the reinforcement mesh is pressed into the base-coat and and more basecoat applied on top and “scratched up” to provide a good key for the top coat. The end result is a white finish which dries rock-hard and completely hides the insulation and mesh.
Corner and bell-cast beads are stuck in place (with a bit more base-coat) ready for the final render coat. We specified a through-colour render because we don’t want to have to paint (and repaint and repaint…) it. Parex can supply their own top coat but Bespoke were advised that they can use a traditional sand-cement render, which will less expensive. The render will use a yellow powder dye, four parts local yellow-brown builders sand and one part white cement. We experimented to get the quantities of yellow dye right, we want a finished look which blends in with neighbouring houses (mostly sandstone or off-white paints).
The top coat of render was applied the week after Easter. Unfortunately hairline cracks appeared over most of it and work on the rendering has now stopped pending investigation. All walls have these cracks except for the front of the house – was is the first wall to be rendered. What’s different about this wall? It’s the only wall which wasn’t insulated (the base-coat and top coat were applied directly over the existing pebble-dash); it was done on a different day; in different weather (although I think every day was pretty hot and dry); the mix could have been slightly more or less wet (there shouldn’t be any other difference as quantities were measured carefully to ensure that colours match).
There’s no sign of the render coming off, it’s just cracked all over. I took photos a week ago for comparison and I don’t think the cracks have spread or widened. The builders are seeking specialist advice – I’ll update this page when the problem is understood and resolved.
Did you get this sorted in the end then? How?
A couple of cracks were hacked-out and filled with a sand-cement mortar then left for a few weeks to see if more cracks appeared. They didn’t. From this it was concluded that that the cracking was not increasing and had occurred only during the drying process. The render seems sound, there’s no signs of it becoming loose. It is being covered with a tyrolean finish (using Webber’s Cullamix bagged tyrolean mix) which should both fill the hairline cracks and hide them. We wanted a flat render finish, now we will have a tyrolean finish (and we’ve had to wait for some months for Bespoke to get on with it) but at least things are progressing.
The reason for the cracks appearing is still not clear. I think it dried too fast – the render was applied in hot sunny weather and because it was over insulation there was nowhere for the heat to go (without the insulation it would dissipate through the blockwork behind the render). TMaybe the rendering should have been done in cooler conditions or should have been sprayed to slow the drying or shaded from the sun. Bespoke came up with various other reasons relating to the change from the original polystyrene insulation boards which they intended to use to Kingspan insulation.
We used mostly Kingspan ThermaWall TW53 but one wall has Kingspan Kooltherm K5 because the TW53 was thicker than the space available for it on the elevation. Both cracked the same way, maybe slightly more cracking on the K5 but it’s hard to judge.
Time will tell whether the tyrolean finish resolves the problem, I think it will be okay.