Insulating a hard wall

Adding an insulating layer inside a hard wall of brick, block or stone.

For materials you could use, see Insulation.

For the future, insulation will have to be quite thick. Most of the materials come in rigid panels, unlike the soft materials laid in roofs. Most internal walls are 2.1m to 2.7m high unless you live in a palace, so they take the standard sheet size of 2.4m x 1.2m, or sub-divisions of that like 600 x 1200 or 600 x 900mm

To use soft materials, or small panels, you have to dry-line. This also applies if you need very thick insulation of any kind. Wood stud is traditional, but steel channel is easier for DIY fitting. Dry-lining is practical if the insulation depth is 50mm or greater.

For rigid insulation between 10mm and 100mm it is possible to fix the panels directly to a hard wall. Practical methods are :-

1. Gripfill adhesive piped in ribbons on the wall, the panel offered up immediately. The wall must be sound and flat. This is the quickest method, so preferred by tradesmen. Best to use a special primer or a 50% solution of PVA first but then you have to allow drying time.
2. Nylon insulation supports, long spikes with a big head. You offer up the panels and drill through with an 8mm bit. The spike is hammered directly into the wall. It has a barbed tip to grip within the hole. Available only on special order, lengths up to 130mm.
3. Simple nails or screws. Use insulation disks, which are a large washer with a depressed centre. Offer up the panel and drill for a plastic wall plug. Screw with the disk washer through the panel(s) into the plug.

I had a good fix using 3-inch annular shank nails hammered into 6mm wall-plugs on a 300mm grid, to hold 25mm PIR and 12mm plasterboard into a block wall. Cheaper than buying the ready-bonded version but more work. Has to be good hard brick or a firm rendered surface to get a good grip. For thermalite or other light blocks you need a deeper anchor.