We are often asked if it’s possible to bond the Kingspan Kooltherm K118 Insulated Plasterboard to existing plaster, and if so what would be the best method.
The short answer is that insulated plasterboards can be bonded over existing plaster using acrylic sealant adhesive or low expanding PU foam adhesive so long as the plaster is sound, solid and dry. However there are a few more issues that need to be considered before you start.
Existing Cavity Wall
If it is an existing cavity wall e.g. if the construction comprises a 100mm blockwork inner leaf, 25 mm + drained residual cavity, 102.5mm facing brick externally and internally lined with a solid plaster finish then the Kooltherm K118 Insulated Plasterboard can be bonded internally to insulate the existing cavity walls.
Solid Masonry Construction
If the wall is of a solid masonry construction, then provided the outside is cladded externally, rendered or equivalent, plastered or lined with lath and plaster internally then again the Kooltherm K118 Insulated Plasterboard can be bonded internally to insulate the existing solid masonry wall construction.
Using this method of attachment does have some limitations, for example the adhesive will compress down to 2mm - 3mm and the lining will follow the contour of the existing finish, leaving no room for new services, if required, to be surface mounted on to the plaster.
If new services are required then these would need to be fully chased into the substrate provided it is permissible, chases in walls or leaves constructed of solid units, then the depth of horizontal chases should be no greater than one-sixth of the thickness of the single-leaf at any point, and the depth of vertical chases should be no greater than one-third of the thickness of the single-leaf at any point.
It maybe that services such as cabling for electric sockets or light switches are already chased into the wall, therefore only needing to be extended slightly to accommodate for the new wall lining thickness, the back boxes and switches can then be relocated into or onto the surface of the new insulated plasterboard.
Alternatively new services could be located onto internal walls not receiving a new insulated wall lining system.
Always use a qualified electrician, plumber or heating engineer when carrying out any alterations to services.
Fitting the boards
Gloss paint and wallpaper will need to be removed from the existing plaster otherwise this can prevent moisture vapour from diffusing through the structure or act as a food source for mould, any signs of existing mould should be removed and the wall treated with a fungicidal wash, allowed to dry so wont effect the bond.
The easiest way to remove the paint would be to sand the walls down to the existing plaster finish or use a steamer for removal of wallpaper. Once this has all been done, all wall surfaces receiving the insulated plasterboard need to be clean, dry and free from dust in order to provide a good bond for the adhesive.
When using these types of adhesive the existing substrate should be sound and flat to within a 5mm tolerance when measured with / under a 2000mm straight edge. Existing plastered walls should be within this tolerance, if any localised hollows exists then these should be filled before commencing the new lining work.
Dado rails, window boards, skirting boards, existing wall units, switches, surface mounted pipework etc will all need to be removed and relocated once the new lining has been installed.
When using an acrylic sealant or low PU foam adhesive there is, in most cases, no need to use a bonding agent over the plaster first, as these adhesives works well on low, medium and high suction substrates.
Once the above preparation has been carried out, the insulated plasterboard can then be installed.
When using acrylic sealant adhesive apply blobs of adhesive to the plastered wall or they can be applied to the back of the Kooltherm K118 Insulated Plasterboard. Apply the acrylic sealant adhesive approximately 25mm in diameter and they are to be positioned at 300mm centres horizontally and vertically. This type of adhesive grabs instantly and will set very quickly (within the hour).
Low expanding PU foam can also be applied to the plaster or the back of the insulated plasterboard, beads of adhesive should be applied around the perimeter of the board and down the centre of the board, some brands may require an additional bead of adhesive. Always check manufacturers guidelines.
Cut the Kingspan Kooltherm K118 Insulated plasterboard approximately 15mm short of the floor to ceiling height, position the insulated plasterboard in place and gently lift it using a foot lifter until the board is tight against the ceiling lining, the leading edge should be plumb with vertical edges lightly butted.
Insert packing strips (these can be plasterboard off cuts) at the base to wedge the system in place and remove the foot lifter. Use a straight edge to tap the system firmly into place until the face is level with the predetermined guidelines drawn on the floor and ceiling, use a spirit level to ensure the Kooltherm K118 Insulated plasterboard is level and plumb.
Once installed, add no less than 2 appropriate fixings i.e. nailable plugs or equivalent plasterboard to masonry fixings, per board. Increase this figure to 9 if the insulated plasterboard is to receive ceramic wall tiles.
For more information about fitting Kingspan Kooltherm K118 Insulated Plasterboard please visit the product page or watch our short guide.