When using Insulating Flange Kits, the material recommended is a neoprene coated phenolic either as a full face or ring joint, used in conjunction with insulating bolt sleeves which can be supplied in sufficient thinness to permit the use of standard bolts provided that there is a clearance of 3 millimeters between bolt diameter and bolt hole. Tufnol or similar material washers are also used to isolate the bolt and nut head from the flange although in practice it is only necessary to isolate on one side. It is important that the gasket material used should have a high electrical resistance in conjunction with rigidity but at the same time permit sufficient compression by tightening up to overcome any problems of leaking joints. It is particularly necessary to over-wrap the whole joint, immediately after making, in order to prevent dirt and moisture ingress between the flange end faces or between the heads of the nuts and bolts and the pipe itself. Wherever possible this type of connection should be pre-assembled in a works and checked for insulation prior to insertion on site.