A log is provided that has a pair of oppositely directed faces, one of the faces having a pair of tongues separated by a channel and the other face having a pair of grooves separated by a ridge. The tongues, grooves, ridge and channel respectively are dimensioned so as to inter-engage when logs are placed one on top of another. The base of the channel is spaced from the end face of the ridge to accommodate a pair of seals that are maintained laterally spaced apart in the channel so as to define an air space therebetween. A thermal break is thus provided between the seals in the channel to inhibit conduction of heat through the seal.