An improved roof insulation structure and method of making the structure for greatly increasing the insulation quality of a metal building roof. The structure includes a self-supporting low to medium density thermal insulation blanket having elongated lines of weakness in the top thereof. The blanket spans the width between a pair of adjacent and parallel roof purlins with one end of the blanket folded up one side of the purlin, folded across the top of the purlin and folded down the opposite side of the purlin, thereby enclosing the top and opposite sides of the purlin. The blanket provides means for receiving a thick blanket of low density thermal insulation inside thereof and between the purlins for increasing the insulation "R" factor of the insulated roof.

A method of providing insulation between joists 31, 32 during building comprises unrolling a batt of mineral fibre which has, at least at its longitudinal edges, top 35, 45 and bottom 33, 43 layers. The bottom layer 33, 43 can be compressed to fit between joists, whilst the top layer 35, 45 extends across the joists, which adjacent strips 35, 45 abutting one another, to minimize cold bridging. The batt can be formed of two layers 33 and 35, 43 and 45 which may be manufactured by slitting a single web into two layers followed by winding the two layers together to form a roll.

An insulation system for a roof structure which includes a semi-rigid insulation blanket overlying a support structure across roof purlins and having additional insulation material filling a U-shaped trough created by the blanket between adjacent purlins. The semi-rigid insulation blanket is notched by the manufacturer at predetermined points to enable the blanket to easily and securely fold over and around the support structure and roof purlins.

An insulation blanket for the roof of a metal building installed between the roof purlins and roof paneling includes a concealed safety reinforcement of tapes or netting between the insulating material of the blanket and the interior facing thereof. The arrangement enables the reinforced insulation blanket to support the weight of a workman who inadvertently steps on it while installing a roof.

A support grid (10) for roof insulation in a metal building having a roof (12) with a plurality of substantially identical, spaced purlins (16). The support grid includes at least one cross member (26) adapted to extend between adjacent purlins (16). The cross member (26) rests on and is supported by a flange (22, 24) of a first purlin (16) and the upper flange (22) of the adjacent purlin (16). At least one support member (46) may extend longitudinally at angles to the cross members (26) and be engaged therewith. Preferably a first end (30) of the cross member (26) is adapted to rest on the lower flange (24) of a first purlin (16), and a second end (32) is adapted to be hooked, prior to the application of rigid roofing material (18) to the purlins, over the upper flange (22) of the adjacent purlin (16) toward which the lower flange (24) of the first purlin (16 ) extends. The method for insulating of the invention includes installing a support grid (10) on the purlins (16) of a roof (12) from above before the purlins (16) are covered by other parts of the roof (12). Selected insulation material (34) is then placed onto the support grid (10) from above to substantially fill the space between the purlins (16) to a selected level. Then rigid roofing material (18) is fastened to the purlins (16) over the insulation material (34).