July 19, 20215 yr Rafter depth is at 220mm which has left me in a pickle, drawings stated 170mm to fill with 120mm PIR. I've already bought the PIR based on the drawing so intend on making use of it, but have a little more room to fill so may add some more insulation in between the rafters. Had a play with ubakus to get some idea of any improvement. I'm now thinking of putting some frametherm slab on top of the PIR as this seems best bang for buck, I think I can make use of 50mm or 70mm. Drawings state a 50mm airgap. Typically is this airgap between the insulation and the membrane, or is it between insulation and tile? Many thanks.
July 19, 20215 yr as I am not rushing..... The air gap is to let the roof breathe, esp if there is a timber sarking, and esp if that is a large board rather than narrow boards. I haven't done it, but plan on trying just nailing at 50mm for the boards to push against , to save £hundreds on 50 x 25 batten. (£1.50 x 2, per m, per rafter). If anyone has tried and it is a bad idea, please tell.
July 19, 20215 yr Author Ok cool, I understand the purpose of the 50mm airgap. Judging between the indicative U Values I'm not sure I'd notice the difference between 50mm and 70mm insulation on top of the PIR so I'll save the money up front, and put it towards a new tool. Putting some short precut lengths of batten in at strategic intervals (Joins and board centres) was an idea I was mulling over too. I've bought some of the gapotape to have a play with too. Watched some youtube vid, and whilst it's expensive, I think it will make the job a little easier and cleaner.
July 19, 20215 yr 17 minutes ago, crispy_wafer said: short precut lengths of batten in at strategic intervals (Joins and board centres) Good plan, and can poss use up leftover bits. Just a blob of cheap glue to the rafter, as it becomes redundant after ins is fitted? Watched the gapotape video, but will need some convincing that this is worth while.
July 22, 20215 yr Can confirm that short (20cm across each join every 600 mm) bits of 25*25mm batten nailed to studs hold the dense mineral wool (paroc was35) pieces in place just fine. 1/6th the material Vs full length 25*50 and though 1/3rd the bearing area on insulation there shouldn't be any suction loads on this if you are airtight on the inner side.
July 22, 20215 yr Saw this detail today: Any reason that the membrane can't be the thing holding in the insulation (squishy first then capped by firmer insulation in this case) and the air gap is all above membrane? (in this case it was horizontal battens sat on spacers to create a capillary break)
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