another query about the jpb I'm having done on my house. Got a quote for insualting the attic and was looking at insulating between rafters. However the rafters are only 4" (1960's house). So the max insulation I can go between the rafters is 60mm. The cost of going under the rafter with another board is quite high so I am wondering as a cheaper alternative would insulating to the debt of the joists be an option - might not perform as well as between and under rafters but much cheaper.
Does this make sense or are there problems with this option of insulating between the rafters and the joists.
One I can think of is that there is no breaking of the thermal bridging at joist or rafter level.
Would appreciate opinons.
Hi CBGB. If I understand you correctly, you are insulating an unconverted attic.
In my opinion, you'd be much better off insulating at joist level, for a number of reasons.
1. If you insulate at rafter level, you are choosing to heat the attic area and there is no gain from that really, apart from not having to lag your attic pipes and tank. In other words, the heat generated by your heating system will have to heat a much greater area in your house.
2. With only 100mm (4 inch) rafters, you won't get a whole lot of insulation in there without having to add insulated plasterboard/rigid board etc. insulation and this will add to your cost.
3. If you insulate at rafter level, you can use as much insulation as you want.
If your rafters are 4 inch, chances are your joists are the same. I'd recommend you put 100mm of insulation between them and then roll more across the joists, perpendicular to them. There are a lot of insulation types available to you. Not all insulation rolls are the same. What you want is something with a low U-Value. The best available is Knauf Rafter Roll 32. It has a U-Value of 0.32 but, when I went to look for it, I found it almost impossible to source. So I used the next best thing which is Moy Metac. It has a U-Value of 0.34. The 100mm product comes in 1200mm wide rolls and is 7m in length. And, of course, you can cut the roll with a saw before you use it. It's about €60 per roll but well worth the money.
You will need to achieve a U-Value of 0.16. If you cross lay another layer of 100mm Metac, you'll achieve 0.17. I'd recommend you go thicker than that with the cross layer, however. The various Metac U-Values are here..... [broken link removed]= (I have no connection with any insulation company but have done a lot of research on it).
And do remember that compressing this type of insulation is of no benefit to you as it needs the air within it to perform to an optimum level.
Hope all this helps...........