Insulating between rafters - depth of mineral wool required vs rigid PU?

[apple1]

Folks,

Just finished helping the brother insulate his dormer roof. He fitted 100mm rigid PU between rafters (400mm centres). While I think we did a reasonable job, there are invariably still many slight gaps between the boards & rafters. We didn't seal these gaps with anything before the slabbers came in (his call!). Ahead of my own build then, I was wondering if its possible to use a combination of mineral-wool (between studs to get a good fit) & insulation backed plasterboard (to have a "sealed" envelope) but still get as good a u-value as using rigid between & under rafters? If it is, what depth of mineral-wool would I need to use between the rafters (9" x 2")? Thanks in advance, apple1

 

[Mystic Oil]

apple1

Just to clarify... are you talking about new build? If so (or if you are re-slating/re-tiling), far better to insulate from the outside, a "warm roof", keeping roof timbers on the warm side of the insulation.

If you are insulating an existing roof, then the highest performance is a rigid PIR or phenolic board between rafters, crossboarded with (say) 25mm PIR board taped on all edges with foil tape to maintain vapour control barrier. Minimum air gap of 50mm recommended above PIR boards for roof ventlation, assuming felt sarking.

Look at the Kingspan Insulation (or Quinn Therm or Celotex) website for pretty pictures illustrating the above.

I can't think of an advantage in using glassfibre in the scenario described.

 

[davidoco]

While I think we did a reasonable job, there are invariably still many slight gaps between the boards & rafters.

Look up Xtratherm Rafterloc. You'll have a perfect fit on the rafters. No cutting but it's not as easy as it looks fitting it - there is a fair bit of bending and beating it in - depending on the accuracy of your carpenter.

 

[apple1]

Thanks both. Just to clarify....it is a new build so I have a little more flexibility. Regarding the option of insulating on the outside of the rafters, where can I find more details on this method. Interested in the detailing at valleys, around chimney, along ridges etc. Also, isn't there a risk in the duration that the insulation is exposed to the elements before being covered? Thanks in advance, apple1

 

[Mystic Oil]

....where can I find more details on this method. Interested in the detailing at valleys, around chimney, along ridges etc. Also, isn't there a risk in the duration that the insulation is exposed to the elements before being covered? Thanks in advance, apple1

Suggest you have a look at the websites of the manufacturers - the various common scenarios are detailed in online documentation. The technical helplines for these companies are very good ime.

For starters you could look at..



..(from page 24 onwards) for the detailing of ridges, valleys etc. This info is on the Quinn website, other manufacturers data will be broadly similar.