Why is this a bad idea?

[acresswell]

I'm sure this must be a bad idea, because I've never seen it done and it seems sensible to me!

Why shouldn't I insulate my loft space with glassfibre (or similar) between the joists, but also use celotex between the rafters?
If I get the balance right between the two layers of insulation (putting most of it between the rafters) it ought to keep the loft frost-free but I'd have space for more overall insulation.

I've got a nagging feeling it's to do with condensation...  can anyone explain simply? Does it make any difference where the airtightness barrier goes?

Thanks!

[A.L.]

hello,

if there is any ventilation into the loft from outside, air movement will render the between rafter insulation ineffective - turn any lights out and look for light coming in, usually around gutter level.

celotex, polyisocyanurate, has sufficient vapour resistivity that as long as the edges are well sealed condensation on the cold underside of the roof should not be a problem

Does it make any difference where the airtightness barrier goes?

- if you mean 'vapour control layer' it should go on the warm side of any insulation, so ideally under the fibreglass. Otherwise on the warm side of the celotex but you probably do not need it there.

[Loir]

Your loft should either be treated as part of the outside, or part of the house. The reason is to do with water vapour and condensation.

If it is part of the outside then the temperature will be close to the outside temperature and there must be sufficient ventilation to remove the water vapour that will inevitably come up from the house, otherwise there will be condensation and problems.

If it is part of the house then its temperature will be close to the rest of the house and less ventilation will be needed to remove water vapour as there will be less condensation.

Having the roof space somewhere between the two, so a bit warmer then outside but insufficient ventilation (to keep it warmer) will mean problems.

Loir

[dhaslam]

I have 100mm insulation  between the rafters, allowing a 50mm gap to the breathable membrane under the slates.  There is fibreglass between the 225mm joists as well as most of the space  being floored with  loads of items stored.    The attic space has a temperature range of less than  ten degrees  from day to night and usually the temperature is over 10C.    There is quite a lot of ventilation because of the eave vents and also  the ventilators over the Velux windows.  There is a good bit of leaked heat from the buffer tank and pipes

[acresswell]

dhaslam, it sounds like you've done something similar to me, except the bulk of your insulation is between the joists, and in my case it'd be between the rafters.

A.L. It's a new build, so I can't do the light test yet...   As currently drawn, the main insulation is between rafters, with a "superquilt" stapled to the underside of the rafters.  There will therefore be the minimum possible ventilation into the loft, exactly to avoid bypassing the insulation between rafters.   I'm paying a builder to do the main building work because I have a relatively busy job and SWMBO insists that the building time needs to be measured in months rather than decades.  The insulation between rafters will therefore be installed by the builder.

I was hoping that I could add maybe 100mm of glass fibre between the joists (myself) before l board the loft floor.  This wouldn't lose me any more headroom, but would slow the heat transfer from the main house into the loft.... hopefully keeping the loft at a minimum of about 5-10 degrees C. I wouldn't want to add 200mm of insulation between the joists because it might make the loft too cold in winter. If it's OK to have some rooms colder than others within a house (e.g an unheated pantry with a blockwork wall between it and the main house) I'm still not sure why I can only have 1 sheet of plasterboard between the main house and the loft

Is there any calculation I can do...?