Insulating loft

[DomP]

I'm giving my loft/attic an overhaul due to mouse infested fibreglass and no boarding for storage. The main area will be 270mm of Knauf earth wool but my plan for the storage area is to put 100mm between the 100mm ceiling joists then 100mm PIR over the joists and 18mm chipboard loft floor over the top.

Main reason is my loft is in the eaves so very tight on head room, the joists are 100 x 70mm supported halfway along their 2400 length by stud walls below and storage is only for light items/Christmas crap.

Eaves will be vented, it's a pretty draughty loft so should have good air movement, will this work without condensation under the pir?

 

[Terry - Somerset]

Will this stop the mice or just make them warm and cosy

 

[DomP]

Will this stop the mice or just make them warm and cosy

I think I've got them I'm under control now....I hope!

 

[Fitzroy]

My concern with your design is that the glass wool is vapour permeable whereas the PIR board is not. The risk is you will drop below the dew point of the air on the warm side of the PIR, but as the PIR is not vapor permeable this condensation is trapped. With an extra thick layer of glass wool you still have the risk of dropping below the dew point somewhre in the middle of the insulation, however evaporate and dryout in warmer periods.

Looking at a simple heat transfer model (https://pickedshares.com/en/heat-transfer-through-a-multilayer-wall/) you can model the wall, internal and external HTCs of 10-20 will be about right and not make a huge difference. The results show a PIR internal surface temperature of 12-13deg C, with an internal temp of 20 and attic temp of 0degC. In our house windows with an internal temperature of below 10degC (i got a thermal camera for xmas) tend to get condensation on them at times. I'd be concerned but not overly worried. If it was my only solution I'd install a few areas where I could lift the floor and PIR easily and check a few times during the first winter.

 

[DomP]

Thanks Fitzroy, I think you suggestion of an trial area that I can lift to check is a good plan. I'll do it ASAP as we should get the perfect conditions between now and spring I'd imagine.

 

[Fitzroy]

Thanks Fitzroy, I think you suggestion of an trial area that I can lift to check is a good plan. I'll do it ASAP as we should get the perfect conditions between now and spring I'd imagine.

With insulation the devil is in the detail, if you have a draft under the PIR it will be rendered largely pointless. An overfilling of glass-wool that the PIR compresses a little would be a solution.

 

[DomP]

With insulation the devil is in the detail, if you have a draft under the PIR it will be rendered largely pointless. An overfilling of glass-wool that the PIR compresses a little would be a solution.

I think the later is likely due to the joists being 100mm absolute may so 100mm earthwool should be under slight compression, I could also close the ends off to prevent a draught under the pir