My house has a brick outer wall and a aerated block inner wall. The inner wall is one block taller than the outer wall (maybe 8 inches?) and the roof structure rests on the inner wall. The soffit rests on the top of the outer wall and has air grills in it. The roof slopes down outside the wall line to meet the soffit. I do not know whether this is common or weird. It seems a bit strange to call the space outside the inner wall, above the soffit and below the roof the eaves but maybe thats what it is.

The gap between the inner and outer walls is insulated but this stops at the top of the outer wall. The loft is insulated reasonably well but this leaves the outer side of the top block of the inner wall open to the air coming in through the soffit, that is at outside temperature.

We therefore have a warmish ceiling (insulated), warmish lower walls (insulated) but a band of cold blocks around the top of the upstairs rooms. In the bathroom this attracts condensation like mad and the damp causes some mold.

The space between the wallplate and the underside of the roof is very narrow and if I carry the insulation over the wall plate and down to the soffit, I am liable to block the airflow into the loft from the soffit space. That same narrowness makes it very hard to even see into that space and insulation done that way would have gaps where the rafters prevent the loft insulation going. The only way I can think of getting in there would be to remove tiles and battens and tile underlay and use polystyrene blocks.

Is there a standard, easy way to insulate the outside of those top blocks?

  • Mark Brinkley

    What you describe is a fairly conventional eaves detail and its cold bridge. The cavity insulation should extend to the top of the masonry inner wall: it frequently doesn’t, which results in the block-height cold bridge in your bathroom.

    On the other hand, you have noticed that the roof itself needs to be vented and it is good practice not to simply stuff insulation into the eaves because this blocks the roof ventilation. There are proprietary eaves vents that you can fit over the rafters that maintain a vent channel whilst allowing you to pack insulation underneath.

    Problem is access. Ideally, you need to take off the roof cover and make good the detail from above, but this is not straightforward. I don’t know of any shortcut which would enable you to do the job effectively from inside the loftspace.

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