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Our house is constructed so that the upper floor has large cavities between the wall and the ridged roof running on each side from front to back.
These cavities are not insulated and total 360 cubic feet.
I wish to insulate them and can gain access.
I have a choice of Vermiculite or polystyrene chips. The former is considerable more expensive than the chips.
What would an expert recommend?

swansea

Is the area of wall in question above your top floor ceiling level? It sounds like you may be referring to the gable wall, but it is not quite clear.

Is the inside face fo the wall to a room or the loft space?

Thank you for the response.
The roof extends down to ground floor ceiling level.
The upper floor is built within the sloping roof and thus is narrower than the ground floor below; this creates a triangular profile between the ground floor ceilings, the upper room side wall and the roof.
The inside face of the walls is to the upper rooms.
The loft space is above and is adequately insulated.
I would post a diagram but am uncertain how this might be done.

So you have a Chalet bungalow style house where the first floor is effectively built in the roof.

If the walls that your refer to in your original message are the low level ones between the bedroom and the loft void, these can be easily insulated by wedging polystyrene between the studwork. Assuming the studs are in the region of 100mm then you should be able to achieve a level of insulation almost equivalent to current building regs. The floor in this loft area (above the ground floor ceilings) if not already insulated can be insualted with mineral quilt (fibre glass), vermiculite, or polystyrene, take you pick.

Hope this helps.

LCL

Thank you for your response.
I should have mentioned that access to the cavities can be gained either
1) by making a hole through the through the inner brick walls or
2) by removing roof tiles from outside.
In both situations pouring loose-fill insulatiion is the only practical method.