Cavity insulation or warmboards

RoryMurphyJnr wrote: »

Hi looking at getting an extension done and we're torn between building with cavity blocks and then pumping insulation into them or using warmboards.

Getting advice from both camps

Anyone got any opinions?

Assuming your are talking about a cavity wall scenario and not taking construction costs into account:

Warmboard pros:

• room heats quicker

Warmboard cons:

• room cools quicker when heat source turned off due to lack of thermal mass
• impossible to achieve a good and longterm level of air tightness
• reduces room size
• brings the dew point of the wall into the room (increased risk of damp/mould at back of insulation
• thermal bridging at all internal wall junctions with external wall
• can be difficult to mechanically fix heavy objects to
• can hide shoddy workmanship at the back of the boards

Cavity wall pros:

• Room stays warmer for longer due to thermal mass (stored heat)
• External walls will take up and store any available solar radiation available for later release
• Using an airtight plaster, superior airtightness can be achieved
• Dew point of wall stays in the cavity - where it should be
• No thermal bridging at internal / external wall junctions
• Simpler to mechanically fix heavy objects to
• No loss of room space

Cavity wall cons:

• room heats up slower

OP, have you considered block on flat with ewi (external wall insulation) or timberframe?

In my experience, getting a good level of air tightness is key (note, the backstop level in the b.regs is very poor)