What exactly is the problem with Cavity Wall Insulation?
Failure Modes in Blown Fibre Insulation
I suspect that the day of reckoning is fast approaching for irresponsible Cavity Wall Insulation installers but to date the campaign and public outcry against CWI failures has primarily centered on poor specification and installation. However, I think there is another very important question that should be asked… for those properties that were subject to a reasonable level of due diligence, responsible pre-survey work and sound installation methods, is it possible that even those installations may have failed despite all the installers best efforts? Sadly, the answer is a resounding yes and it’s an answer I give based on pragmatic experience on what I’ve been finding on site for a number of years.
I have been warning my clients against using blown fibre cavity wall insulation for quite some time because as part of the damp investigation process I frequently open up cavities or at least inspect them with a borescope. I firmly believe that blown fibre cavity wall fill is responsible for the vast majority of failures relating to CWI installation and I generally discuss two modes of failure in my reports.
- Dry Slump: Dry slump occurs within a very short period of having the blown fibre installed and how much it slumps depends on how well it is installed. A loose fill will slump significantly more than a tightly packed fill and you would be amazed at just how quickly this slumping can occur. I have rechecked properties within a week of first inspecting them with a thermal imaging camera and found that the walls now contain zero fill within the top 12 inches of the wall. Where a previous problem existed with cold surface condensation, then homeowners find that the problem revisits them but with a focus on the top sections of the internal walls that no longer benefit from CWI due to its slumping effect.
Realistically, all properties would be best having a follow up visit to top up the fibre CWI after it has slumped but this rarely, if ever occurs.
- Wet Slump: To my mind, this is the biggest problem with blown fibre CWI because when this mode of failure occurs it creates a problem above and beyond the obvious lack of thermal improvement to the building fabric. Blown fibre CWI, unlike blown polystyrene beads, is not inherently waterproof and I consistently find it completely slumped to the bottom of the cavity in a wet mushy mess. If you imagine holding up a bed quilt filled with blown fibre, holding it by two corners. You’ll probably find that the fibre is evenly distributed throughout the quilt. Now if you were to get an assistant to saturate the quilt with a hose-pipe you would quickly find that the saturated blown fibre slumps to the bottom of the duvet case in a wet soggy mess. This is essentially exactly what happens within the cavity wall, particularly for those properties that have an issue with penetrating damp to the outer leaf of brickwork. The resultant technical problem is that you will potentially have a very serious wall base damp problem caused by bridging of the cavity wall and physical damp proof course and of course no benefit whatsoever from the alleged improvement in insulation.
Aside from wet or dry slump, I consistently see the problem of inconsistent and patchy fill. This problem is fairly obvious under thermal imaging and it is more unusual to find solid and consistent levels of fill than it is to find empty voids in the cavity. I think this problem has been fairly well reported and I’ve seen a number of thermal images online aside from my own that highlight the problem.
I notice that the ‘all damp is caused by cavity wall insulation’ party are using this piece to promote their cause and whilst I have no objection to that it should be read in conjunction with this piece to bring a sense of balance to the discussion… Is all cavity wall insulation bad?