Cork and cork and cork and cork

[Garald]

1. As people who've come across my posts know, my lodgings are the greater part of a rowhouse of sorts (1930s) in the Paris area. I have brought up the energy rating from a F to a B, and improving matters further is an ongoing task. Skip the following two paragraphs if you already know the context.

 

Now that I've made things as airtight as I can, the weakest part of the insulation is clearly the roof. By rights, the (heated!) attic should be the warmest part of the house in winter, and it is not. The roof was insulated by the owners before the previous owners, some 15 years ago. When we had new skylights installed, the insulation next to the skylights was redone - it was in poor condition. We also had the combles perdues filled with cellulose insulation. It's still the case that the insulation of most of the attic ceiling was done 15 years ago, to dubious standards - and it is a certainty that cold air (or, in summer, hot air) manages to circulate in the insulated space between the ceiling and the roof tiles.

 

The question is what to do. Opening up the ceiling and redoing the insulation with an airtight barrier would be 10k eur. Insulating from the outside (perhaps raising the ceiling by a couple of centimeters in the process) would be very expensive - around 45k eur. It would make more sense to wait 5-10 years and a couple of year-long academic invitations to better-paying countries, and then hire a structural engineer to restructure the entire roof, gaining a lot of living space. But that's not the subject now.

What to do for now? My parquet guy is worried that the attic gets too cold when I put the heating on low (as I do when I'm travelling or simply not using half of the attic). He suggests lining the coldest attic room entirely with cork.

 

That's a tempting suggestion (and one that risks giving my girlfriend the fits; she hates cork - the parquet guy says it can simply be painted over). At the same time I wonder how much of an effect it can really have. 1cm of cork adds just 0.25 to the R. Or does having cork exposed (and separated from the insulation cavity) give extra gains in any way? The parquet guy even suggests using 1cm only in the coldest bits, and using 3mm on the rest, in part due to cost considerations and in part not to give up on any precious headroom. Believing that 3mm helps (on anything that is not, say, metal) sounds unscientific.

(Of course 1cm cork could help a lot with sound insulation, and that's valuable given that I live not far from an airport, but that's a different issue.)

 

Here are some thermal-camera pictures for context:

 

 

Paradoxically, as you can see, some of the coldest parts are the low walls of the combles perdues. I know I didn't get cheated on the insulation - I actually used a laparascopic camera to be sure - so I wonder what is going on there.

 

 

 

 

The insulation on the side walls is new but not fantastic:

 

2. A friend just sent this link to me. Insulating from the outside using cork is nothing new - the issue is that the ranking by price is polystyrene << rock wool << wood fibre << expanded cork. What is new here is that expanded cork is being used in a brutalist way. Is that really viable? How do these folks protect their cork? And does not hiding the cork lead to significant savings? Could it make insulating on the outside with cork more competitive?

 

https://www.theguardian.com/environment/2025/feb/18/enveloping-calming-london-house-wrapped-cork?fbclid=IwZXh0bgNhZW0CMTAAAR2Zk58djcTehOcpjhUDnBMdP3brmAF1ODNiXyjXOqS42IV4R10Upgs3_YI_aem_I90joDLxrFtqV6BSf4iJjA

 

[Big Jimbo]

I like the house in Tottenham.

[Mike]

Cork is expensive, but a remarkable insulation material, however...

 

6 hours ago, Garald said:

Opening up the ceiling and redoing the insulation with an airtight barrier would be 10k eur.

...if you have enough depth between the rafters, or are prepared to sacrifice head height, then that would b my preferred option.

 

6 hours ago, Garald said:

Paradoxically, as you can see, some of the coldest parts are the low walls of the combles perdues. I know I didn't get cheated on the insulation - I actually used a laparascopic camera to be sure - so I wonder what is going on there.

If it was installed without an external airtightness membrane, then it could be due to air movement.

[Garald]

8 hours ago, Mike said:

Cork is expensive, but a remarkable insulation material, however...

 

...if you have enough depth between the rafters, or are prepared to sacrifice head height, then that would b my preferred option.

 

Can’t sacrifice head height - there isn’t enough as it is. As for depth: there’s somewhere between 15cm and 20cm to work with - not enough for current standards (whether I use rock wool or wood fibre), but enough to do a decent job for now, I’d think.

 

 

8 hours ago, Mike said:

 

If it was installed without an external airtightness membrane, then it could be due to air movement.

 

Exactly. I’m asking the old contractor but I’ll be lucky if I get a response.