Roof insulation on the inside - quotes - discussion

[Mike]

3 hours ago, Garald said:

I ask the insulators to, say, drop a layer of mass-loaded vinyl somewhere in the ceiling?

I've no experience of MLV, but my hunch is that you'd be better off with that second layer of high-density plasterboard I suggested above, but your roof windows will be the weakest point.

[Garald]

15 minutes ago, Mike said:

I've no experience of MLV, but my hunch is that you'd be better off with that second layer of high-density plasterboard I suggested above, but your roof windows will be the weakest point.

 

Right - and sound is not like heat; the weakest point will matter a lot. Might eventually need some sort of inside sliding doors (whatever would be the name for them?) on the south-side windows (notice there are only three of them, on purpose). I could easily handcraft something temporary out of wood and cork to see if that makes a difference. I doubt the windows are the weakest point right now - I suspect it's the entire structure that is transmitting noise.

(What *is* the density of high-density plasterboard? I'm still think of whether I would be best off with two layers of that, or with a layer of that plus a 15cm layer of decorative cork.)

Edited April 13 by Garald

[Garald]

In fact, why shouldn't one be able to paste an extra layer of heavy, high-transparency plexiglass 6cm away from the skylight's glass? There's an indentation that makes this very tempting. I suppose that the inside could get very hot. I guess that indentation is there to accomodate Velux's own blinds, but I don't see anything noise-reducing in their product line.

[Garald]

I suppose the alternative is to get new, triple-glazed Velux for the three southern-facing skylights, and move those three double-glazed Velux to create new skylights on the southern side, thereby giving me a nice view from my desk... but I'm starting to think that most people here are not friends of having lots of skylights.

[Mike]

4 hours ago, Garald said:

Right - and sound is not like heat; the weakest point will matter a lot.

Correct.

 

4 hours ago, Garald said:

Might eventually need some sort of inside sliding doors (whatever would be the name for them?) on the south-side windows

Secondary glazing in English, but as French windows open inwards there's not a direct equivalent in France. The best solution I've seen were two sets of double windows in the same opening - 1 set opening out, the other in - which was very effective. I seem to recall secondary external sliding windows being used too, which is closest to the UK solution but exposed to the weather.

 

4 hours ago, Garald said:

why shouldn't one be able to paste an extra layer of heavy, high-transparency plexiglass 6cm away from the skylight's glass?

Worth a try.

 

21 minutes ago, Garald said:

I suppose the alternative is to get new, triple-glazed Velux for the three southern-facing skylights

Yes; the combination of glass panes makes a difference to the acoustic performance too; not all triple glazing is identical.

 

Also consider the structure of the hole that they sit in, which may also be a weak point.

 

4 hours ago, Garald said:

What *is* the density of high-density plasterboard?

At 13mm thick, regular plasterboard is typically between 7 & 9 kg/m². High density plasterboards such as SolidRoc and GypsoOmnia / GlypsoLignum are around 12.5kg/m². Fermacell is denser still at 15kg/m², but isn't a traditional plasterboard.

Edited April 13 by Mike

[Mike]

4 hours ago, Garald said:

a 15cm layer of decorative cork

15mm perhaps?

 

Maybe you have a small room that you could use as a test, now?

[Garald]

8 minutes ago, Mike said:

15mm perhaps?

 

Maybe you have a small room that you could use as a test, now?

 

Ah sorry, meant 15cm!

 

Test for what - for the look?

[Garald]

17 minutes ago, Mike said:

Secondary glazing in English, but as French windows open inwards there's not a direct equivalent in France. The best solution I've seen were two sets of double windows in the same opening - 1 set opening out, the other in - which was very effective. I seem to recall secondary external sliding windows being used too, which is closest to the UK solution but exposed to the weather.

 

Oh, I solved my first-floor noise problem (expensively!) by having a new set of sliding windows (heavy single glazing, high transparency) installed 15cm away from existing double-glazed windows (which had good specs, but were not top-of-the-line; the fake architect got in the way of that, unfortunately). So: outer set sliding sideways, inner set opening inwards.

 

But that's not what I meant! These are skylights, which open by tilting.

 

17 minutes ago, Mike said:

Also consider the structure of the hole that they sit in, which may also be a weak point.

 

Aha? What do you mean? I can share pictures (and a plan) if that helps.

[Mike]

1 hour ago, Garald said:

But that's not what I meant! These are skylights, which open by tilting

Then triple glazed with a spec focusing on sound insulation is likely your best option.

 

1 hour ago, Garald said:

2 hours ago, Mike said:

Also consider the structure of the hole that they sit in, which may also be a weak point.

 

Aha? What do you mean?

I mean that where the window meets the reveal your 180mm of insulation between inside and outside falls to very little; you're potentially relying mostly on the thickness & density of the plasterboard (unless the plasterboard is right up against a 180mm deep rafter):

 

 

 

Edited April 14 by Mike

[Garald]

5 hours ago, Mike said:

Then triple glazed with a spec focusing on sound insulation is likely your best option.

 

I mean that where the window meets the reveal your 180mm of insulation between inside and outside falls to very little; you're potentially relying mostly on the thickness & density of the plasterboard (unless the plasterboard is right up against a 180mm deep rafter):

 

 

 

 

Right, I've thought about that. Is there any solution to that? (In a future society, maybe windows would be surrounded by a layer of vacuum insulation, but I doubt a random insulation person who is trying to charge me twice what he should would agree to it.)

[Garald]

On liege24: who on earth charges so much for two *samples*?

 

https://www.liege24.fr/shop/42-echantillons-de-liege/1161-eacutecorce-de-liege-decoratif-mural-virgin-amp-cameleon---ensemble-dechantillons---2-pcs/

 

Elsewhere, prices are more reasonable - see, e.g.,

 

https://www.toutleliege.com/22-les-decos-double-epaisseur-6-mm-et-plus

 

Or yet again

 

https://magasindeliege.fr/liege-mural-echantillons-virgin-river.html

 

Wonder how to compare the quality in an objective way.

Edited April 14 by Garald

[Mike]

14 hours ago, Garald said:

Right, I've thought about that. Is there any solution to that?

Just a matter of doing what you can in the space you have available. Airtightness (membrane / tape / foam) + X layers of high density plasterboard + any insulation you can get in there.

[Garald]

On 14/04/2025 at 22:26, Mike said:

Just a matter of doing what you can in the space you have available. Airtightness (membrane / tape / foam) + X layers of high density plasterboard + any insulation you can get in there.

 

Come to think of it, those spots you mention might be even more critical when it comes to sound insulation (since sound, again, finds the weakest point - and the skylights themselves are double-glazed; one might hope, but I can't know, that the frame is made so that it is not the weakest point itself). In that case, the thing to do might be just to put a few cm of cork around them, behind the plasterboard. Probably a wiser choice than putting PIR, say, which would insulate more but do nothing for noise.

[Mike]

1 hour ago, Garald said:

Come to think of it, those spots you mention might be even more critical when it comes to sound insulation

Should have been clearer - that was what I was trying to say...

Yes, cork would be good as one material - it all depends on how much space you have.

[Garald]

6 hours ago, Mike said:

Should have been clearer - that was what I was trying to say...

Yes, cork would be good as one material - it all depends on how much space you have.

 

Well, around the windows, I can put as much cork as I want. The minimal spacing between windows is 10cm. Right now the insulation between pairs of windows close together is as thick as elsewhere (which is idiotic and not what was planned: it makes one hit one's head); I think 5cm should suffice, no?

 

If an insulation company tries to overcharge me for it, I can tell them I can buy the cork and paste it myself. There's no magic to it, no? All one needs is cork (expanded or not - would it matter?), glue and a cutter.

Edited April 16 by Garald

[Garald]

Come to think of it: what would be wrong with just having expanded cork *exposed*, without a plaster finish, at least in sections of the ceiling? On top of that, one would still need an airtight membrane (and a watertight membrane? - or can one membrane fulfil both roles? I think the crucial thing would really be to keep water from filtering down...). 

 

By "sections of the ceiling" I really have in mind the lower part of the ceiling in half the attic - the part that is my home office. If I did that to the bathroom or my girlfriend's home office, she would murder me with an axe (or whatever vegans use to murder).

 

(Would dust get in expanded cork's interstices? I have allergies.)

Edited April 16 by Garald

[Mike]

6 hours ago, Garald said:

what would be wrong with just having expanded cork *exposed*, without a plaster finish

Nothing wrong with that in principle. If it's somewhere it may get knocked then you may need to make some repairs in future, but that's easy enough if you keep a little spare.

 

6 hours ago, Garald said:

she would murder me with an axe (or whatever vegans use to murder)

A sharpened carrot? A lobbed turnip?

 

7 hours ago, Garald said:

Would dust get in expanded cork's interstices? I have allergies

I've not heard of that being a problem, and some cork finishes are very smooth. Dust is also much less likely to accumulate on the underside of a sloping surface.

 

6 hours ago, Garald said:

On top of that, one would still need an airtight membrane (and a watertight membrane? - or can one membrane fulfil both roles?

They perform different roles. You always need an airtightness membrane (or some other material performing that function - such as a parge coat on masonry) on the outside of the insulation somewhere. Although cork can be airtight, the joints wouldn't be, and if it's on the internal surface it's probably not in the right place to serve that function

 

Provided the airtightness membrane is properly airtight, then the air pressure inside the room and inside the insulation will be more-or-less identical, so there's nothing to drive warm moisture laden into the insulation. That limits moisture transfer to that caused by fluctuations in internal relative humidity, and you may need a vapour control membrane to limit that. You'd need WUFI modelling to know whether or not that is necessary. If / when you get MVHR the risk will go down significantly - quite likely to zero in the absence of external sources of moisture - as I'm sure we must have discussed before.

[Garald]

48 minutes ago, Mike said:

They perform different roles. You always need an airtightness membrane (or some other material performing that function - such as a parge coat on masonry) on the outside of the insulation somewhere. Although cork can be airtight, the joints wouldn't be, and if it's on the internal surface it's probably not in the right place to serve that function

 

Provided the airtightness membrane is properly airtight, then the air pressure inside the room and inside the insulation will be more-or-less identical, so there's nothing to drive warm moisture laden into the insulation. That limits moisture transfer to that caused by fluctuations in internal relative humidity, and you may need a vapour control membrane to limit that. You'd need WUFI modelling to know whether or not that is necessary. If / when you get MVHR the risk will go down significantly - quite likely to zero in the absence of external sources of moisture - as I'm sure we must have discussed before.

 

Understood. (The insulation company just sent me an Ubakus model - I can do that myself in a second, and so can anybody else. WUFI sounds a bit trickier - don't know whether they handle that themselves.)

 

What about, um, keeping any water that gets past the tiles from dripping its way past the insulation? Does the airtightness membrane also take care of that?

[Mike]

45 minutes ago, Garald said:

What about, um, keeping any water that gets past the tiles from dripping its way past the insulation? Does the airtightness membrane also take care of that?

Maybe. In my case I chose Pro Clima Mento 1000 as my airtightness layer partly for that reason (the slates on my mansard weren't entirely watertight at the time, though they have since been renewed above a new external sarking membrane). Although it is perfectly airtight ("windtight"), Mento is marketed as an advanced highly vapour-permiable sarking membrane, with a 'high' resistance to ageing & heat (provided it's not exposed to UV light for extended periods), and it's suitable for exposure to the elements for up to 3 months, which was perfect for during the re-roofing. An ideal choice in my case.

 

Of course if it does catch any drips, the membrane needs to channel them to the outside, or at least somewhere that can cope with some intermittent wetting without creating a problem.

[Garald]

Update:

 

ISOVEO just answered:

- yes, they can line all the skylights with 2cm cork within the plasterboard... and charge me 1300 eur more for it (OK, there's lots of skylights, but still);

- to my question of whether we wouldn't need an airtight membrane outside the insulation in addition to the vapor-proof membrane below the insulation, they answer:

 

"L’air doit circuler sous la toiture, mais pas dans l’isolant bien sur. Il ne circule pas dans l’isolant car nous prenons soin de ne pas laisser de passage d’air là où nous posons l’isolant. Nous veillons à bien calfeutrer partout avec l’isolant.

Enfin, votre toiture protège de la pluie et à moins de déposer la toiture, il n’est pas possible d’installer un pare pluie qui se situe au-dessus des liteaux."

 

TF;DR: no such thing is needed, in part because it's not really possible (without undoing the roof, which would cost a fortune) and in part because they take care of "calfeutrer" (literally: caulking; meaning filling a space so that it is impermeable) everything with the insulating material.

(In fact, they had told me verbally before that that's why they prefer to work with wood fibre rathern than cork.)

Does that make any sense?

I'll try to coax a more reasonable quote from elsewhere.

[Mike]

On 30/04/2025 at 21:23, Garald said:

Does that make any sense?

This bit does: votre toiture protège de la pluie et à moins de déposer la toiture, il n’est pas possible d’installer un pare pluie qui se situe au-dessus des liteaux

 

But not the fist part: Nous veillons à bien calfeutrer partout avec l’isolant. You can't make a roof airtight using only wood fibre insulation. To do make it airtight from the inside requires a proper airtight layer. Pro Clima show one way of doing it on their website, using Mento, at https://fr.proclima.com/produits/etancheite-exterieur/solitex-mento-1000/mise-en-oeuvre#sub_navigation (see Pose ultérieure d’une sous-toiture par l’intérieur). If the tile already have a sarking membrane/felt beneath them that's in good enough condition to catch any rain that bypasses the tiles, then I'd omit the central batten that creates the V shape. And probably omit the other spacer battens if the sarking membrane is vapour-permiable; I guess yours may well be? Of course this requires doing with care, especially to make sure that the airtightness remains at the ridge, eaves, skylights, etc.

 

It would, however, be much better (present fewer potential leakage paths) to use a continuous membrane instead of cutting & sealing it at each every rafter. In other words, very similar to installing a membrane when insulating a floor (but doing it overhead) - for example here (though, as above, potentially omitting the VCL):

 

 

Edited May 2 by Mike