Attic insulation: air space between wool and plasterboard?

[CYRIL.59200]

hello everyone there is a little question about the insulation of the attic is it annoying that there is a space between the glass wool which is between the rafters and the placo here is a little diagram thank you in advance for your help

[Christophe]

Hi and welcome here!

Small diagram indeed!

It is not at all annoying provided that the blade is not ventilated.

So on the contrary if the fixing slats of the placo are placed horizontally in order to create boxes it will contribute to improve insulation overall piece!

2 cm of still air = 4 cm glass wool equivalent.

[Christophe]

ps: you can read these topics https://www.econologie.com/forums/isoler-des ... t8971.html

https://www.econologie.com/forums/panneau-sa ... 12068.html

and my work in progress: https://www.econologie.com/forums/chantier-d ... 12139.html

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[Obamot]

Welcome.

[...]

It is not at all annoying provided that the blade is not ventilated.

So on the contrary if the fixing slats of the placo are placed horizontally in order to create boxes it will contribute to improve insulation overall piece!

2 cm of still air = 4 cm glass wool equivalent.

Absolutely.

In the above case it is therefore necessary to complete the insulation by making it airtight. It is quite simple to achieve with polyurethane foam, very little since it is not a question of filling but of creating a "plug" - if we can easily access the areas to be treated - and / or put in the interstices wherever there is air that can escape (so for that it is necessary to have the possibility of disassembling the plasterboard, since one cannot predict the hermiticity of the glass wool before 'having made the "inventory" of the insulation).

Here is the type of foam:



You have to go soft, because it swells, and it's just a question of putting a cap on each end. I have already done so in similar circumstances. For the bottom, the foam stays down by itself by gravity, for the top it is more difficult, because it would be necessary to put a cardboard formwork, so that the foam holds, but we cannot always access. .. Neither remove the plasterboard and put it back again (which is the ideal solution in this type of implementation)

If you can't access it, then you just have to fill ALL the space with this foam. Because otherwise, it is utopian to think that we are going to "immobilize the blade of air" as if by magic!
- Air circulates all the time and everywhere,
- If there are differences in temperature between two zones (outside / inside) it will be inevitable that a draft will occur: and there it does not isolate at all.
- If all the glass wool insulation is hidden, then we do not know if it is well sealed all around.
- ditto for the placo or air can pass.

So if I am summarizing - and assuming that it is glass wool including its vapor barrier - you have to dismantle the plasterboards, to seal all the perimeter of this glass wool insulation, put "plugs" at the ends, where air can still escape, then reattach the plasterboard by gluing it with this foam around the perimeter carefully so that no air passes. There yes, the desired effect will be assured.
When I did this at home, I finally filled it all with foam, because the glass wool ages and loses its intrinsic insulation capacity, so if we intervene in an area, it is best not to half ... And the advantage is not to have to dismantle everything, we just make a hole every meter, from bottom to top, for each box (not to create too much pressure or judged), we blow the foam slowly in small doses, it will come out through the holes above. We will then plug the bottom hole with these dowels provided to hold a screw by forcing it with a hammer (otherwise the foam will push it out because of the pressure) and we will continue through the upper hole - hence, and through which, will flow back the foam a little - and in which we will put the nozzle to continue the progression upwards ...

BE CAREFUL THAT THE FOAM MAY CONTINUE INFLATING SEVERAL HOURS!
And so since it easily doubles or triples in volume, you should not put too much so that it has time to fill the space without compressing the glass wool too much! We must therefore complete the last step very carefully without putting too much, just so that the foam begins to come out through the last hole that we drilled at the top of the area to be insulated.

one thing is certain, it is better to leave everything "as is", rather than not to do it carefully.

[CYRIL.59200]

hi is thank you for your response just to sum up I prescribe that nothing is done yet
I put the ldv between the rafters
I stuff expansive foam where the air infiltrates
then put the placo
everything seems correct and thank you again for your response

[Obamot]

Well yes, if nothing is done, then your idea is excellent!

It's simple:
- you use rock wool (rather than glass wool) that which is stuck on a sheet of strong paper (this paper acts as a vapor barrier and must be side down, the wool oriented roof side).
- you cut it so that you have to compress it a little so that it fits all by itself (how much space do you have between each joist?)
Please note: respiratory mask, coveralls and gloves are mandatory!
- then with the polyurethane foam, you place a small bead of at least one inch all around, between the paper and the structure, which you will smooth and which will act as a glue / sealant (as you would with a sausage of modeling clay so you should not smooth everything, but keep "material", so not big, for that you have to reduce the size of the nozzle by heating it with a lighter. Attention, the bomb is to be used in 1 TIME, because it dries ... If it is not finished, turn it over to purge the nozzle, like any spray, but it does not say that the next day it still works ...).
- after the placo idem, a small sausage so that the air does not pass.
- and of course at the top and bottom there must be no air passage, so block everything if necessary ...
- thus your air will be truly prisoner, it will only be able to pass infinitely little, through the paper, that's all we ask of it.

Mébon, little note, given the danger of glass wool / rock, why not take another insulating material of the same cost? (Our chemist tells us that it would potentially be as dangerous as asbestos ...)

[CYRIL.59200]

hi and thank you again it must be 45 cm between two chevrons the choice of glass wool because it is already bought at 70% of the surface if not what materials you thought I will attach a small diagram should you put tape between the ldv thank you in advance