ᐅ Insulation between roof rafters using Armaflex?? Question!!!!
Created on: 31 Jul 2018 09:55
K
Kiwi&Tina
Hello, I have a question about insulation between rafters.
We removed the ceiling on the upper floor and want to insulate and cover the sloped roof in some rooms.
From the outside, there are slate tiles, underneath are boards with bitumen membranes attached. My idea is to install 19mm Armaflex from the inside, then add fiberglass insulation on top of that.
I am an insulator but mainly in industrial settings, not roofs. Armaflex is vapor diffusion tight, which also concerns me...
The assembly from outside to inside would be: slate – bitumen membranes – boards – Armaflex – 140mm (5.5 inches) fiberglass insulation – vapor barrier – counter battens – 30mm (1.2 inches) fiberglass insulation – interior cladding.
Maybe someone has an opinion on this and could kindly share their thoughts.
Best regards
We removed the ceiling on the upper floor and want to insulate and cover the sloped roof in some rooms.
From the outside, there are slate tiles, underneath are boards with bitumen membranes attached. My idea is to install 19mm Armaflex from the inside, then add fiberglass insulation on top of that.
I am an insulator but mainly in industrial settings, not roofs. Armaflex is vapor diffusion tight, which also concerns me...
The assembly from outside to inside would be: slate – bitumen membranes – boards – Armaflex – 140mm (5.5 inches) fiberglass insulation – vapor barrier – counter battens – 30mm (1.2 inches) fiberglass insulation – interior cladding.
Maybe someone has an opinion on this and could kindly share their thoughts.
Best regards
K
Kiwi&Tina1 Aug 2018 09:17Caspar2020 schrieb:
First, describe the complete current conditionThe current condition is the rough construction above the ceiling. Since I had a roof window installed, I know there is roofing felt on the outside. Below the ceiling, the rafters are insulated with Styrofoam (which will be removed).
In a Bauhaus video, which seemed quite professional, they showed insulating the rafters, putting a vapor barrier on, sealing it tightly, installing battens for drywall, and then adding insulation fleece between the battens.
That seemed a bit strange to me.
I'll try to attach a picture of the current state.
The question now is whether to leave out the Armaflex entirely and just use 160mm (6.3 inches) of mineral wool. Would a ventilation gap be necessary then?
G
garfunkel1 Aug 2018 15:47The construction method you described is problematic but not uncommon. This doesn’t mean you can or should just do it without careful consideration.
Having insulation wool compressed between the battens for the drywall is not an issue as long as the insulation behind the vapor retarder or vapor barrier is thick enough. You can calculate when this is sufficient yourself, for example using u-wert.net, which I find to be a very good tool!
Basically, the insulation is adequate when, even at the coldest outdoor temperatures, the dew point is not reached within the compressed insulation in front of the vapor retarder.
For example, I entered the following values in the calculator out of personal interest:
Drywall 12mm (0.5 inches)
Compressed insulation wool 40mm (1.5 inches)
Vapor retarder
Insulation wool 160mm (6.3 inches)
Spruce wood 40mm (1.5 inches)
Bitumen membrane 5mm (0.2 inches)
Brick including battens.
You will then have a condensation problem on the sheathing or roofing felt. Increasing the insulation thickness alone will not fix this. That makes sense since nothing passes through the roofing felt.
If there is an air gap or better yet, ventilation with outside air between the sheathing and the insulation, these problems are resolved. This is logical because any moisture that could enter the insulation from inside can then dry out.
The only question is how to implement this in practice and whether it is a common method. I believe it is not, as most older buildings are insulated directly against the sheathing. Perhaps it is simply assumed that if the vapor retarder or vapor barrier is airtight, the problem is solved.
From what it looks like in your case, you have ventilation openings in the gable wall. Therefore, I assume you once had an unventilated (cold) roof? This means the top floor ceiling was insulated, but the space between the rafters was not.
It also seems that the polystyrene only extends a short distance above the floor of the upper story. Why do you want to insulate all the way up to the ridge beam, or have I misunderstood?
It would be simpler to just insulate the upper floor ceiling again, allowing moisture to leave the insulation and avoiding issues with the roofing felt on the sheathing.
I recommend consulting a professional. Get advice on the correct construction method. Such consultation is usually not expensive and well worth it.
Having insulation wool compressed between the battens for the drywall is not an issue as long as the insulation behind the vapor retarder or vapor barrier is thick enough. You can calculate when this is sufficient yourself, for example using u-wert.net, which I find to be a very good tool!
Basically, the insulation is adequate when, even at the coldest outdoor temperatures, the dew point is not reached within the compressed insulation in front of the vapor retarder.
For example, I entered the following values in the calculator out of personal interest:
Drywall 12mm (0.5 inches)
Compressed insulation wool 40mm (1.5 inches)
Vapor retarder
Insulation wool 160mm (6.3 inches)
Spruce wood 40mm (1.5 inches)
Bitumen membrane 5mm (0.2 inches)
Brick including battens.
You will then have a condensation problem on the sheathing or roofing felt. Increasing the insulation thickness alone will not fix this. That makes sense since nothing passes through the roofing felt.
If there is an air gap or better yet, ventilation with outside air between the sheathing and the insulation, these problems are resolved. This is logical because any moisture that could enter the insulation from inside can then dry out.
The only question is how to implement this in practice and whether it is a common method. I believe it is not, as most older buildings are insulated directly against the sheathing. Perhaps it is simply assumed that if the vapor retarder or vapor barrier is airtight, the problem is solved.
From what it looks like in your case, you have ventilation openings in the gable wall. Therefore, I assume you once had an unventilated (cold) roof? This means the top floor ceiling was insulated, but the space between the rafters was not.
It also seems that the polystyrene only extends a short distance above the floor of the upper story. Why do you want to insulate all the way up to the ridge beam, or have I misunderstood?
It would be simpler to just insulate the upper floor ceiling again, allowing moisture to leave the insulation and avoiding issues with the roofing felt on the sheathing.
I recommend consulting a professional. Get advice on the correct construction method. Such consultation is usually not expensive and well worth it.
C
Caspar20202 Aug 2018 07:53The U-value calculator even recognizes Armaflex. If you enter all your layers there—as gypsum plasterboard, mineral wool with battens, vapor retarder, mineral wool, Armaflex, spruce wood, roofing membrane, slate—you’ll see four issues (if you want to use Armaflex).
You have to be extremely careful that the vapor retarder is truly airtight, so no moisture can get into the insulation (if using Armaflex and no ventilation as in your original proposal).
It must also not be too airtight because you have an airtight layer above and below (if the Armaflex is behind the insulation).
Without ventilation behind the layers due to your existing spruce/bitumen layer, it won’t work.
Regardless, I wouldn’t place the vapor retarder between the two mineral wool layers. Under unfavorable indoor humidity conditions, condensation may occur within the 40mm (1.6 inches) insulation layer.
Actually, the build-up should ideally be as follows:
Gypsum plasterboard, vapor retarder, mineral wool with battens, mineral wool ventilation gap, roofing membrane, slate.
You have to be extremely careful that the vapor retarder is truly airtight, so no moisture can get into the insulation (if using Armaflex and no ventilation as in your original proposal).
It must also not be too airtight because you have an airtight layer above and below (if the Armaflex is behind the insulation).
Without ventilation behind the layers due to your existing spruce/bitumen layer, it won’t work.
Regardless, I wouldn’t place the vapor retarder between the two mineral wool layers. Under unfavorable indoor humidity conditions, condensation may occur within the 40mm (1.6 inches) insulation layer.
Actually, the build-up should ideally be as follows:
Gypsum plasterboard, vapor retarder, mineral wool with battens, mineral wool ventilation gap, roofing membrane, slate.
K
Kiwi&Tina2 Aug 2018 10:08First of all, thank you. I'll write more this evening; for now, I'm writing a lot on my phone...
K
Kiwi&Tina4 Aug 2018 12:32Hello, this took a bit longer.
It was a cold roof, with insulation wool between the ceiling joists, and on top of that there was a full layer of Styrofoam.
Yes, the rafters are currently insulated with Styrofoam, and on the left side of the picture you can still see a part of the old sloped ceiling, which will remain like that. In front of it, there will be a Heraklit panel.
The plan is as follows: the beams in the ceiling should remain visible, it’s the bedroom, which might stay cooler in summer because of the high ceiling. I’ve also considered installing a fan under the ceiling to exhaust air outside...
Where can I find a reliable specialist for this?
How could ventilation behind the insulation be implemented? Because if I put wool between the rafters, it will naturally press down at the bottom, even if it were about 2cm (1 inch) narrower.
I guess I can forget about Armaflex, although I think I could probably make it airtight. I’m an insulator by profession, but in industrial settings, not in residential construction.
Best regards, Kevin
garfunkel schrieb:
From what it looks like, you have ventilation openings in the gable wall. So I assume you had a cold roof?
It was a cold roof, with insulation wool between the ceiling joists, and on top of that there was a full layer of Styrofoam.
garfunkel schrieb:
It also looks like the Styrofoam only extends just over the floor structure
Yes, the rafters are currently insulated with Styrofoam, and on the left side of the picture you can still see a part of the old sloped ceiling, which will remain like that. In front of it, there will be a Heraklit panel.
garfunkel schrieb:
Why do you want to insulate all the way up to the ridge beam, or did I misunderstand that?
The plan is as follows: the beams in the ceiling should remain visible, it’s the bedroom, which might stay cooler in summer because of the high ceiling. I’ve also considered installing a fan under the ceiling to exhaust air outside...
Where can I find a reliable specialist for this?
How could ventilation behind the insulation be implemented? Because if I put wool between the rafters, it will naturally press down at the bottom, even if it were about 2cm (1 inch) narrower.
I guess I can forget about Armaflex, although I think I could probably make it airtight. I’m an insulator by profession, but in industrial settings, not in residential construction.
Best regards, Kevin
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