ᐅ Attic Conversion – Insulating a Pitched Roof from the Inside – Completely Unsure
Created on: 28 Aug 2021 23:22
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ww_chris1984
Good evening everyone!
My girlfriend and I are currently renovating my grandparents’ house to move in this autumn. It’s a matter close to our hearts, and we are doing our very best to make the most of the existing building fabric (built in 1952, solid construction with pumice stone, originally built as housing for refugees after WWII).
The next major step is converting the attic. Since we’re both more or less beginners in this area, I’m currently quite unsure and hope to get some advice and feedback from the community.
We receive different recommendations from various parties regarding the procedure and I’m unsure about how to proceed.
It concerns the insulation of the pitched roof from the inside. The plan is to install both intermediate and counter-rafter insulation (is that the correct term?). According to the contractor, the following build-up makes sense and will be implemented by the selected company: intermediate rafter insulation, then a vapor retarder, followed by counter-rafter insulation, topped with OSB boards. Finally, drywall will be installed. The insulation material is Knauf WLG32 mineral wool. The vapor retarder is a product called Knauf Insulation LDS FlexPlus.
My main question is this: The contractors say the insulation layers need ventilation behind them. For example, if there is 14 cm (5.5 inches) of insulation, there should be 16 cm (6.3 inches) of space. However, the building materials supplier, a friend who is an architect, and another contractor in my circle of friends all said that this is no longer common practice and that 14 cm (5.5 inches) of space is sufficient for 14 cm (5.5 inches) of insulation.
What is your assessment? Is this something that can be answered generally? Are there important arguments against this overall build-up? I have read about quite different construction methods.
I don’t want to presume to tell a contractor how to do their work, but I think my uncertainty is understandable. Also, I would simply like to understand it better. Losing 4 cm (1.6 inches) of space on each side of the pitched roof amounts to quite a lot, and if it is not necessary, I would prefer to avoid it.
If you need any further information to better assess the situation, I’m happy to provide it.
Thank you very much in advance for your help!
My girlfriend and I are currently renovating my grandparents’ house to move in this autumn. It’s a matter close to our hearts, and we are doing our very best to make the most of the existing building fabric (built in 1952, solid construction with pumice stone, originally built as housing for refugees after WWII).
The next major step is converting the attic. Since we’re both more or less beginners in this area, I’m currently quite unsure and hope to get some advice and feedback from the community.
We receive different recommendations from various parties regarding the procedure and I’m unsure about how to proceed.
It concerns the insulation of the pitched roof from the inside. The plan is to install both intermediate and counter-rafter insulation (is that the correct term?). According to the contractor, the following build-up makes sense and will be implemented by the selected company: intermediate rafter insulation, then a vapor retarder, followed by counter-rafter insulation, topped with OSB boards. Finally, drywall will be installed. The insulation material is Knauf WLG32 mineral wool. The vapor retarder is a product called Knauf Insulation LDS FlexPlus.
My main question is this: The contractors say the insulation layers need ventilation behind them. For example, if there is 14 cm (5.5 inches) of insulation, there should be 16 cm (6.3 inches) of space. However, the building materials supplier, a friend who is an architect, and another contractor in my circle of friends all said that this is no longer common practice and that 14 cm (5.5 inches) of space is sufficient for 14 cm (5.5 inches) of insulation.
What is your assessment? Is this something that can be answered generally? Are there important arguments against this overall build-up? I have read about quite different construction methods.
I don’t want to presume to tell a contractor how to do their work, but I think my uncertainty is understandable. Also, I would simply like to understand it better. Losing 4 cm (1.6 inches) of space on each side of the pitched roof amounts to quite a lot, and if it is not necessary, I would prefer to avoid it.
If you need any further information to better assess the situation, I’m happy to provide it.
Thank you very much in advance for your help!
S
Seven198429 Aug 2021 10:30In my opinion, your contractor is absolutely right.
By the way, I also built my new house (KFW55 standard) with a ventilated roof. That might not be the newest trend, but the newest trend isn’t always the best. Without ventilation, you risk your roof rotting if there’s any construction error or if moisture accumulates for any reason.
This topic was briefly discussed in our case as well. The building materials supplier recommended a warm roof without ventilation. The structural engineer and building engineer involved strongly recommended ventilation. A construction expert I know eventually agreed that ventilation is preferable after a thorough discussion, although he said both options are basically possible.
However, the safer choice is ventilation. With the other method, you have zero tolerance for errors in execution.
By the way, I also built my new house (KFW55 standard) with a ventilated roof. That might not be the newest trend, but the newest trend isn’t always the best. Without ventilation, you risk your roof rotting if there’s any construction error or if moisture accumulates for any reason.
This topic was briefly discussed in our case as well. The building materials supplier recommended a warm roof without ventilation. The structural engineer and building engineer involved strongly recommended ventilation. A construction expert I know eventually agreed that ventilation is preferable after a thorough discussion, although he said both options are basically possible.
However, the safer choice is ventilation. With the other method, you have zero tolerance for errors in execution.
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ww_chris198429 Aug 2021 10:37What you’re saying reflects my current level of knowledge. But that is just “internet knowledge.” You always read that the vapor barrier must be installed perfectly. I can’t watch the contractors all the time, nor can I assess whether the work is done correctly.
If I lose some space in the room but gain the confidence that I won’t have problems later on, that is absolutely fine with me.
I think tradespeople should be able to explain and justify this professionally. I can’t get much out of a statement like “that’s just how it’s done.”
If I lose some space in the room but gain the confidence that I won’t have problems later on, that is absolutely fine with me.
I think tradespeople should be able to explain and justify this professionally. I can’t get much out of a statement like “that’s just how it’s done.”
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Seven198429 Aug 2021 10:48As I explained, my decision regarding the execution was not based on information from the internet.
ww_chris1984 schrieb:
I don't understand how house plans help to assess the insulation build-up.Well, then I guess I must have asked out of senile boredom *rolleyes*https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/
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pagoni202029 Aug 2021 12:04Another opinion or experience report, although I do not want to contradict the other opinion:
When I built my new house in 1990, there was no internet, so there was no way to gather such extensive information.
At that time, we also insulated between the rafters, and I additionally installed a layer of boards on the outside of the rafters before finally putting on the roof tiles.
The rafter depth was 14cm (5.5 inches) or 16cm (6.3 inches), and even back then there was the question of whether to fully insulate the space in between or leave an air gap.
At that time, the KfW funded a simple “expert report” or an objective assessment from an institute for wood processing or a similar organization. I had this done (the simplest form of today’s KfW calculation), and they gave advice regarding wall thickness, window orientation, insulation, etc.
From them, I was explicitly told that I could and even should fully insulate the space in between. Even if this did lead to the often feared occasional formation of moisture accumulation or similar, overall it was considered absolutely negligible (from memory, this was the wording...). This had been determined through various studies.
Contrary to the opinion of my carpenters and my carpenter uncle at the time, I fully insulated it and have lived in the house for 30 years without any problems arising from it.
I think that in this matter, there is no definitive right or wrong, and based on this experience, I would do it the same way again. Back then, on the inside, only an ordinary PE foil was installed as a vapor retarder—the topic of airtightness was not really addressed yet… 🤨
When I built my new house in 1990, there was no internet, so there was no way to gather such extensive information.
At that time, we also insulated between the rafters, and I additionally installed a layer of boards on the outside of the rafters before finally putting on the roof tiles.
The rafter depth was 14cm (5.5 inches) or 16cm (6.3 inches), and even back then there was the question of whether to fully insulate the space in between or leave an air gap.
At that time, the KfW funded a simple “expert report” or an objective assessment from an institute for wood processing or a similar organization. I had this done (the simplest form of today’s KfW calculation), and they gave advice regarding wall thickness, window orientation, insulation, etc.
From them, I was explicitly told that I could and even should fully insulate the space in between. Even if this did lead to the often feared occasional formation of moisture accumulation or similar, overall it was considered absolutely negligible (from memory, this was the wording...). This had been determined through various studies.
Contrary to the opinion of my carpenters and my carpenter uncle at the time, I fully insulated it and have lived in the house for 30 years without any problems arising from it.
I think that in this matter, there is no definitive right or wrong, and based on this experience, I would do it the same way again. Back then, on the inside, only an ordinary PE foil was installed as a vapor retarder—the topic of airtightness was not really addressed yet… 🤨
The construction is set up as follows: on the inside, it should be as airtight as possible, while being open to the outside. If moisture from the inside reaches the insulation, it must be able to dry out again at the back.
If there were a breathable membrane under your roof tiles, the insulation could be placed directly against it.
However, since you have wood there, moisture would accumulate and start to cause mold.
In this case, your contractors are absolutely right. By the way, I also really like the overall design. It provides a good airtight layer and offers relatively effective heat protection in summer.
If there were a breathable membrane under your roof tiles, the insulation could be placed directly against it.
However, since you have wood there, moisture would accumulate and start to cause mold.
In this case, your contractors are absolutely right. By the way, I also really like the overall design. It provides a good airtight layer and offers relatively effective heat protection in summer.
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