ᐅ Roof Insulation for New Hip Roof Construction to Meet KfW 40 Standard
Created on: 8 Oct 2021 14:07
D
Dany250Hello dear community,
we are currently planning our new KfW40+ single-family house with a prefabricated house provider. Although the contract is not yet signed, we are about 98.9% sure that this will be the one. The offer somewhat reveals the manufacturer’s construction method, which leads to my question.
I have absolutely no knowledge of house construction or anything similar, so here is the consideration of a complete layperson. The offer states the following:
For an attic that is not intended for expansion, a nail plate roof truss structure is built in traditional craft-style according to static calculations. The attic space is not accessible with this roof structure.
Construction from inside to outside:
21 x 76 mm and 30 x 80 mm substructure
200.0 mm (8 inches) thermal insulation at ceiling level
solid nailed truss according to calculation
roof underlay membrane
counter battens 40 x 60 mm (1.5 x 2.5 inches)
roof battens 40 x 60 mm (1.5 x 2.5 inches) or 40 x 80 mm (1.5 x 3 inches)
Increase of rafters and insulation: 200 mm to 280 mm (8 to 11 inches) → U-value = 0.14
(for nailed plate roof bottom chord and insulation)
Does this mean that the “standard roof” insulation thickness is being changed from 200 mm to 280 mm?
Can you estimate if this is a reasonable and good level of insulation? Or could you go even further? What confuses me is that the insulation is not applied on the roof itself, but rather only in the uppermost floor ceiling. Does this mean that the roof is not insulated at all?
As someone who grew up in an attic, I am quite sensitive to this. It’s less about the thermal insulation for me and more about summer heat protection. I am approaching my forties and have always slept directly under the roof throughout my life. For the rest of my life, I would simply want to avoid spending summers “cooked” in my own heat.
Our bedroom is upstairs, and I am hoping that with KfW40, underfloor cooling, and of course good shading, my sleep quality will improve significantly in summer.
I am also a bit worried about the:
which is recessed into the attic when not in use
Isn’t this also a “gap” allowing heat to pass through?
I would greatly appreciate constructive feedback. Maybe my concerns are completely unfounded and everything is fine. I just don’t want to save in the wrong place when it comes to the biggest investment of my life.
Thank you very much and best regards
Dany
we are currently planning our new KfW40+ single-family house with a prefabricated house provider. Although the contract is not yet signed, we are about 98.9% sure that this will be the one. The offer somewhat reveals the manufacturer’s construction method, which leads to my question.
I have absolutely no knowledge of house construction or anything similar, so here is the consideration of a complete layperson. The offer states the following:
- Nail plate roof truss, roof pitch => 23°
For an attic that is not intended for expansion, a nail plate roof truss structure is built in traditional craft-style according to static calculations. The attic space is not accessible with this roof structure.
Construction from inside to outside:
21 x 76 mm and 30 x 80 mm substructure
200.0 mm (8 inches) thermal insulation at ceiling level
solid nailed truss according to calculation
roof underlay membrane
counter battens 40 x 60 mm (1.5 x 2.5 inches)
roof battens 40 x 60 mm (1.5 x 2.5 inches) or 40 x 80 mm (1.5 x 3 inches)
- Increase of rafters and bottom chords in nailed plate roofs from 200 mm to 280 mm
Increase of rafters and insulation: 200 mm to 280 mm (8 to 11 inches) → U-value = 0.14
(for nailed plate roof bottom chord and insulation)
Does this mean that the “standard roof” insulation thickness is being changed from 200 mm to 280 mm?
Can you estimate if this is a reasonable and good level of insulation? Or could you go even further? What confuses me is that the insulation is not applied on the roof itself, but rather only in the uppermost floor ceiling. Does this mean that the roof is not insulated at all?
As someone who grew up in an attic, I am quite sensitive to this. It’s less about the thermal insulation for me and more about summer heat protection. I am approaching my forties and have always slept directly under the roof throughout my life. For the rest of my life, I would simply want to avoid spending summers “cooked” in my own heat.
Our bedroom is upstairs, and I am hoping that with KfW40, underfloor cooling, and of course good shading, my sleep quality will improve significantly in summer.
I am also a bit worried about the:
- ceiling retractable staircase as a space-saving folding ladder,
which is recessed into the attic when not in use
Isn’t this also a “gap” allowing heat to pass through?
I would greatly appreciate constructive feedback. Maybe my concerns are completely unfounded and everything is fine. I just don’t want to save in the wrong place when it comes to the biggest investment of my life.
Thank you very much and best regards
Dany
If you place the insulation on the floor, you will have a cold roof. This means it will be hot in summer and cold in winter. This is a very common construction method. Unfortunately, I cannot provide any details about the insulation itself.
Underfloor cooling will only lower the temperature by about 2–3 degrees. If you really want it cool, you will need an air conditioner. One thing to keep in mind is that a well-insulated house retains heat for a long time. In summer, there comes a point when the interior has warmed up significantly and it becomes very difficult to cool down. That’s when the air conditioner comes into play. From a planning perspective, you can also orient your bedroom towards the north to avoid direct sunlight.
Underfloor cooling will only lower the temperature by about 2–3 degrees. If you really want it cool, you will need an air conditioner. One thing to keep in mind is that a well-insulated house retains heat for a long time. In summer, there comes a point when the interior has warmed up significantly and it becomes very difficult to cool down. That’s when the air conditioner comes into play. From a planning perspective, you can also orient your bedroom towards the north to avoid direct sunlight.
Dany250 schrieb:
We had basically given up on the whole project due to my age and the increasingly shorter financing options. Dany250 schrieb:
I’m approaching my fourth decade of life soon. To be honest, I could imagine that you might be expressing yourself unclearly in other areas as well. When I read about a 23° roof pitch and consider your desire for a safe, maximum room temperature of around 25°C (77°F) in the attic, I can’t fully shake the feeling that there may have been some misunderstanding between roof angle degrees and Celsius degrees (?).
KfW40 is an advanced insulation standard for enthusiasts and is by no means necessary if the goal is primarily or only to reliably avoid an overheated attic space. Insulation aimed at preventing heat loss from heating and insulation aimed at preventing overheating from external heat are two different things. Besides, an attic with such a low roof pitch would in any case not be a living space attic, where I would rather close the thermal envelope at the upper floor ceiling than at the roof surface.
I strongly advise you to start a thread linked here – with the completed questionnaire from the top of the floor plan section in the opening post – in which you discuss your construction project as a whole. It’s best to include a site plan together with the completed questionnaire and also state the development plan (without providing a link!).
https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/
Dany250 schrieb:
I'm approaching my fourth decade and have always slept directly under the roof. You actually don’t in this case. Based on your description, I assume you’re referring to a so-called townhouse with a hip roof. The roof is not convertible, so the ceiling of the upper floor is insulated. This is a common design, as mentioned above. Whether you need to add another 40 (years, centimeters, or something else) is debatable, but probably not necessary.
So, you’re not really sleeping under the roof 😉
@11ant
You’re really amazing (in the best way) 🙂, you recognized my other thread right away, incredible! I often read here (usually without logging in), and you’re almost always active with the most pleasant responses!
You’re of course right—I mixed up the decades, I’m actually entering my 50s 😱. 🙂
But I didn’t make a mistake with the angle measurements; the indented text in my original post is a direct copy from the quote (word for word, including every shown arrow). The roof actually has a 23° pitch and my target indoor temperature is below 25° Celsius 🙂.
You’re also correct about my wording when I said “under the roof.” Sorry, as I said I’m still learning about house construction but I’m making an effort. Our little house will have a basement, a ground floor, an upper floor, and of course a non-habitable attic (I didn’t count such an attic as a floor because it’s too low). By “sleeping under the roof” I meant the upper floor. For me as a layperson, the space under the roof is the attic where I sleep, and that’s exactly where I’m worried about the heat.
Because if the roof isn’t insulated directly, the space underneath becomes like a sauna in summer—and I sleep under that sauna.
@11ant
You wrote that heat and cold protection are two different matters. I know you understood me 😉, but could you please explain where the difference lies and what I could do better specifically for heat protection?
I’ve read your last paragraph several times now, but I just don’t understand it 😕. First post, questionnaire, floor plan section? 🤨
Unfortunately, we don’t have the floor plans for our house yet. Our builder has always drawn them by hand and presented them to us. However, he only wants to give them to us once we actually sign the contract. That sounds worse than it really is. After the first meeting (interview), he really put a lot of thought into the drawings (not just random CAD modifications of an existing house) and created plans that absolutely blew us away. All our wishes were incredibly well thought out and implemented with dedication—really old-school and very sympathetic. We understand that he doesn’t want us sharing these ideas around.
But don’t worry, especially since we lack experience, as soon as I get these floor plans, my first step is to post them here. That’s when we’ll need advice from professionals like you… We can still make changes then…
But I want to clarify things like possible additional insulation beforehand (with you) to negotiate them during our last conversation (before signing), since it should still be possible to get an “extra discount.”
That’s why I’d appreciate it if you could tell me if you would do anything differently or recommend changes.
By the way, I’m already in contact with the prefab house experts, initially for contract review. You also had some positive impressions of them (at least from their website and posts).
You’re really amazing (in the best way) 🙂, you recognized my other thread right away, incredible! I often read here (usually without logging in), and you’re almost always active with the most pleasant responses!
You’re of course right—I mixed up the decades, I’m actually entering my 50s 😱. 🙂
But I didn’t make a mistake with the angle measurements; the indented text in my original post is a direct copy from the quote (word for word, including every shown arrow). The roof actually has a 23° pitch and my target indoor temperature is below 25° Celsius 🙂.
You’re also correct about my wording when I said “under the roof.” Sorry, as I said I’m still learning about house construction but I’m making an effort. Our little house will have a basement, a ground floor, an upper floor, and of course a non-habitable attic (I didn’t count such an attic as a floor because it’s too low). By “sleeping under the roof” I meant the upper floor. For me as a layperson, the space under the roof is the attic where I sleep, and that’s exactly where I’m worried about the heat.
Because if the roof isn’t insulated directly, the space underneath becomes like a sauna in summer—and I sleep under that sauna.
@11ant
You wrote that heat and cold protection are two different matters. I know you understood me 😉, but could you please explain where the difference lies and what I could do better specifically for heat protection?
11ant schrieb:
I strongly advise you to start a thread linked here—in the first post with the completed questionnaire from the top of the floor plan section—where you discuss your entire building project concept. With the completed questionnaire, it’s best to also attach a plot plan and name (without linking!) the zoning/building regulations.
I’ve read your last paragraph several times now, but I just don’t understand it 😕. First post, questionnaire, floor plan section? 🤨
Unfortunately, we don’t have the floor plans for our house yet. Our builder has always drawn them by hand and presented them to us. However, he only wants to give them to us once we actually sign the contract. That sounds worse than it really is. After the first meeting (interview), he really put a lot of thought into the drawings (not just random CAD modifications of an existing house) and created plans that absolutely blew us away. All our wishes were incredibly well thought out and implemented with dedication—really old-school and very sympathetic. We understand that he doesn’t want us sharing these ideas around.
But don’t worry, especially since we lack experience, as soon as I get these floor plans, my first step is to post them here. That’s when we’ll need advice from professionals like you… We can still make changes then…
But I want to clarify things like possible additional insulation beforehand (with you) to negotiate them during our last conversation (before signing), since it should still be possible to get an “extra discount.”
That’s why I’d appreciate it if you could tell me if you would do anything differently or recommend changes.
By the way, I’m already in contact with the prefab house experts, initially for contract review. You also had some positive impressions of them (at least from their website and posts).
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