Roof insulation hip roof new building KfW 40

Hello dear community,

we are currently planning our new KfW40+ single-family house with a prefabricated house provider, the contract is not yet signed, but we are 98.9% sure that it will be the one. The offer somewhat reveals the manufacturer's construction, and this is where my question comes in.

I have absolutely no idea about house construction or anything similar, so here is the consideration of an absolute layperson. The offer states the following:

[*]Nail truss roof, pitch => 23°

For a non-extendable attic, a nail truss roof construction is made in traditional craftsmanship according to structural calculation. The attic space is not accessible with this roof construction.

Construction from inside to outside:
21x76 & 30x80 mm substructure
200.0 mm thermal insulation at ceiling level
solid nail truss calculation
roof underlay membrane
counter battens 40/60
roof battens 40/60 or 40/80

[*]Increase of rafters and bottom chords for nail truss roofs from 200mm => 280mm

Increase of rafters and insulation: 200mm to 280mm --> U=0.14
(for nail truss bottom chord and insulation)

Does this now mean that the "standard roof" insulation thickness is changed from 200mm to 280mm?

Can you estimate if this is a sensible and good insulation? Or could it be improved even more? What confuses me is that the insulation is not done on the roof but only in the top floor ceiling. Does this mean that the roof itself is not insulated at all?

As an old attic child, I am very sensitized here. It is less about thermal insulation for me, rather I am interested in summer heat protection. I am soon entering my fourth decade of life and have always slept directly under the roof my whole life. For the rest of my life, I would simply wish not to have to spend the summer in my own sweat.
Our bedroom is upstairs and I expect a significant improvement in my sleep quality in summer through KfW40, underfloor cooling, and of course good shading.

I am also a bit afraid of the:

[*]ceiling retractable staircase as a space-saving folding staircase,

which is recessed into the attic space when not in use

isn’t that also a “joint” that lets heat through?

I would be very happy about constructive contributions. Perhaps I am worried for no reason and all of this is good. I would just rather not want to save in the wrong place for the biggest investment of my life.

Many thanks and best regards
Dany

If you put the insulation on the floor, then you have a cold roof. This means hot in summer and cold in winter. This is a very common construction method. Unfortunately, I can't say anything about the insulation itself.

The underfloor cooling will only bring you about 2-3 degrees. If you really want it cool, then you need an air conditioner. One thing must be clear: a well-insulated house retains heat inside for a long time. In summer, there comes a point when the interior has warmed up nicely and you can hardly cool it anymore. Then the air conditioner comes into play. From a planning perspective, you can also orient your bedroom to the north to at least avoid direct sunlight.

we had actually written off the whole project due to age and the increasingly shorter financing

I’m soon entering my fourth decade of life

Honestly: I could imagine that you might also be expressing yourself ambiguously elsewhere. When I then read about a 23° roof and think of your desire for a safe, preferably maximum room temperature of 25° in the attic, I can’t quite shake the feeling that maybe even inclination and Celsius degrees have been talked past each other (?)

KfW40 is a super insulation level for avant-garde enthusiasts and by no means necessary if it is only or primarily about safely avoiding a sauna attic. Insulation against heating energy loss and insulation against heating from outside are two different things. Moreover, an attic with such a low roof pitch would be a non-habitable attic anyway, where I would rather close the thermal envelope at the top floor ceiling than at the roof surface.

I strongly advise you to start a thread linked here — with the completed questionnaire in the opening post from the plan section header — in which you discuss your building project in its overall conception. With the completed questionnaire, it’s best to also include a site sketch and name (without link!) the development plan.

I am soon entering my fourth decade of life and have always slept directly under the roof,

You don't do that here. From your description, I assume a so-called city villa with a hipped roof. The roof is not expandable, so the ceiling of the upper floor is insulated. Common practice, as stated above. Whether you have to add 40 plus to that is a matter of debate. But rather no, you don't have to. So you don't sleep under the roof ;)

You are really awesome (meant very positively) :), you immediately recognized my other thread, amazing! I often read along here (mostly not logged in) and you are almost always around and also have the most pleasant answers!
Of course you're right, I got the decades wrong, I'm actually already entering the 5th :eek:. :)

But I did not make a mistake with the degree indications, the indented text in my original post is a transcript from the offer (with every word and even every depicted arrow), the roof actually has 23° and my desired room temperature is below 25° Celsius :).

Of course you are also right regarding my choice of words for "under the roof," excuse me, as I said I am still learning regarding the topic of house construction, but making an effort. Our little house is supposed to have a basement, a ground floor, an upper floor, and of course a non-accessible attic (I did not count such a roof as a floor because it is too low). I actually meant with sleeping under the roof the upper floor. For me as a layman the space under the roof is therefore the attic beneath which I sleep, and this is exactly where I am afraid of the heat.
Because if the roof is not insulated directly, the space under the roof in summer is like a sauna, and I sleep under this sauna.


You write that heat and cold protection are two different things. I know you understood me ;), could you please show me where the difference lies and what I could do better as heat protection?

I strongly advise you to start a thread linked here – in the initial post with the completed questionnaire from the head of the floor plan section – in which you discuss your construction project in its overall concept. Along with the completed questionnaire, you should best also include a site sketch and name (without a link!) the development plan.

I have already read your last paragraph a few times, but I just don’t understand it :confused:. Initial post, questionnaire, floor plan section? o_O
Unfortunately, I don’t have the floor plans of our little house yet, our house builder always draws them by hand and presents them to us; he only wants to give them to us once we really sign with him. Sounds worse at first than it actually is. He really used brainpower after the first appointment (interview) and created drawings (not just some CAD changes of an existing house) that really blew us away. All our wishes were incredibly well thought out and drawn with dedication, insanely good, really old school, and very likeable. We understand that he doesn’t want us to go around with these ideas.
But don’t worry, precisely because we lack experience, my first plan after receiving these floor plans is to post them here. We need help from professionals like you here… We can always still change things…

But exactly things like possibly additional insulation I want to discuss with you beforehand so that I can negotiate it at our last meeting (before signing), it should still be possible to get an “additional discount” for that.
Therefore I would be happy if you could tell me if and what you would do differently.

By the way. I am already in contact with the "prefab house experts," first for contract review. You have also spoken positively about them (even if only your impression from the homepage and their posts).

Wanted to edit my post, but unfortunately you can only do that for 10 minutes.

EDIT:
I think I found the thread you mentioned, there are many points! But it will definitely still be done, but as I said, probably not sensible right now without floor plans, right?