á Attic insulation upgrade from KfW 55 to KfW 40 standard applied to the attic floor
Created on: 5 Feb 2020 12:18
A
annab377
Hello everyone,
We are planning to build a single-family house in Baden-WĂźrttemberg with the following current key specifications:
- approximately 12x12 m (39x39 ft) external dimensions
- basement with about 16 cm (6 inches) perimeter insulation and a good layer (exact height unknown) of glass foam gravel underneath (within the thermal envelope)
- 49 cm (19 inches) Poroton exterior wall filled with perlite (U-value around 0.16)
- windows/doors adapted to the exterior wall (U-value currently unknown, but roughly at the same level as the facade)
- gable roof (attic outside the thermal envelope)
- domestic hot water heat pump or geothermal heat pump with an appropriate annual performance factor to qualify for BAFA incentives
- photovoltaic system on the roof
- controlled ventilation system with heat recovery
Now, regarding the gable roof, I have the following question. For a KfW-certified house, besides the exterior walls, basement, and openings in the facade (windows, doors), the roof also needs to be properly insulated. Is it sufficient to insulate the attic floor itself (about 30-40 cm (12-16 inches) of insulation material, then covered with wood to allow access to the atticâthe technical term escapes me), or is it necessary to insulate directly under the roof (between or below the rafters) to meet KfW 55 or, ideally, KfW 40 standards?
I believe that insulating the attic floor would be much easier as a DIY project and thus more cost-effective than insulation between or below the rafters. Or are the costs for both methods comparable?
Which material would you recommend for roof insulation in a KfW 40 house?
Are we mistaken in thinking that with the materials mentioned above, achieving a KfW 40 standard is fairly straightforward? Or is the effort of upgrading from KfW 55 to KfW 40 economically unjustified?
So far, we have not yet had a conversation with an energy consultant from the list.
Thanks in advance,
Best regards
Ann.
We are planning to build a single-family house in Baden-WĂźrttemberg with the following current key specifications:
- approximately 12x12 m (39x39 ft) external dimensions
- basement with about 16 cm (6 inches) perimeter insulation and a good layer (exact height unknown) of glass foam gravel underneath (within the thermal envelope)
- 49 cm (19 inches) Poroton exterior wall filled with perlite (U-value around 0.16)
- windows/doors adapted to the exterior wall (U-value currently unknown, but roughly at the same level as the facade)
- gable roof (attic outside the thermal envelope)
- domestic hot water heat pump or geothermal heat pump with an appropriate annual performance factor to qualify for BAFA incentives
- photovoltaic system on the roof
- controlled ventilation system with heat recovery
Now, regarding the gable roof, I have the following question. For a KfW-certified house, besides the exterior walls, basement, and openings in the facade (windows, doors), the roof also needs to be properly insulated. Is it sufficient to insulate the attic floor itself (about 30-40 cm (12-16 inches) of insulation material, then covered with wood to allow access to the atticâthe technical term escapes me), or is it necessary to insulate directly under the roof (between or below the rafters) to meet KfW 55 or, ideally, KfW 40 standards?
I believe that insulating the attic floor would be much easier as a DIY project and thus more cost-effective than insulation between or below the rafters. Or are the costs for both methods comparable?
Which material would you recommend for roof insulation in a KfW 40 house?
Are we mistaken in thinking that with the materials mentioned above, achieving a KfW 40 standard is fairly straightforward? Or is the effort of upgrading from KfW 55 to KfW 40 economically unjustified?
So far, we have not yet had a conversation with an energy consultant from the list.
Thanks in advance,
Best regards
Ann.
annab377 schrieb:
If you go to, for example, the Schlagmann website and select the T8 49er brick -> 0.16 W/(m2K) U-value (approximate price 8 EUR per brick). The newer T7 49er brick has a 0.14 U-value (price about 8.5 EUR per brick). Is the 50 cent difference per brick really worth the 0.02 improvement in U-value?I have to admit, I was going too fast and confused the lambda value with the U-value. Nevertheless, I would consider whether it's worth going for the 49 cm (19 inches) thickness. We have a 36.5 cm (14 inch) wall filled with mineral wool and achieve a U-value of 0.18.
And donât forget you lose quite a bit of floor area with the thicker wall.
Forget the prices. You donât know where your builder sources from or what materials he prefers to work with. That will affect the price. Plus, if the wholesaler currently âpushesâ a certain brick, that one becomes cheaper. You can see that happening in our area right now. Of those who build with Poroton, 90% are using Bellenberg MZ70.
I canât give you exact numbers on this. It also certainly depends on your individual situation. But I would base the decision on whether to insulate yourself on the carpenterâs quote. Looking at our case, insulation was a relatively small part of the overall cost. (We insulate the roof between and above the rafters.) If you only insulate above the floor ceiling, the effort is even less, and the carpenter shouldnât charge inflated prices for that.
Yes, that is true as well. But when you send the input plan with the requests for quotes to the construction companies, you specify that you want to install a xx-wide brick wall insulated with air, perlite, or wool, right?
As I said, I donât want to aim for KfW 40 just to be able to say âyeah, my conscience is clear now because I have a KfW 40 house,â but more because of the higher repayment subsidy from KfW (from 55 to 40, specifically 6,000 EUR more). And of course, with more insulation (a lower KfW value), you save more heating costs over the coming decades (since no one knows how prices will increase), thatâs clear too. But as I said, I wanted to know if with the specifications I mentioned, Iâm more likely to reach KfW 40 or if itâs just around KfW 55.
As I said, I donât want to aim for KfW 40 just to be able to say âyeah, my conscience is clear now because I have a KfW 40 house,â but more because of the higher repayment subsidy from KfW (from 55 to 40, specifically 6,000 EUR more). And of course, with more insulation (a lower KfW value), you save more heating costs over the coming decades (since no one knows how prices will increase), thatâs clear too. But as I said, I wanted to know if with the specifications I mentioned, Iâm more likely to reach KfW 40 or if itâs just around KfW 55.
Yes, thatâs the principle of a tender, so you should first discuss with your architect or energy consultant where you want to go with the project.
However, itâs not a problem to include alternative options. For example, you can specify one brick type and ask for the price per square meter for two alternative options. Then you can compare.
You wonât know the exact figures until you do the calculations. KFW 40 standard with a basement is very demanding, and you will likely need to invest extra money in insulating the basement. Whether that is financially worthwhile is debatable; I tend to think itâs less so.
The general trend of your wall construction is clearly towards KFW 40. I wouldnât build half a meter (1.6 feet) thick walls if I wasnât aiming for that standard.
As I said, think it over carefully depending on your goalsâeconomic, ecological, sustainability, self-sufficiency, etc. There are many motivations.
And you will really lose usable floor area. Depending on the design, you might lose about 5 square meters (54 square feet) of floor space. If you want to regain that by increasing the overall external dimensions, it will cost more.
Itâs better to consider investing more in basement or roof insulation instead.
Thatâs just my opinion.
However, itâs not a problem to include alternative options. For example, you can specify one brick type and ask for the price per square meter for two alternative options. Then you can compare.
You wonât know the exact figures until you do the calculations. KFW 40 standard with a basement is very demanding, and you will likely need to invest extra money in insulating the basement. Whether that is financially worthwhile is debatable; I tend to think itâs less so.
The general trend of your wall construction is clearly towards KFW 40. I wouldnât build half a meter (1.6 feet) thick walls if I wasnât aiming for that standard.
As I said, think it over carefully depending on your goalsâeconomic, ecological, sustainability, self-sufficiency, etc. There are many motivations.
And you will really lose usable floor area. Depending on the design, you might lose about 5 square meters (54 square feet) of floor space. If you want to regain that by increasing the overall external dimensions, it will cost more.
Itâs better to consider investing more in basement or roof insulation instead.
Thatâs just my opinion.
face26 schrieb:
You should consider investing more in basement or roof insulation instead.Donât you think that 16 cm (6.3 inches) of perimeter insulation for the basement exterior walls is already quite a lot?
Why do you advise against the 49 cm (19.3 inches) thick walls? Is it purely for aesthetic reasons?
Iâve often read from people who also have walls thicker than 42 cm (16.5 inches) that they are glad about their wall thickness once they saw how many pipes, cables, and chases ended up inside the exterior walls.
Personally, I find thick exterior walls more visually appealing than thin ones (and wider exterior walls usually mean deeper window sills). The same applies aesthetically to the roof overhangâgenerally, a longer overhang looks better than one flush with the exterior wall.
N
nordanney5 Feb 2020 15:50annab377 schrieb:
how many pipes / cables / grooves went into the exterior walls.As far as I know, the wiring is usually installed beneath the floor, with only the last few centimeters (inches) routed into the wall. So that shouldnât be a problem. annab377 schrieb:
Personally, I find thick exterior walls more visually appealing than thin ones.Even if that results in smaller windows that look more like arrow slits? When I think of "thick" walls, I always imagine the effect of the small windows in castles.annab377 schrieb:
Donât you think that 16cm (6 inches) of perimeter insulation for the exterior basement walls is already quite a lot?I havenât looked at it that closely, but itâs fine if you have the appropriate thermal conductivity value (WLG).
Also under the slab?
annab377 schrieb:
Why do you actually advise against 49 cm (19 inches) thick walls? Is it purely for aesthetic reasons?
Iâve read from several people here who also have walls thicker than 42 cm (17 inches) that they are glad to have the larger wall thickness once they saw how many pipes, cables, and chases were installed in the exterior walls.
Personally, I find thick exterior walls more visually appealing than thin walls (plus wider exterior walls usually provide deeper window sills). The same applies aesthetically to the roof overhang, which I prefer to be somewhat larger rather than flush with the wall.- Because I personally donât find it attractive. 42 cm (17 inches) is already substantial; 49 cm (19 inches) is 7 cm (3 inches) more.
- Because it reduces the floor area. For a 12 x 12 m (39 x 39 ft) house, you have 48 m (157 ft) of exterior wall. Multiplying by an additional 0.125 m (5 inches) wall thickness means losing 6 m² (65 ft²) of floor space.
- Because I doubt the cost-benefit ratio makes sense. I donât believe you save much by choosing one or two steps down in thickness to get better insulation if thatâs so important to you.
- Because, as @nordanney already mentioned, small window dimensions look like gun slits to me. That might have charm when renovating an old farmhouse, but for a new build, I donât personally like it. Run cables under the floor or into the ceiling, and/or install a service shaft if it extends across floors. A few remaining sockets on exterior walls arenât worth sacrificing floor area for.
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