ᐅ Roof Insulation for New Hip Roof Construction to Meet KfW 40 Standard
Created on: 8 Oct 2021 14:07
D
Dany250
Hello 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
Dany250 schrieb:
I just don’t quite understand yet why this option is better than letting the heat escape directly through the insulation right at the roof, out of the house. This is called a cold roof and is the most cost-effective option for the roof structure. It involves one surface area that is smaller than the total area of the four sides of a hip roof. Installing insulation between the rafters is much more complex.
As always, you have the choice between a non-convertible attic (the standard version) or a higher-quality upgrade with usable space, although the height and roof pitch will be important factors.
If it’s just a flat hip roof design without standing height anyway, then there’s no need to include the roof within the thermal envelope.
Snowy36 schrieb:
Why do you insist on the 40Plus standard?
I don’t really see any advantages besides the costs. Not quite, for our prioritized home builder the additional costs between KfW 55 and 40 are extremely low (<30,000€). Since we are planning with a granny flat, a subsidy of 80,000€ from KfW is more than attractive… (We want to have photovoltaics anyway to qualify for the Plus standard.)
@ypg
Thanks for the explanation! So, it’s just about costs!? Is the insulation in the upper floor ceiling much worse than between the rafters?
Finishing the attic doesn’t make sense for us since we need a basement because of the slope, so we have more storage space than we will ever need. On top of that, we’re also limited by the maximum allowed building height of 8 meters (26 feet), so we can’t build much higher.
Thanks for the explanation! So, it’s just about costs!? Is the insulation in the upper floor ceiling much worse than between the rafters?
Finishing the attic doesn’t make sense for us since we need a basement because of the slope, so we have more storage space than we will ever need. On top of that, we’re also limited by the maximum allowed building height of 8 meters (26 feet), so we can’t build much higher.
Dany250 schrieb:
Is the insulation in the upper floor ceiling significantly worse than between the rafters?No, it is not. In your case, there is absolutely no reason to consider a different roof structure or alternative insulation.O
Oetzberger10 Oct 2021 19:11Dany250 schrieb:
Would additional insulation between the rafters improve anything?You might not fully understand yet. When the ceiling of the upper floor is insulated, the roof remains ventilated and uninsulated. If you want to negotiate for something, a thicker insulation layer on the top floor ceiling would be recommended. The question is whether that would still provide significant benefits given the already good U-value. The marginal benefit of extra insulation is very small.Similar topics