Hello,
I'm pondering a bit about a roof construction for an attefallshus. The idea is to have as flat a roof as possible, without consuming too much roof height since I'm planning a loft.
The lowest roof slope, as I understand it, is achieved by using roofing felt. I'm looking for something I can lay myself, and found the following self-adhesive option:
http://www.icopal.se/Produktsortiment/branta/yttertak/tatskikt.aspx
Is it possible to go below a 3-degree slope in any way?
When it comes to the construction of the actual roof, it should be somewhat well-insulated while not adding too much to the height. How thin should it be built? Are there any recommendations that are standard?
Thankful for any answers!
I'm pondering a bit about a roof construction for an attefallshus. The idea is to have as flat a roof as possible, without consuming too much roof height since I'm planning a loft.
The lowest roof slope, as I understand it, is achieved by using roofing felt. I'm looking for something I can lay myself, and found the following self-adhesive option:
http://www.icopal.se/Produktsortiment/branta/yttertak/tatskikt.aspx
Is it possible to go below a 3-degree slope in any way?
When it comes to the construction of the actual roof, it should be somewhat well-insulated while not adding too much to the height. How thin should it be built? Are there any recommendations that are standard?
Thankful for any answers!
Double-seamed sheet metal roofing should withstand a slope of 1:30, which is about 1.9°. If the right sheet metal quality is used, it lasts for 100 years. However, it will probably be more expensive than felt. The modern types of felt are, however, very good. The best can handle 3° and last perhaps 40 years.
All modern insulation materials have about the same insulating capability. In an Attefall house, the roof should have a maximum U-value of 0.13. This means about 340 mm of insulation material. If you want to save space in height, it is appropriate to use a hygroscopic material such as cellulose fibers; otherwise, more space is needed for ventilation.
All modern insulation materials have about the same insulating capability. In an Attefall house, the roof should have a maximum U-value of 0.13. This means about 340 mm of insulation material. If you want to save space in height, it is appropriate to use a hygroscopic material such as cellulose fibers; otherwise, more space is needed for ventilation.
So it will be 3 degrees minimum with felt then. Is there no solution with double or triple-coating?
Looked at this one:
http://www.beijerbygg.se/store/priv...pu-al-2400x1200mm-spontad-kant?artikel=416243
With a lambda of 0.023 W/mK, you need 0.023/0.13 = 0.177 m = 177mm insulation
Compared to mineral wool with 0.045 mW/mK lambda: 0.045/0.13 = 346mm insulation
Anyone have tips on insulation with an even lower lambda?
Looked at this one:
http://www.beijerbygg.se/store/priv...pu-al-2400x1200mm-spontad-kant?artikel=416243
With a lambda of 0.023 W/mK, you need 0.023/0.13 = 0.177 m = 177mm insulation
Compared to mineral wool with 0.045 mW/mK lambda: 0.045/0.13 = 346mm insulation
Anyone have tips on insulation with an even lower lambda?
One can of course use membrane insulation, i.e., some form of welded bitumen mat with suitable surface coverage, as is commonly used on roof terraces and similar places. It can be laid with a minimal slope (for drainage purposes), e.g., 1:100, which is very close to zero degrees. But it will probably not be cheap and not something you should do yourself.
0.023 is a really great lambda value. Polyurethane foam is what refrigerator manufacturers use. I think it's hard to go below that.
0.023 is a really great lambda value. Polyurethane foam is what refrigerator manufacturers use. I think it's hard to go below that.
Thanks for the answers - leaning towards PIR insulation instead.
Now I'm wondering how to approach the construction - there's no point in using thin insulation if you still have to use thick studs.
Considering the following:
Paper
22mm tongue and groove
Air gap (25-30mm?)
120mm stud with PIR insulation in between
45mm stud + mineral wool
11mm OSB board
13mm Gypsum board
It ends up somewhere around 240mm in thickness - which is quite good. If it holds, that is, it should span about 3.4m, with a small part going 1.8m further, but then it has a wall to support it, so it's 3.4+1.8 there. I've tried to find different ways to calculate the durability online and see if it would suffice, but can't find anything reasonable. Anyone got tips?
Now I'm wondering how to approach the construction - there's no point in using thin insulation if you still have to use thick studs.
Considering the following:
Paper
22mm tongue and groove
Air gap (25-30mm?)
120mm stud with PIR insulation in between
45mm stud + mineral wool
11mm OSB board
13mm Gypsum board
It ends up somewhere around 240mm in thickness - which is quite good. If it holds, that is, it should span about 3.4m, with a small part going 1.8m further, but then it has a wall to support it, so it's 3.4+1.8 there. I've tried to find different ways to calculate the durability online and see if it would suffice, but can't find anything reasonable. Anyone got tips?
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Is it a roof structure you are describing? A 120 mm stud in a roof structure with close to zero degree slope cannot handle any significant span. The snow load can be considerable, especially if you live in Norrland.
Your load-bearing joists should be at least 45x170 mm in class C30 to ensure it holds. On flat roofs, one must be able to go up and shovel snow if needed. If you place the distance for the air gap correctly, this measurement does not have to mean that the total height of the construction increases. If you go up to 45x195 C24, you can place the crossing 45x45 joists as recessed nogging without an extra addition in height.
Very good suggestion! Thank you very much!
How do you suggest I should set the distance for the air gap?
Is it sufficient to attach the 45x45 battens to the 45x195 with angle brackets from both sides? I figure they don't carry anything anyway.
How do you suggest I should set the distance for the air gap?
Is it sufficient to attach the 45x45 battens to the 45x195 with angle brackets from both sides? I figure they don't carry anything anyway.
It sounds good! From what I can see on Isover's website, the fabric is placed between the two insulations/studs. In this case, I want to inset the 45-studs into the 195s as justusandersson suggested. Where should the fabric be placed then?
This is actually a bit of a tricky question because it involves an unusual roof construction. One must apply basic building physics to get it right.
As I understand it, PIR insulation is normally placed on top of the roof panel and provided with fully glued felt. However, I assume that the construction height becomes too large in your case. If you instead want to place the boards between the rafters under the roof panel, you should keep a few things in mind.
1 PIR seems to be in the format 600x1200 mm. The c/c distance of the rafters will then be 645 mm.
2 To achieve optimal fit, the rafters must be installed with great precision.
3 An air gap is needed between the roof panel and the PIR insulation to allow the roof panel to dry out. If the latter had been carpenter dry (takes at least 6 months without a drying facility), the air gap could have been avoided.
4 The simplest way to create an air gap is to attach battens, e.g., 25x25 mm at the top against the roof and the rafters.
The 45x45 rafters are fixed either by diagonal nailing/screwing into the beams or by nailing/screwing at the ends through the beams. No fittings. When the last insulation is in place, everything is covered with an airtight but vapor-permeable membrane.
As I understand it, PIR insulation is normally placed on top of the roof panel and provided with fully glued felt. However, I assume that the construction height becomes too large in your case. If you instead want to place the boards between the rafters under the roof panel, you should keep a few things in mind.
1 PIR seems to be in the format 600x1200 mm. The c/c distance of the rafters will then be 645 mm.
2 To achieve optimal fit, the rafters must be installed with great precision.
3 An air gap is needed between the roof panel and the PIR insulation to allow the roof panel to dry out. If the latter had been carpenter dry (takes at least 6 months without a drying facility), the air gap could have been avoided.
4 The simplest way to create an air gap is to attach battens, e.g., 25x25 mm at the top against the roof and the rafters.
The 45x45 rafters are fixed either by diagonal nailing/screwing into the beams or by nailing/screwing at the ends through the beams. No fittings. When the last insulation is in place, everything is covered with an airtight but vapor-permeable membrane.
Thank you, excellent explanation!
Now I plan to use OSB+Gypsum - do I still need the air gap then since I don't have any wood that needs drying? I assume OSB doesn't need drying then.
Any opinion on Isover's Vario membrane mentioned above? Doesn't seem like it works in this case, since you'll need to nail through the membrane if it is placed between PIR and 45-insulation. Maybe it's just as well to go with a regular air gap against the raw wood instead.
Now I plan to use OSB+Gypsum - do I still need the air gap then since I don't have any wood that needs drying? I assume OSB doesn't need drying then.
Any opinion on Isover's Vario membrane mentioned above? Doesn't seem like it works in this case, since you'll need to nail through the membrane if it is placed between PIR and 45-insulation. Maybe it's just as well to go with a regular air gap against the raw wood instead.
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You should have an air gap between råsport and PIR. Nowhere else. I don't think that Isovers Vario membrane is suitable here when you use PIR insulation.
Construction plywood is lighter and stronger than OSB, but more expensive. When you want a screwable substrate inside the drywall, it is superior.
Construction plywood is lighter and stronger than OSB, but more expensive. When you want a screwable substrate inside the drywall, it is superior.