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BungaSeppel20 Jan 2023 11:59Hello everyone,
we are soon planning the renovation of a typical bungalow from the late 1960s. Regarding energy efficiency, we are open-minded, but due to budget constraints, we want to focus on quick wins if possible. The previous owner has mostly already replaced the windows with very decent triple glazing, the facade is clad with facing bricks and in good condition, unfortunately without an air gap – so we want to avoid touching that and preserve its good state.
The roof, on the other hand, seems to have potential. We do not yet know the exact condition – technically, it was renovated about 15 years ago (new waterproofing on top), but we fear that insulation was not a priority at that time. We assume it is a cold roof with moderate insulation above the ventilation gap, probably installed just to accommodate the slope. Oh, and it is a timber beam construction, with a membrane covered by gravel on top.
A contractor, who will be doing some other work there, has proposed cellulose blown-in insulation combined with sealing the parapet (attic wall). The whole offer sounds surprisingly affordable (in the four-digit range).
What are the potential pitfalls here? How effective is this approach? Will it be structurally relevant in terms of weight? I am concerned that no one will guarantee the exact condition of the roof structure; apart from a visual inspection, it can hardly be determined. Or is that irrelevant, because cellulose, to put it simply, is not significant in terms of weight?
Sorry if this topic has already been discussed, but I have searched back to 2017 and found nothing. Also, technology evolves...
we are soon planning the renovation of a typical bungalow from the late 1960s. Regarding energy efficiency, we are open-minded, but due to budget constraints, we want to focus on quick wins if possible. The previous owner has mostly already replaced the windows with very decent triple glazing, the facade is clad with facing bricks and in good condition, unfortunately without an air gap – so we want to avoid touching that and preserve its good state.
The roof, on the other hand, seems to have potential. We do not yet know the exact condition – technically, it was renovated about 15 years ago (new waterproofing on top), but we fear that insulation was not a priority at that time. We assume it is a cold roof with moderate insulation above the ventilation gap, probably installed just to accommodate the slope. Oh, and it is a timber beam construction, with a membrane covered by gravel on top.
A contractor, who will be doing some other work there, has proposed cellulose blown-in insulation combined with sealing the parapet (attic wall). The whole offer sounds surprisingly affordable (in the four-digit range).
What are the potential pitfalls here? How effective is this approach? Will it be structurally relevant in terms of weight? I am concerned that no one will guarantee the exact condition of the roof structure; apart from a visual inspection, it can hardly be determined. Or is that irrelevant, because cellulose, to put it simply, is not significant in terms of weight?
Sorry if this topic has already been discussed, but I have searched back to 2017 and found nothing. Also, technology evolves...
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BungaSeppel3 Apr 2023 11:28I’m bumping this topic up again since there have been no replies so far, but it’s worth a try.
We have made some progress in analyzing the existing structure—old drawings were found, and a visual inspection more or less confirmed them:
From inside to outside, the structure is as follows: plaster, 5cm (2 inches) Heraklith (wood fiber insulation board), 2.5cm (1 inch) glass wool, then comes the ventilation space from the outside—the gap between the joists. Above that, it becomes vapor permeable. You can see corrugated cardboard from below, which isn’t in the drawings; maybe it’s the old sheathing. Above that is probably the “new” roof with an insulating mortar that, due to the ventilation, likely has no thermal function.
When I model this in Ubakus (as mentioned, everything above the ventilation layer has no effect), the U-value comes out to about 0.4 W/m²K. That’s not great but clearly better than “uninsulated.”
If I blow in cellulose insulation there (Ubakus unfortunately doesn’t have cellulose, so I used wood fiber loose-fill), the U-value drops to around 0.16 W/m²K, but the dew point then lies within the upper layer of the blown-in insulation. With 133 days of drying time, 25% wood moisture content, and over 2 kilograms of water per square meter, Ubakus strongly advises against this. Makes sense—basically, we have an interior insulation layer for which the rest of the assembly is not designed.
A vapor retarder beneath the Heraklith would help here; even the thinnest form would push everything into a safe range—but that isn’t installed, and removing all the plaster would likely increase costs toward a conservative roof renovation.
What do you think? This issue should affect everyone treating their roofs this way; otherwise, this type of blown-in insulation wouldn’t exist. Or can cellulose handle this strong moisture load?
We have made some progress in analyzing the existing structure—old drawings were found, and a visual inspection more or less confirmed them:
From inside to outside, the structure is as follows: plaster, 5cm (2 inches) Heraklith (wood fiber insulation board), 2.5cm (1 inch) glass wool, then comes the ventilation space from the outside—the gap between the joists. Above that, it becomes vapor permeable. You can see corrugated cardboard from below, which isn’t in the drawings; maybe it’s the old sheathing. Above that is probably the “new” roof with an insulating mortar that, due to the ventilation, likely has no thermal function.
When I model this in Ubakus (as mentioned, everything above the ventilation layer has no effect), the U-value comes out to about 0.4 W/m²K. That’s not great but clearly better than “uninsulated.”
If I blow in cellulose insulation there (Ubakus unfortunately doesn’t have cellulose, so I used wood fiber loose-fill), the U-value drops to around 0.16 W/m²K, but the dew point then lies within the upper layer of the blown-in insulation. With 133 days of drying time, 25% wood moisture content, and over 2 kilograms of water per square meter, Ubakus strongly advises against this. Makes sense—basically, we have an interior insulation layer for which the rest of the assembly is not designed.
A vapor retarder beneath the Heraklith would help here; even the thinnest form would push everything into a safe range—but that isn’t installed, and removing all the plaster would likely increase costs toward a conservative roof renovation.
What do you think? This issue should affect everyone treating their roofs this way; otherwise, this type of blown-in insulation wouldn’t exist. Or can cellulose handle this strong moisture load?
BungaSeppel schrieb:
If I now blow in wood insulation here (cellulose is unfortunately not available from Ubakus), the U-value drops to about 0.16, but the dew point then lies within the blown insulation under the outer layer. With 133 days drying time, 25% wood moisture, and over 2 kg of water per square meter, Ubakus strongly advises against it. It makes sense; basically, we now have interior insulation, which the rest of the structure is not designed for.No one here (nor any other neutral person) can give you a definitive answer on that. Converting ventilated insulation into "interior insulation" is always critical when the outward vapor diffusion is not clearly resolved. For blown-in insulation in masonry walls, this is usually fairly easy to assess, and leaks towards the masonry are also easy to detect.
Flat roofs are twice as risky. Firstly, because the outer layer (bitumen or EPDM) is usually vapor-tight, meaning that moisture that enters has difficulty escaping. Secondly, due to the heterogeneous structure of the roof with penetrations and connections, indoor air often enters the insulation layer, introducing more moisture.
Personally, I would avoid this approach in this case and, if it must be done, definitely inspect and, if necessary, repair or renew the roof waterproofing. Especially connection points at walls and cable penetrations should be checked to ensure everything is airtight. The latter also doesn’t hurt even if no insulation is blown in. So far, moisture can vent out, but heat loss remains.
Alternatively, installing 30mm (1.2 inch) PIR insulation plus drywall underneath would be a solution. Not ecologically perfect, but physically safe.
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BungaSeppel3 Apr 2023 16:08Nida35a schrieb:
We have a new bungalow with half a warm roof (open living space) and half a cold roof used for storage.
If you choose a warm roof, insulate the roof from the inside;
if you choose a cold roof, insulate the ceiling cavity above the rooms.
Both are different—
which one do you want? Thanks for the answer, but sorry, I don’t quite understand the question. The roof is as it is; it’s not a matter of choice. I think this roof structure is generally called a cold roof (roof surface cold, because insulation and ventilation are underneath). The question was where the risks are if I fill the ventilation cavity with blown-in insulation.
dertill schrieb:
No neutral party here (or elsewhere) can give you a definitive answer on this. Converting ventilated insulation to “internal insulation” is always critical if vapor diffusion to the outside is not clarified.
With blown-in insulation in masonry walls, this is usually quite easy to assess, and leaks to the masonry can be detected easily.
Flat roofs are doubly risky. First, because the outer layer (bitumen or EPDM) is usually vapor-tight, so any moisture that gets in has a hard time escaping. Second, due to the uneven construction of the ceiling with penetrations and connections, indoor air often enters the insulation layer, bringing in more moisture.
Personally, I would avoid this in your case, and if you still want to do it, I would definitely check and if necessary repair or renew the ceiling waterproofing. Especially connection points at the walls and cable penetrations should be inspected to ensure everything is truly airtight. This latter step is advisable even if no insulation is blown in—moisture can vent, but heat loss still occurs.
Alternatively, installing 30mm (1.2 inches) PIR rigid foam insulation plus plasterboard on the underside is an option. Not ideal from an ecological standpoint, but physically safe. Okay, thank you very much! So basically a classic internal insulation from below? Why are blown-in insulations for roofs offered on such a large scale then? Friends of ours also have a bungalow, and they were offered this too. But you can’t find any user experiences on the internet.
BungaSeppel schrieb:
Why are these blown-in insulations for roofs even offered on a large scale then?If you have a hammer, everything looks like a nail.
You can do it that way, and in many cases, it is a cost-effective and simple solution. It might work fine for you as well. But nobody can really inspect inside the roof, especially if it’s blown in. If you install PIR rigid foam boards on the underside or at least connect a vapor retarder airtight, you can have the cavity blown in as well. The current construction method is just safer from a physical standpoint.
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