ᐅ Insulating a cold roof with compressed fiberglass insulation, cost
Created on: 15 Feb 2019 17:39
N
NordlysHello friends,
We have a bungalow with a slab measuring 13.5 by 10 meters (44 by 33 feet), and a hipped roof with a 30° pitch along the length and 45° on the short sides. So far, the roof is not insulated. The roof area is approximately 160 square meters (1,722 square feet), possibly a bit more. The ceiling between the floors is insulated. We use the attic not as living space but as a cellar substitute. It is accessible by a staircase, not a hatch, since the stairs are easier to use. A patio door separates the living area from the unheated attic well, providing good thermal separation. Unfortunately, this floor gets very cold in winter during frost — I’ve even had paint freeze — and very warm in summer. Now that we have some funds available, we have asked a carpenter to provide a quote for insulating the unheated attic floor (ceiling below the attic).
He is quoting us 8,500 gross for installing 200 mm (8 inches) of cavity insulation between the rafters, along with a vapor barrier and drywall lining, without plastering or painting.
Is this a fair price? Too expensive? Or even a good deal?
Karsten
11ant schrieb:
....that the insulation in the attic is visible, meaning there is no sealed cavity between it and the indoor climate of the loft....Hello 11ant,
Just to be sure: By cavity, you mean the case where there is no covering with OSB, drywall, or similar. In that case, the insulation would need to be as deep as the rafters to avoid creating cavities.
If we don’t install any covering but only apply the vapor barrier membrane, the cavity shouldn’t be an issue, right? 🙂
netuser schrieb:
If we don’t add cladding and only install the vapor barrier, the cavity shouldn’t be a problem, right? By installing a “tightly stretched” ;-) membrane, you basically create the cavity that you then have to deal with (???). I’m not sure what the point of that is…
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https://www.linkedin.com/company/bauen-jetzt/
Obviously, the details here are important, and I don't yet fully understand them.
A brief excerpt from "Bauwissenonline":
Moisture Protection:
"Insulation materials must be especially protected against moisture, as moisture reduces their insulating effectiveness and can lead to long-term building damage (e.g., mold). Protection against external moisture is ensured by the roofing covering in combination with the underlayment.
However, moisture can also develop inside the roof structure due to water vapor from the interior, which can cause condensation. Water vapor passing through diffusion-permeable insulation materials does not cause damage as long as no condensation occurs. To prevent this, almost every pitched roof assembly requires a special vapor retarder or vapor barrier layer on the interior side of the roof structure, which must be matched to the specific roof construction. In a diffusion-open roof assembly, water vapor from the indoor air can diffuse outward through the insulation. In the case of a vapor-tight roofing membrane without ventilation, a vapor barrier or a variable-permeability vapor retarder must be installed to prevent damage within the insulation layer."
Furthermore, regarding condensation due to water vapor convection:
"To prevent internal condensation due to water vapor convection, a continuous air barrier layer must be ensured. Even very small leaks in vapor-tight membranes (vapor barriers) can cause severe moisture damage because water vapor convection transports significantly more moisture than diffusion alone."
Ventilation Cavity:
In the past, vapor-tight underlayments were commonly used, which meant insulation thickness was chosen to be less than the rafter depth to provide a ventilation cavity directly above the insulation, connected to outside air. This is called a ventilated roof. This allowed moisture accumulating under the vapor-tight underlayment to dry out.
As far as I understand, our construction is a "ventilated roof assembly."
Therefore, a vapor RETARDER as a diffusion-restrictive layer should be installed on the interior side of the roof assembly—or am I mixing things up here?
A brief excerpt from "Bauwissenonline":
Moisture Protection:
"Insulation materials must be especially protected against moisture, as moisture reduces their insulating effectiveness and can lead to long-term building damage (e.g., mold). Protection against external moisture is ensured by the roofing covering in combination with the underlayment.
However, moisture can also develop inside the roof structure due to water vapor from the interior, which can cause condensation. Water vapor passing through diffusion-permeable insulation materials does not cause damage as long as no condensation occurs. To prevent this, almost every pitched roof assembly requires a special vapor retarder or vapor barrier layer on the interior side of the roof structure, which must be matched to the specific roof construction. In a diffusion-open roof assembly, water vapor from the indoor air can diffuse outward through the insulation. In the case of a vapor-tight roofing membrane without ventilation, a vapor barrier or a variable-permeability vapor retarder must be installed to prevent damage within the insulation layer."
Furthermore, regarding condensation due to water vapor convection:
"To prevent internal condensation due to water vapor convection, a continuous air barrier layer must be ensured. Even very small leaks in vapor-tight membranes (vapor barriers) can cause severe moisture damage because water vapor convection transports significantly more moisture than diffusion alone."
Ventilation Cavity:
In the past, vapor-tight underlayments were commonly used, which meant insulation thickness was chosen to be less than the rafter depth to provide a ventilation cavity directly above the insulation, connected to outside air. This is called a ventilated roof. This allowed moisture accumulating under the vapor-tight underlayment to dry out.
As far as I understand, our construction is a "ventilated roof assembly."
Therefore, a vapor RETARDER as a diffusion-restrictive layer should be installed on the interior side of the roof assembly—or am I mixing things up here?
You can certainly "fold" the vapor barrier and attach it directly to the insulation. However, if you stretch it over the rafters and the insulation does not fully fill the cavity depth, you create a separate air space between the insulation and the vapor barrier that no longer mixes with the air in the attic. This risks having different climates on either side of the vapor barrier within those air volumes. In such cases, I would expect condensation to form on the insulation side of the vapor barrier. Using insulation as thick as the rafters avoids this problem (or alternatively "folding" the vapor barrier, which in my opinion is a bit of a tedious task). But be careful – my expertise lies in drawing clerk insults, windows, and a handful of other fields—not in insulation stuff with all its various side issues!
https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/
https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/
11ant schrieb:
But be careful—I’m the expert here for criticizing Zeichenknecht, windows, and a handful of other topics, but not for insulation stuff with all its side issues!I’m aware of that and take it into account in my risk analysis 😉
Thanks anyway!
Hmm, in that case, I probably would have been better off with 24cm (9.5 inches) insulation thickness (rafter depth 24cm).
Too bad I already went with the 18cm (7 inch) option.
Is there any technical reason against installing the insulation flush on the inside and sealing it with the vapor barrier, so the cavity remains on the ventilated outside? Nonsense or a possible alternative solution?
netuser schrieb:
It's unfortunate that I stocked up on the 18mm (0.7 inch) panels on short notice. From my perspective, this isn’t a major issue, just a bit inconvenient: bending the membrane at each rafter bay, following the insulation contour, and then realigning to the rafter plane adds about 2 x 6cm (2.4 inches) more membrane width per bay and takes quite a bit more effort, but I don’t see any real problems.
netuser schrieb:
Is there any technical reason not to apply the insulation flush on the interior side now and finish with the vapor barrier, leaving the cavity on the ventilated exterior side? Nonsense or a possible alternative solution? I’ll leave that to those who can contribute more knowledgeable opinions on the matter.
https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/
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