ᐅ Insulating a cold roof with compressed fiberglass insulation, cost
Created on: 15 Feb 2019 17:39
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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
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boxandroof18 Feb 2019 22:22Thank you for the response.
I have windows in the gable. I can ventilate, heat, and dry the space. The question is how to approach this systematically and identify any potential issues. It is also clad, not plastered.
I have windows in the gable. I can ventilate, heat, and dry the space. The question is how to approach this systematically and identify any potential issues. It is also clad, not plastered.
B
boxandroof18 Feb 2019 22:48This is a new build, occupied since July 2018. Solid construction with a wooden pitched roof.
Intermediate ceiling: plasterboard, vapor barrier, 20cm mineral wool (8 inches) plus OSB board.
Roof structure: inside plasterboard with vapor barrier, insulation between rafters, membrane, roof tiles.
Access: insulated pull-down stair.
Rooms below: several bedrooms, bathroom.
Ventilation: irregular manual ventilation; during the first winter, occasional electric heating and use of dehumidifiers as a precaution (without significant water extraction).
Intermediate ceiling: plasterboard, vapor barrier, 20cm mineral wool (8 inches) plus OSB board.
Roof structure: inside plasterboard with vapor barrier, insulation between rafters, membrane, roof tiles.
Access: insulated pull-down stair.
Rooms below: several bedrooms, bathroom.
Ventilation: irregular manual ventilation; during the first winter, occasional electric heating and use of dehumidifiers as a precaution (without significant water extraction).
The same basic principle applies here as with Nordlys: an insulated roof with a vapor barrier is rather problematic if the space is not heated.
In your case, the OSB used as a floor covering makes it even more complicated. This means your top floor ceiling is completely sealed (it also acts as a vapor barrier). Especially if the vapor barrier below—as you mentioned—was not installed correctly, there is a significant risk to the structure. Moisture can enter from below but cannot escape upwards.
Due to the complexity of the situation (OSB, mechanical ventilation with heat recovery), I’m reluctant to give specific advice. However, in my opinion, the floor covering should at least be replaced with tongue-and-groove boards (which are vapor-permeable). Alternatively, it might be sufficient to cut out strips of the OSB panels or drill larger holes in them. Please consult an independent expert about this—they will also be responsible for their recommendations.
To be clear: I see an urgent need for action here. This might last for decades without issues, but it can also lead to rotting ceiling joists within a very short time. Personally, I would consider the risk too high.
In your case, the OSB used as a floor covering makes it even more complicated. This means your top floor ceiling is completely sealed (it also acts as a vapor barrier). Especially if the vapor barrier below—as you mentioned—was not installed correctly, there is a significant risk to the structure. Moisture can enter from below but cannot escape upwards.
Due to the complexity of the situation (OSB, mechanical ventilation with heat recovery), I’m reluctant to give specific advice. However, in my opinion, the floor covering should at least be replaced with tongue-and-groove boards (which are vapor-permeable). Alternatively, it might be sufficient to cut out strips of the OSB panels or drill larger holes in them. Please consult an independent expert about this—they will also be responsible for their recommendations.
To be clear: I see an urgent need for action here. This might last for decades without issues, but it can also lead to rotting ceiling joists within a very short time. Personally, I would consider the risk too high.
B
boxandroof18 Feb 2019 23:58Thank you for the assessment. Okay – I understand where problems might occur. In that case, ventilating in the attic won’t help.
Sorry, I realize I’m sharing limited information myself: I had an expert on site (called in too late) to discuss the moderate results from the blower door test. He said I should monitor it over the first winters (but how exactly? hence my original question), but didn’t offer any other suggestions. Of course, he advised against a complete dismantling – the blower door test was passed. Regarding the balanced mechanical ventilation system, he generally saw the risk of moisture damage as lower than without it.
However, it’s also possible that the balanced mechanical ventilation might be pushing moist air upwards somewhere.
Sorry, I realize I’m sharing limited information myself: I had an expert on site (called in too late) to discuss the moderate results from the blower door test. He said I should monitor it over the first winters (but how exactly? hence my original question), but didn’t offer any other suggestions. Of course, he advised against a complete dismantling – the blower door test was passed. Regarding the balanced mechanical ventilation system, he generally saw the risk of moisture damage as lower than without it.
However, it’s also possible that the balanced mechanical ventilation might be pushing moist air upwards somewhere.
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