Drywall at €2.45/m2: the trap of insulating a damp house
Drywall board at €2.45/m2 at Brico Depot and XPS at €3.25/m2: the prices don't solve rising damp or condensation in a stone house without a cavity.
English · Original discussion in Spanish · Published
Drywall at €2.45/m2: the price doesn't hide the damp
A price of €2.45/m2 for a 260x120x13 mm board sounds like a bargain. And it is, if the goal is to finish a wall. The problem starts when that drywall is used to "insulate against damp" an 80 m2 upper floor in a 50-year-old Galician stone house in Spain, with no air cavity and mould lifting the paint. The price of the material is the easy part. The hard part is knowing where the water comes from.
How much a square metre of drywall costs in 2015
The starting point is a Brico Depot offer: €2.45/m2 for the standard 260x120x13 mm board and €4.95/m2 for the moisture-resistant one in the same size. In the same flyer, extruded polystyrene (XPS) comes out at €3.25/m2, 4 cm rock wool at €2.25/m2 and 6 cm mineral wool with kraft paper at €1.25/m2.
Is it a steal? Some people recall seeing the board at 2 euros in promotions at Leroy Merlin and Brico Depot. Another construction budget quotes €8.10 per board of 2500x1200x13 mm, about €2.7/m2. In other words: the price of the board isn't the problem. Transport is —they're bulky and heavy—, and cutting too: a 260 cm board has to be cut in 250 cm walls.
Rock wool or extruded polystyrene against damp
Here the consensus is uncomfortable. Rock wool is not suitable where there is damp: when it gets wet it loses much of its insulating capacity. The kraft paper it comes with is a vapour barrier that goes on the inside, to stop moisture from the room reaching the wool. If the water comes from outside, that paper is useless.
XPS withstands contact with water better. A real case: a north-facing room, cold and mouldy, was lined with extruded polystyrene held by metal furring channels —screwed through the insulation— with drywall on top. Four years later, without repainting, while the rest of the house was still being scrubbed with bleach every summer. The key: leaving an air gap between the drywall and the insulation and avoiding thermal bridges.
Why drywall can make the problem worse
If the damp is rising damp —the kind that climbs from the foundations through the pores of the wall—, covering it with a drywall lining is a mistake. The water is still there, with no way out. The only real solution involves permanent ventilation or cutting off the moisture path at the source. And rising damp rarely reaches an upper floor: it escapes into the air first.
The other hypothesis is condensation. A closed house, unheated and with airtight double-glazed windows, accumulates vapour. The test is simple: leave a window slightly open for a few days. If the mould recedes, it wasn't the wall. If literal water appears on the floor under a window with the heater on, the diagnosis points to condensation.
What the numbers say about savings
For an 80 m2 floor, the difference between the cheap board and the moisture-resistant one is €2.5/m2. On the total, about 200 euros. Insulation adds up: XPS at €3.25/m2, rock wool at €2.25/m2, mineral wool with kraft at €1.25/m2. The reflective insulation roll for floors comes out at €2.86/m2.
The full calculation, line by line, gives a difference that surprises more for what it doesn't include —labour, mould treatment, ventilation— than for the material itself.
At these prices, lining 80 m2 costs less than many painting quotes. The question isn't how much you save on drywall. It's whether you know where the water you're going to seal behind it comes from.
A price of €2.45/m2 for a 260x120x13 mm board sounds like a bargain. And it is, if the goal is to finish a wall. The problem starts when that drywall is used to "insulate against damp" an 80 m2 upper floor in a 50-year-old Galician stone house in Spain, with no air cavity and mould lifting the paint. The price of the material is the easy part. The hard part is knowing where the water comes from.
How much a square metre of drywall costs in 2015
The starting point is a Brico Depot offer: €2.45/m2 for the standard 260x120x13 mm board and €4.95/m2 for the moisture-resistant one in the same size. In the same flyer, extruded polystyrene (XPS) comes out at €3.25/m2, 4 cm rock wool at €2.25/m2 and 6 cm mineral wool with kraft paper at €1.25/m2.
Is it a steal? Some people recall seeing the board at 2 euros in promotions at Leroy Merlin and Brico Depot. Another construction budget quotes €8.10 per board of 2500x1200x13 mm, about €2.7/m2. In other words: the price of the board isn't the problem. Transport is —they're bulky and heavy—, and cutting too: a 260 cm board has to be cut in 250 cm walls.
Rock wool or extruded polystyrene against damp
Here the consensus is uncomfortable. Rock wool is not suitable where there is damp: when it gets wet it loses much of its insulating capacity. The kraft paper it comes with is a vapour barrier that goes on the inside, to stop moisture from the room reaching the wool. If the water comes from outside, that paper is useless.
XPS withstands contact with water better. A real case: a north-facing room, cold and mouldy, was lined with extruded polystyrene held by metal furring channels —screwed through the insulation— with drywall on top. Four years later, without repainting, while the rest of the house was still being scrubbed with bleach every summer. The key: leaving an air gap between the drywall and the insulation and avoiding thermal bridges.
Why drywall can make the problem worse
If the damp is rising damp —the kind that climbs from the foundations through the pores of the wall—, covering it with a drywall lining is a mistake. The water is still there, with no way out. The only real solution involves permanent ventilation or cutting off the moisture path at the source. And rising damp rarely reaches an upper floor: it escapes into the air first.
The other hypothesis is condensation. A closed house, unheated and with airtight double-glazed windows, accumulates vapour. The test is simple: leave a window slightly open for a few days. If the mould recedes, it wasn't the wall. If literal water appears on the floor under a window with the heater on, the diagnosis points to condensation.
What the numbers say about savings
For an 80 m2 floor, the difference between the cheap board and the moisture-resistant one is €2.5/m2. On the total, about 200 euros. Insulation adds up: XPS at €3.25/m2, rock wool at €2.25/m2, mineral wool with kraft at €1.25/m2. The reflective insulation roll for floors comes out at €2.86/m2.
The full calculation, line by line, gives a difference that surprises more for what it doesn't include —labour, mould treatment, ventilation— than for the material itself.
At these prices, lining 80 m2 costs less than many painting quotes. The question isn't how much you save on drywall. It's whether you know where the water you're going to seal behind it comes from.
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Summary of a discussion on Burbuja.info - Foro de economía, actualidad y política., translated from Spanish and reviewed before publication. Read the full discussion (20 replies).
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