28-08-2012, 07:22 PM
Hi Gary,
That's extremely wise of you
Jeff Howell is also very vocal about cavity wall insulation, and believes that there is no satisfactory way to retro-fit insulation to a cavity wall. OK, it's fine if the walls were originally built that way - they have a much larger gap, and the sheets of insulation are firmly bonded to the inner leaf with the wall-ties - but anything used to fill the gap will cause bridging...
Cavity walls were first used in coastal areas to reduce wind-driven rain ingress. The idea was that water would penetrate the outer skin, then trickle down the inside of the outer leaf. As the cavity is (or should be) well-ventilated, the water would harmlessly evaporate away.
Interestingly, a single-skin (4.5") wall is guaranteed to leak, no matter how well constructed it might be. The problem is thermal expansion; when the bricks cool down and shrink slightly, gaps open up in the vertical sections of mortar (the "perps").
Anyway, cavity walls were never intended to be thermally efficient, and indeed, they're not especially. If the cavity is ventilated, the air in the cavity is not much warmer than the outside air, so you're relying on the inner leaf to provide the bulk of the thermal insulation (some of the modern blocks used are pretty good compared to traditional red brick). As an aside, I wouldn't be surprised if a house built with solid 9" walls was better than a house made with early cavity walls - providing the walls are totally dry...
Of the cavity wall insulation that is generally fitted, the most common is mineral fibres that are blown in via small holes. These work like fibreglass and Rockwool; by trapping air. But these only work when they are dry - as soon as the fibres become wet, they lose all their insulation properties. And, because the cavity is no longer ventilated, the fibres take a long time to dry. The result is a cold spot on the inner leaf, which inevitably attracts condensation.
Another problem is caused by gaps - invariably there are areas that don't get any of the infill - especially around obstructions like windows and doors, and wall ties or lumps of mortar that fell into the cavity during construction can cause these problems. These also cause cold-spots on the inner leaf.
Also, mineral-fibre insulation can settle with age, causing gaps at the tops of the walls. And as the lower levels compress, less air is trapped, so the insulation value falls away. This can be a problem with timber-framed houses too - frequently mis-diagnosed as a roofing or penetrating damp problem.
Yet another problem is that a lot of houses have their joists socketed in the inner leaf, so when you block up the cavity, you run the risk of rot forming in the ends. More modern houses use metal joist hangers, of course.
At least fibres can be removed (expect a massive industry to emerge over the next few years, specialising in removing them). Some people inject expanding foam, which is more or less impossible to remove - while these should be better when wet, gaps and cold-spots can still occur. Some houses are insulated with tiny polystyrene beads, and these fall out in copious quantities when you drill through the wall!
Finally, over-insulating your loft is a bad idea because the timbers above the insulation will be proportionally colder. Which means they will attract condensation - perhaps not enough to be dripping wet, but damp they will be. And what do we know about damp timber? Yes, back to the topic of this thread - our local beetle population will soon set up home in them. And they will be much more susceptible to rot as well! I really wish the government would listen properly to the people advising them, rather than just picking out the bits they want to hear!
We have 4" of insulation in our loft, and I will be putting no more down on the top of the ceiling. I'm willing to put sheets of foam insulation (Kingspan or similar) between the rafters to insulate the slopes themselves. We have 4" timbers, so 2" sheets will be perfect (building regs call for a 2" gap between insulation and the roof surface. When the place was re-roofed about 15 years back, they used a plastic sarking layer with lots of tiny holes that make it breathable yet waterproof, so I'd be confident that the timbers will remain in good condition. Having a bit of warmth up there will hopefully help my collection of radios and other junk survive until the kids leave home and I can start doing electronics again :-)
All the best,
Mark
That's extremely wise of you
Jeff Howell is also very vocal about cavity wall insulation, and believes that there is no satisfactory way to retro-fit insulation to a cavity wall. OK, it's fine if the walls were originally built that way - they have a much larger gap, and the sheets of insulation are firmly bonded to the inner leaf with the wall-ties - but anything used to fill the gap will cause bridging...
Cavity walls were first used in coastal areas to reduce wind-driven rain ingress. The idea was that water would penetrate the outer skin, then trickle down the inside of the outer leaf. As the cavity is (or should be) well-ventilated, the water would harmlessly evaporate away.
Interestingly, a single-skin (4.5") wall is guaranteed to leak, no matter how well constructed it might be. The problem is thermal expansion; when the bricks cool down and shrink slightly, gaps open up in the vertical sections of mortar (the "perps").
Anyway, cavity walls were never intended to be thermally efficient, and indeed, they're not especially. If the cavity is ventilated, the air in the cavity is not much warmer than the outside air, so you're relying on the inner leaf to provide the bulk of the thermal insulation (some of the modern blocks used are pretty good compared to traditional red brick). As an aside, I wouldn't be surprised if a house built with solid 9" walls was better than a house made with early cavity walls - providing the walls are totally dry...
Of the cavity wall insulation that is generally fitted, the most common is mineral fibres that are blown in via small holes. These work like fibreglass and Rockwool; by trapping air. But these only work when they are dry - as soon as the fibres become wet, they lose all their insulation properties. And, because the cavity is no longer ventilated, the fibres take a long time to dry. The result is a cold spot on the inner leaf, which inevitably attracts condensation.
Another problem is caused by gaps - invariably there are areas that don't get any of the infill - especially around obstructions like windows and doors, and wall ties or lumps of mortar that fell into the cavity during construction can cause these problems. These also cause cold-spots on the inner leaf.
Also, mineral-fibre insulation can settle with age, causing gaps at the tops of the walls. And as the lower levels compress, less air is trapped, so the insulation value falls away. This can be a problem with timber-framed houses too - frequently mis-diagnosed as a roofing or penetrating damp problem.
Yet another problem is that a lot of houses have their joists socketed in the inner leaf, so when you block up the cavity, you run the risk of rot forming in the ends. More modern houses use metal joist hangers, of course.
At least fibres can be removed (expect a massive industry to emerge over the next few years, specialising in removing them). Some people inject expanding foam, which is more or less impossible to remove - while these should be better when wet, gaps and cold-spots can still occur. Some houses are insulated with tiny polystyrene beads, and these fall out in copious quantities when you drill through the wall!
Finally, over-insulating your loft is a bad idea because the timbers above the insulation will be proportionally colder. Which means they will attract condensation - perhaps not enough to be dripping wet, but damp they will be. And what do we know about damp timber? Yes, back to the topic of this thread - our local beetle population will soon set up home in them. And they will be much more susceptible to rot as well! I really wish the government would listen properly to the people advising them, rather than just picking out the bits they want to hear!
We have 4" of insulation in our loft, and I will be putting no more down on the top of the ceiling. I'm willing to put sheets of foam insulation (Kingspan or similar) between the rafters to insulate the slopes themselves. We have 4" timbers, so 2" sheets will be perfect (building regs call for a 2" gap between insulation and the roof surface. When the place was re-roofed about 15 years back, they used a plastic sarking layer with lots of tiny holes that make it breathable yet waterproof, so I'd be confident that the timbers will remain in good condition. Having a bit of warmth up there will hopefully help my collection of radios and other junk survive until the kids leave home and I can start doing electronics again :-)
All the best,
Mark
