Loft condensation –Airbricks perhaps?

[Flyfisher]

Could these polystyrene rafter insulation boards be a possible solution to my loft condensation problem?

http://www.wickes.co.uk/polystyrene-rafter-insulation-boards-pk3/invt/210020/

As one reviewer states: - “The panels have a series of very deep slots running along their length that are cut to match the panel to the width of the rafters. The remaining slots provide compressibility so the panel can be squeezed concertina-like between the rafters and the tension induced holds the panel firmly in place. No tape, glue. tacks or anything else required. A clever, neat, simple and effective design.”

It seems to me that with these panels ‘tightly’ installed between the rafters, there would be no cold slate or roofing felt surface for condensation to form on?

I don't think so.

Presumably your attic is not heated, in which case what will any insulation achieve? All it can do is slow down the speed at which the attic air temperature will equalise with the outside air temperature. Without an internal source of heat the attic cannot remain at a higher temperature than the outside air for very long. Also, consider what will happen when the outside air temperature rises. That same insulation will keep the attic temperature cooler for a while, leading to the possibility of condensation, even on warm, humid summer days.

If your ceilings are so 'leaky' that a significant amount of humid air is escaping into the attic then I suppose that good insulation might help to trap that escaped heat and help to keep the attic warm and might therefore reduce condensation, but I suspect you're not really trying to create a warm attic space.

Instead of insulation, vapour barriers and the like, all you really need is effective ventilation to allow humid air to escape. This won't require a howling gale through the attic, just a gentle and barely perceptible trickle of fresh air.

Also . . . .

It seems to me that with these panels ‘tightly’ installed between the rafters, there would be no cold slate or roofing felt surface for condensation to form on?

Beware of being fooled by 'cold' surfaces. The roof slates will undoubtedly feel colder to the touch than the wood rafters but think about why this should be. How could the wood actually be warmer than the slates in the absence of any form of heating if they are both in the same ambient temperature? The reason, of course, is that our fingers are warmer than both so when we touch them heat is drawn away from our finger and we feel that as a cool sensation. The faster the heat is drawn away, the cooler the surface 'feels'.

Put a wooden-handled hammer in your fridge overnight. In the morning hold it by the handle and also by the metal head. They will feel completely different, even though both the wood and head must be the same temperature.

 

[Zebra]

Could these polystyrene rafter insulation boards be a possible solution to my loft condensation problem?

Sounds a bad idea. Isn't the standard thinking that you should do one or the other, but not both? ie either warm roof construction, with insulation at rafter level, and the attic space thermally part of the house; or cold roof construction, with the insulation at ceiling level, and the attic separated from the house thermally, and ventilated to remove any moisture.

The only problem with this I’m sure is that warm air will no doubt filter through the lath plaster ceiling where the insulation has been pulled out.(we’ll see)

Will it? If warm air could filter through a lath and plaster ceiling, then surely everyone would have condensation in their loft. If there are holes and gaps in your ceiling then perhaps you could patch them up to prevent air passing through.

Any tips for sealing a loft hatch? Again my hatch is made of 25mm thick timber and sits in a timber frame within the ceiling joist structure. There are no hinges, it’s a fairly snug fit and it just pushes up into the loft. Last year I installed strips of fitted carpet in the hatch seating i.e. the hatch seals itself by sitting on the carpet within the frame and I also installed a secondary hinged plywood hatch above the existing timber hatch. This is connected by chain so that it pulls closed when the main hatch is lowered into its seating.

Sounds like your loft hatch is already pretty well sealed! Much better than most loft hatches with that secondary layer. If there is space in the frame and you don't use the loft very often you could also cut a perfectly fitting piece of rigid foam or polystyrene insulation and wedge it in the frame?

 

[Onetap]

To have condensation in a roof space there must be a source of moist air for the water to condense out of when cooled.
either the moist air is from outside (is the outside of the roof covered in condensation?) or from inside the house.

True, probably warm, moist air escaping into the loft from inside the house.

It may be air from outside, which drops condensation when the temperature drops overnight, but outside air in winter is cold and so can carry little water vapour. Look at a psychrometric
chart if you want to get into that in detail.

Similarly, insulation for chilled water (typically at 6 degC flow) must have a continuous vapour barrier on the outside of the insulation or water vapour penetrates the insulation and condenses on the pipe, saturating the insulation.

I've seen photographs of condensation in lofts from the US, or somewhere with seriously cold winters, where the condensation had frozen forming what appeared to be an indoor skating rink.

There's a hole somewhere in your ceiling I suspect (pipes to & from cold water storage tank??) with a warm draught blowing into the loft.

 

[Lime]

To have condensation in a roof space there must be a source of moist air for the water to condense out of when cooled.
either the moist air is from outside (is the outside of the roof covered in condensation?) or from inside the house.

True, probably warm, moist air escaping into the loft from inside the house.

It may be air from outside, which drops condensation when the temperature drops overnight, but outside air in winter is cold and so can carry little water vapour. Look at a psychrometric
chart if you want to get into that in detail.

Unless the outside air is saturated with moisture (blown/drifting off large grassed fields/parks or similar) it is almost certainly moist air from the house escaping into the loft as you say.

Similarly, insulation for chilled water (typically at 6 degC flow) must have a continuous vapour barrier on the outside of the insulation or water vapour penetrates the insulation and condenses on the pipe, saturating the insulation.

Indeed. One of the chief reasons I do not use chest freezers is that the vapour barrier under the chest gets damaged allowing moisture to pass through to the inner freezer skin.
The base of the freezer insulation becomes a frozen lump of ice with disastrous consequences to the insulation R-value.

There's a hole somewhere in your ceiling I suspect (pipes to & from cold water storage tank??) with a warm draught blowing into the loft.

Exactly so.

 

[Onetap]

There was a recent discussion about this on a DIY forum I frequent in which it was mentioned that possible causes could be downlighters or water vapour from a cold water storage tank or heating feed & expansion cistern (which should be cold anyway).

http://www.diynot.com/forums/plumbing/another-loft-condensation-post.351718/