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Perry525 04-15-2010 06:55 AM

Insulating an attic
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A bathroom needs to be comfortable not too hot or too cold and as a bathroom creates a lot of water vapour it need to be ventilated.
I recommend that you do this job properly as water vapour can lead to wood rot.
You need to tear down the drywall.
And fit polystyrene or Blue Board between the framing, pack in at least four inches thick, and it needs to be a tight fit, between the rafters. No gaps or cracks.
Cutting it with a knife works OK, a saw makes even more mess.
Then fit a plastic water proof sheet over the framing taking care to overlap the joints and seal them.
This is to ensure that none of the water vapour gets into the wall/wood and starts rot.

Then fix another two inch thick layer of polystyrene over the plastic and finish with drywall. This last layer is to prevent the heat from the bathroom escaping by conduction through the wood, and it stops the heat from the sun coming in.

A temperature of 51C/124f on one side of a two inch thick piece of polystyrene will often result on a temperature of 38C/102f on the other, so you can see six inches of polystyrene should meet your requirements, with a drop to about 22C/70f.

It makes sense to do the whole attic to the same standard.

Then fit an extractor fan, one with an automatic shutter to keep out the wind and a built in humidistat that will turn it on as required.

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