Cavity Wall Insulation and Under Floor Heating

 I'm about to insulate the top 10 inches of a cavity wall that is in the roof
space. Its 3 3/4 inches deep, I was planning on putting fibre glass
insulation in the cavity and then putting sisalation over it. Would there be
much benefit from making the fibre glass 3 inchs thick and having a 3/4 inch
air space between the fibre glass and the sisalation?

I'm also toying with the idea of some home brew underfloor heating. All the
systems I have seen on the net use PEX or PEX/AL/PEX pipe. I priced PEX in
Australia and its $800 for a 200m roll of 16mm. If I am going to use
plastic, stainless steel or brass for the fittings, tank and pump etc, is
there any other reason that I would need to use PEX pipe instead of normal
poly pipe?
Thanks

 

There can be a downturn with under floor heating in Oz, depends where you
live.

The problem seems to occur when you have a fairly large swing in
temperature, between day and night.

You heat the floor to warm up the house, but if the outdoor temperature gets
quite warm during the day, that large slab of hot concrete overheats the
house, and there is no way you can shut it off.

Yes, you can shut down the actual hot water heating, but a hot cement floor
keeps heating the house until it cools down.

This of course is not a problem if the outdoor temperature remains cold.

One way to overcome it is to use the under floor heating to bring up the
indoor temperature below what is needed, and use another form of heating to
'top up' the areas you are wanting to be warmer, thus the cement floor does
not need to be so hot.

Just thought this might be of assistance to you.

 RamRod Sword of Baal wrote:

    You use both, exterior and interior thermostats. You adjust the
set-points, to account for the lag time, by experience for local conditions.

 

Thanks. I'm on a timber floor which I'm about to insulate so need to put the
pipes in now. It won't be the primary heat source, at least I don't think it
will be, will have to see how it goes.