Somebody has asked me to put a loft aerial in their loft as a temporary measure until they decide what they want to do regarding Sky or Freesat or BT vision etc.
The trouble is they have foil insulation between the rafters and the roof tiles.
Will an aerial work under the foil, I highly doubt it but I thought I would ask anyway?

There are quite a few layers of foil (I did not count then but I would guess about 3).

I put a loft aerial in my selfbuild. The roof insulation between the rafters is two layers of celotex so four layers of foil in all. With the insulation in place the signal levels were exceptionally poor as I expected (my background is in electronics). Some multiplexes were totally missing.

Before climbing on the roof to fix it to the chimney I removed just the one rafter bay of insulation in line with the aerial as an experiment. I was surprised to find I got a usable signal. Not full strength, but usable. This is pure luck though. You might cut a hole in the insulation and find you still don't get a signal.  

If you are reasonably close to a main transmitter almost any "digital" aerial might work after a fashion but it may not be the best aerial to use. Aerials sold as "Digital" aerials are generally wide band aerials but not everyone needs a wide band aerial. Narrow band or "Grouped" aerials can sometimes work better because they have more gain. It depends where they are and what transmitters they can see. They might have to use a wideband to get both digital and analogue.

Perhaps bash the postcode into here..

Digital
http://www.wolfbane.com/cgi-bin/tvd.exe?

Analogue
http://www.wolfbane.com/cgi-bin/tva.exe?

If in doubt tell us their postcode and we can give some idea how difficult reception will be in that area.

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As soon as you say masthead amp you kind of know the strength is going to be poor.

I think if his area needs a big aerial and a mast head amp he'd be advised not to waste money on a loft aerial. Thing it is so unpredictable. Perhaps worth a read...

http://www.aerialsandtv.com/loftaerials.html

Is there a reason why you can't run a wire from the existing aerial through the loft? I would persuade him to install a distribution amp immediatly below the existing aerial in the loft and run an output around in the loft to where he wants the new TV point.

The only issue is to check how the existing mast head amp is powered. Some are powered up the UHF down lead so cutting that and inserting a distribution amp would be more complicated. I think some Blake distribution amps provide power for a mast head amp up the UHF aerial input but you would have to find out what the mast head amp needed.

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That indicates a distribution amp of some kind is already fitted.

Can you not just use that?

That's possible. Some mast head amps have multiple outputs. How many wires can you see coming out of it?  If there are three then there might be a spare one.