Buying a synthetic sleeping bag would be a waste of money in my opinion (heavier, bulkier, less comfortable, less durable, unnecessary). As Pete says, dew is not a big deal. The only time when getting a sleeping bag damp is a problem is when doing Alpine-style serious routes over a number of days. There are a few things to think about: 1) Where has the moisture come from? The Xero 550 is a pretty warm bag for summer use, so it may well be from you sweating into the bag. If its coming from you then you need to sweat less - ie. wear less clothes, open the zip, etc.. Moisture is more likely to get stuck in a bag if there's high relative humidity in the tent, so keep the tent ventilated. Tents with mesh inners are a good idea, and for UK-use a two-skin tent is a must. 2) If there's very high humidity outside the tent, and it's warm, then getting moisture out of the tent is going to be difficult as there will be little in the way of vapour pressure gradient. In this case, venting will make no difference. 3) How wet is the bag getting? If it's moisture on the outside of the bag there's no problem at all. If it's getting into the down then this is not a disaster: down dries out. Down is way more durable than most people think, and it does not turn to 'porridge' unless it is completely soaked (ie. drenched), and then compressed while it is soaked. Down is amazing stuff - despite its greater density, it floats on water and to get it really wet requires quite a concerted effort. If you're worried about damaging the bag or the filling, don't be unless it is really really wet. Even then, you'll have to crush the down super-tight to make any lasting impression on it. As far as thermal properties go, a damp Xero 550 will still massively outperform any synthetic bag on a warmth-weight ratio. 4) A synthetic bag of the same warmth as a down one will not absorb as much moisture, but as a result may well feel a lot sweatier. When wet, both synthetic and down lose some warmth. | 
Mr Fuller
