Higher wattage heat mat?

At the moment I'm using a 13.5W heat mat under a wood viv. It's on 24 hours with an incandescent bulb on for 12 hours during the day. The heat mat on it's own is not keeping ambiente temperature up quite enough at night. Would a 20W mat create more heat? My daytime temps are fine at the moment - would a 20W mat maybe bring my daytime temps too high?

Also my humidity increases from about 50% during the day to over 70% at night. Does anyone else experience this at all? Is it a problem?

The 20 watt will put out more heat yes, but you ahve to have it on a thermostat.. Undertank heat is really inefficent at raising the ambient temps in a cage. That is really something that you would have to try to see if it works(I know that sucks). I cant stress enough a good thermostat in a wood cage.. FIRE:( You dont want the pad to be able to creste enough heat to cause any problems....