Gordon Ross kicked five penalties as London Welsh beat 14-man London Irish in the battle of the Exiles.
Ross kicked Welsh into an early lead before Irish No.8 Chris Hala'ufia was sent off after only 10 minutes.
Three more Ross penalties sent the hosts in at half-time with a 12-6 lead, Tom Homer and Ian Humphreys replying with the boot for the visitors.
Ross and Homer exchanged kicks after the break, although Irish were left to rue three missed shots at goal.
Welsh's victory lifted them 13 points clear of bottom-of-the-table Sale and seven above second-from-bottom Irish.
The match was a dreadful spectacle with both sides lacking attacking invention.
It was the hosts who took the lead through Ross's boot after Irish were penalised for dragging down a line-out drive.
Welsh then lost centre Hudson Tonga'uiha to a shoulder injury and it was his replacement, Seb Jewell, who was the victim of a tip tackle that resulted in Hala'ufia being shown the red card.
Ross kicked the resulting penalty and another after 15 minutes to give Welsh a 9-0 lead.
Homer and Humphreys were on target to reduce the deficit, but Ross's fourth penalty ensured Welsh led by six points at the break.
It was a margin they were to maintain in the second half as Ross and Homer exchanged penalties, although Homer and Humphreys missed three other attempts between them as Welsh hung on.
VIEWS FROM THE DRESSING ROOM
London Welsh director of rugby Lyn Jones:
"There was no malice in the tackle [by Hala'ufia] but the referee's hands are tied. It was very similar to the Sam Warburton incident in the World Cup when the Welsh captain was sent off.
"All wins are good wins but our ill-discipline cost us dearly.
"We were good value for the win and they scrapped hard for their bonus point.
"Our defensive structure was very good and we are targeting six wins for survival, so with four wins so far we are well on target."
London Irish director of rugby Brian Smith:
"I thought it was a perfectly executed tackle. The referee made a big call as he was not 100% sure of the decision as he had to check with his touch judge.
"I would like to see the rugby league system brought in where a tackle like that goes on report and is assessed afterwards.
"It was an unbelievably gutsy effort and could have been a gutsy victory.
"We are not happy but we have salvaged a bonus point.
"I'm not going to blame the referee for our loss as we weren't precise enough and we lost 15 points with missed kicks."
Arscott, Caprice, Parker, Tonga'uiha, Scott, Ross, Keats, Montanella, Briggs, Ion, Mills, Kulemin, Browne, Jackson, Balding.
Jewell for Tonga'uiha (6), Bristow for Montanella (61), Tideswell for Ion (74), Corker for Browne (55), Bateman for Balding (58). Not Used: To'oala, Stevenson, Davies.
Homer, Ojo, Tagicakibau, Geraghty, Watson, Humphreys, Allinson, Lahiff, Blaney, Halavatau, Skivington, Evans, Danaher, Garvey, Hala'ufia.
Shingler for Watson (65), Moates for Allinson (65), Yanuyanutawa for Lahiff (49), Ellis for Blaney (55), Aulika for Halavatau (41), Low for Skivington (52), Gibson for Danaher (65), Sinclair for Garvey (67).
JP Doyle (RFU).