I have a gas inset fireplace, one of those with coal stones. When cold/windy outside I get drafts in my living room. I cannot access the chimney directly from inside because the shape of the fire inset (otherwise I could push one of those inflatable balloons to seal the chimney. Q: would one of those "H" shaped chimney cap help to prevent down-draft? Q: do I need to post a job lookiing for a roofer? I live in Harrow.