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Thread: New Forced air heat/ a/c concerns due to water heater.

  1. #16
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    Blowing cellulose over old batts isn't a problem if they're not moldy-stinking full of mouse nests or something. A top-cap of cellulose "fixes" the inherent convection loss issues with low-density fiberglass. To get R50 you need at least 14-15" from the ceiling-gypsum to the top of the insulation. The weight of the higher-density overblow will compress the batts a bit, but as long as it's 15" deep on day 1 that will be your approximate R value. To hit R60 you'd be looking at ~17-18", which may be tough to do at the soffits as the roof deck slopes in. Rigid-board iso runs ~R6-6.5/inch of thickness, so even with 2x8 joists it's usually possible to stack in some iso to keep the R-values up to at least R40-R45 right up to and over the top plate of the studwall, whereas with batts or cellulose it might thin out to less than R30 at the ends, which will rob overall performance, and increase the ice-dam risk. With typical eastern/central MA snow loads and temps R50 is enough to prevent big ice dams from forming unless you have big heat-leaks from flues, plumbing vents, skylights, etc. (This year has been good test, eh?)

    R38 can even be enough to prevent ice dams if it's installed perfectly, but during cold snaps R38 fiberglass is performance is less than R30. The cost of even a 3" overblow of cellulose on R38 fiberglass is worth it, since it raises the cold-weather performance by nearly a factor of 2, even if it only improves the April performance by 25% or so. By filling in all voids & leveling out compressions it raises the true performance by more than what might be implied by the rated-R values of what was added and what was already there.

    A clever insulation contractor can usually insulate the cantilevered sections adequately attacking it from the exterior rather than in interior, with low overall mess & repair. In some instances it might be worth going with slow-rise "half pound pour" foam since it seals better than cellulose, but cellulose blown into feed-bags to keep it from filling up the entire joist & ceiling bay is a tried-& true methodology as well.

    Air sealing contractors typically run a calibrated blower door to come up with a ACH/50 numbers both before and after air-sealing, and will usually guarantee some minimum fractional improvement. The more you can seal the big and obvious leaks yourself (like abandoned or undampered flues), the better value you get out of them. Some will use infra-red cameras to find more subtle leaks during the initial blower door test, by de-pressuring the house and looking for the cold spots, then spin the fan the other way to pin-point the leaks using a smoke-pencil. The latter you can do yourself with a stick of incense and a window fan. If you're really into it you can buy a $75-100 pistol-grip infrared thermometer and find the cold-spots indicating leaks with the house de-pressured, but it's a lot quicker/easier with an IR camera (which is essentially unaffordable for a DIY-er) than a thermometer.

    I hope your leg heals quickly (and I hope it wasn't from falling off a ladder while cutting some channels in ice dams, as happened to an acquaintance of mine on Saturday... :-( )

    On the remote bedrooms, make sure it's not the returns being blocked, it may not be a supply duct problem at all. In many older homes the returns are commoned in a hallway or something, and if the doors are closed the "return" is via the great-outdoors through whatever infiltration leakage you have. If that's the case here it may be possible to use a partition wall cavity as a return path, with a grill at the bottom of the cavity on the room side, and at the top on the other side. If the air flow to the remote bedrooms continues to be an issue with a right-sized furnace and additional insulation, with a heat exchanger and some plumbing it's possible to use your hot water heater as a heating boiler to run baseboards or small radiators to heat those areas.

  2. #17
    DIY Member m3rdpwr's Avatar
    Join Date
    Dec 2008


    My joists are 2x6 I believe, not 2x8, but I plan on R19 and probably an R38.
    I've debated on the blow in iso, but it's something I'd have to hire out and I'm not sure the cost differance.

    Thanks for all your time, you've been more than patient with me. lol


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