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Attic ventilation

user-5208075 | Posted in General Questions on

2,100 sf ranch style house on a slab was flipped by contractor prior to our purchase in December 2014. Gas hot air furnace and HVAC unit, new in attic, replaced outdated equipment. Flipper added blown in insulation in attic to R38 and when testing of ducts failed — the ducts were brought up to code. Living space was also evaluated for air tightness and leaks were resolved.

House is shaped like the letter L with the short end over the MBR. This Massachusetts home experienced loud noises over the MBR during the winter nights and the entire roof ( from gutters to the ridge vent ) had ice dams. Because the roof has recently experienced hail damage our contractor has evaluated the attic ventilation ( full ridge vent, soffits around, two gable vents and a powered attic ventilation fan that is thermostatically controlled ) prior to replacing the roof. Contractor is advising that the numerous methods of venting are working against each other and that hot air is not escaping but is simply being ‘blown around’. His suggestion is to eliminate the gable vents and the ridge vent and then either use an attic ventilation fan over the furnace, another over the MBR and a third over the remaining living area – or keep the ridge vent and insure the soffits are all clear.

What do you suggest that will reduce the noise over the MBR and solve the ice dam problem?

Thank you,
Ray Gendreau

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  1. GBA Editor
    Martin Holladay | | #1

    In Massachusetts, a furnace does not belong in an attic. That's what is causing your ice dams. Ventilation changes won't solve the problem.

    If you want to keep your furnace in your attic, you will have to change your attic from a vented attic to an unvented conditioned attic.

    Here are two articles you should read:

    Creating a Conditioned Attic

    Prevent Ice Dams With Air Sealing and Insulation

  2. charlie_sullivan | | #2

    I don't agree with the circulating hot air theory, and closing the vents and adding fans are both bad ideas.

    The problem is the furnace in the attic. The best solution would be to move it out of the attic, but another solution would be to create a small insulated utility room in the attic, with insulated walls and ceiling, and at least a few inches vent space between that insulation and the roof deck, surrounding the furnace and, if possible, the main ductwork.

    One thing to check on is whether the new insulation blocked the soffit vents. You mentioned keeping them clear but I'm not sure what you were thinking might block them. There should be a baffle to hold back the insulation and keep a clear channel from the soffit vent up to to attic.

    Other contributing factors could be remaining duct leaks, air leaks through the ceiling, and heat loss from the ducts. There can be air paths from the outside, up through walls into the attic, that are hard to detect with a conventional blower door test.

  3. Expert Member
    Dana Dorsett | | #3

    The contractors' assessment and proposed solution is out to lunch.

    If you're re-roofing, the real (but not cheap) solution to the ice dam issue is to put R20 rigid foam over the top of the roof deck and R30 of any type of insulation tight to the underside of the roof deck, seal off all of the soffit & ridge vents,then insulate & seal the gables. There are multiple vendors of reclaimed roofing foam in MA to take the financial sting out of the material cost, but if the contractor is clueless as to how to install it you might want to find another contractor. (Green Insulation Group in Worcester and Nationwide Foam in Framingham are the biggest vendors of reclaimed & factory seconds foam that I'm aware of.) Don't do the whole thing as polyiso- the outer inch or two will underperform in mid-winter. But 2" of polyiso with 2" of EPS or XPS on top would get you to a reliable R20+ performance. In homes with trussed attics 7-8" of open cell spray foam on the interior is probably the quickest way to hit the R30 range. It can be sprayed with intumescent paint rather than covering it to meet fire codes. (Open cell foam is a quick & easy way to air-seal the soffits too.)

    That brings both the ducts and the air handler fully inside of conditioned space, reducing the thermal impact of any duct or air handler leakage and it will pretty much eliminate ice damming except for the most extreme circumstances (like 5' cornices & drifts on the leeward side of the ridges after successive nor'easters, like I had this past winter.)

    Are the "loud noises" air-handler rattle, furnace burner noises or something else?

  4. Expert Member
    Dana Dorsett | | #4

    BTW: If you're in National Grid's service territory you may be able to get subsidy money for a "roof only" deep energy retrofit, taking it to an even higher-R

    Bear in mind that when you go higher than R50 total R for the roof you need to keep the exterior-R at least 40% of the total for dew point control at the roof deck, which means a somewhat thicker roof as seen from above.

    See also:

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