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Attic ventilation in saltbox

Conor Stinson | Posted in Energy Efficiency and Durability on

I’m trying to retrofit my 1978 salt-box style house. High on the list is improving the insulation in the attic. In my mind this includes air sealing, solid attic hatch insulation and construction, venting, and then blown cellulose insulation.

The house is two floors. In plan it resembles a U-shape, as some of the rear salt box portion was cut out to make a deck (and I think get more natural light into the first floor). The roof line cuts through the second floor such that there are two knee wall attic spaces, accessible via doors on the second floor.  This means the soffits on the rear side of the house are down by the first floor for part of the roof, then several feet below the attic for the bottom of the “U”. The front of the house has soffits that are essentially part of the attic. The roof has a ridge vent. 

The sloped ceilings of the knee wall spaces and the interior sloped walls of two second floor rooms are insulated with fiberglass insulation. I can’t really speak to the state of their ventilation. 

My question is: can I use vents at the front of the house – soffit vents as so richly described in these pages – and the ridge vent as sufficient ventilation for the attic? 

Sorry if I’m not clear, if pictures would help I can try to accommodate. I’ve been hovering on these forums for months and am just now dipping my toe in. Thanks so much for everyone out there, you’ve already been helpful! I just need a little more specific advice!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Conor,
    A saltbox roof has a long side and a short side. You mention the "front" of your house, but you never told us whether the long side of the roof faces the front or the back.

    To answer your question, though: For soffit vents and ridge vents to work for attic venting, you need to verify that there is a continuous air gap in each rafter bay from the soffit vent to the attic space. If you have that air gap -- ideally, a gap that is at least 1 inch deep, although 2 inches is better -- then your soffit vents will work.

    Ideally, the air gap needs to be well defined -- which means you need a vent baffle above your insulation. The baffle defines the bottom of the vent space; the underside of the roof sheathing defines the top.

    For more information on these issues, see these articles:

    "Site-Built Ventilation Baffles for Roofs"

    "All About Attic Venting"

    When insulating the small triangular attic behind a kneewall, you want the insulation to follow the roofline. For more information, see “Two Ways to Insulate Attic Kneewalls.”

    1. Conor Stinson | | #2

      Of course! The front of the house is the short side and the rear is the long.

      Those articles are indeed helpful but I guess I'm wondering if the ventilation needs to be symmetrical? If I can easily get the front/short side, can I leave the rear alone? Or if not, can I set up similar vents at the back/long side that, later on, I can retrofit from below. For the venting work on the back/long side to have continuous definition would require the removal of sloped wall drywall, etc. So if I don't want to do that now, can I try and set myself up to do it later, but without getting into the attic...

      Thanks so much!

  2. GBA Editor
    Martin Holladay | | #3

    Conor,
    Q. "If I can easily get the front/short side, can I leave the rear alone?"

    A. The purpose of the ventilation gap is to keep the sheathing safe -- to dry out the sheathing if necessary. So you can't ignore half of your roof. Every rafter bay needs to be detailed the same way.

    To do it right, you need to remove the interior drywall for proper access.

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