Don't Try This At Home: Armchair Building Science

Underventilated Attic
by John Walls

Peter,

Speaking of undervenilated attics----

In a hot humid climate, how would you address the following:

The home living space will have an unvented attic design (spray foam insulation walls and roof). However, there are attached 10' wide porches on three sides for shade. These porch attics will not be insulated or conditioned. Local practice is to just box them in with no ventilation. There seems to be no reasonable alternative other than possibly putting ridge vents on the hips (limited to the corners and tricky to do without leakage issues), or just putting in soffit vents at the eaves(sort of a halfway solution), or eave vents plus poking holes in the roof for power vents (ugly and not desired by the owner), or going with a raked porch ceiling (resulting in a real tall exterior walls not desired by the owner).

What's your opinion of just boxing it in and calling it done? Seems to be the way it's done here, but surely there is a better way than just leaving a hot box.

Don't worry
by Martin Holladay, GBA Advisor

I don't think there is any reason to ventilate a porch attic. Just be sure that there are no accidental air leaks or thermal leaks between the conditioned space and adjacent porch attics.

Don't box me in
by Peter Yost

Hard to imagine building a porch ceiling tight enough that you won't get air circulating in this "dead" space. Sort of a shame to put in a ceiling at all, but I understand the aesthetic of a porch ceiling. Almost all materials do better if the temperatures they "see' on their various sides or faces see the same conditions--temperature and humidity. I agree with Martin that you don't have to ventilate this space, but that house wall section you are boxing in will unnecessarily experience higher temperatures from the porch attic getting solar heated. Again, not a terribly big deal, but just something to consider.

How about a leakier Box?
by John Walls

So, do you see any advantage at all in putting in just soffit vents at the eaves? It's cheap to do and has no water leakage potential. It purposely creates a leaky attic. Maybe you could get a little temperature relief from this in the summer? What do you think? Is there any downside to doing this?
Would using a metal roof change any of your thinking?

Soffit to "ridge" venting when shed roofs meet a wall
by Peter Yost

If you want to vent these porch attic spaces soffit to ridge, you could use conventional soffit vents and any one of a number of "half-ridge" products. Cor-a-vent makes one (http://www.roof-2-wall.com/) and DCI Products makes a SmartVent for this application (http://www.dciproducts.com/html/shedroof.htm).

If it fits the porch aesthetic, using any of the Cool Roof products from EPA Energy Star will certainly also keep these unvented porch attic spaces from getting as hot: http://www.energystar.gov/index.cfm?c=roof_prods.pr_roof_products.

attic moisture
by Paul Morin

I have some additional armchair building science observations. Your article did not discuss the source of moisture in the home. I would bet that a 5000 cfm50 house with high humidity levels is running has a humidifier in the furnace duct work. I have also noticed, when investigating attic moisture issues in Minneapolis, that condensation and frost form on the coldest surfaces that are sometimes far away from the source. Often times I find that the area near a large bypass will be the only location in the attic without frost because it is warmer at that location. In the photo provided, the location with the moisture issue may be the coldest surface in the attic because there is adequate ventilation from the cold exterior. Ventilation may be the cause, not the cure.