Old roof vent & insulation question

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Old 09-30-20, 05:01 PM
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Old roof vent & insulation question

I have a 1928 duplex bungalow in Detroit. The attic insulation is original Ė barely there. Itís expensive to heat in the winter and cool in the summer, so Iíve had quotes to upgrade this insulation from local installers who would blow in insulation over the current insulation. Currently there are vents along the 4 ridges of the roof but no intake vents. (?!) The insulation installers have also quoted adding roof vents at the same time. I assume these would be intake vents, installed where they would feed the attic directly, about halfway up the front and rear roofs, and at the bottom of the large ďdormerĒ roofs at the front and rear of the house. These would be relatively quick and easy for them to install, but Iím wondering if something else would be better.

I could create intake vents by cutting openings in the eaves and above the front porch from the outside, and in lumber that currently closes off any gap between the rafters in the attic. At the same time I could try to slide in rigid insulation, to sit on top of the 4 finished areas under the front & rear roofs (3 closets + the staircase) while allowing airflow over top of the insulation.

Iíve searched online but havenít found any examples of adding insulation to a similar roof. Iím curious if anyone has any experience dealing with something like this.
 
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Old 09-30-20, 05:45 PM
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Older houses were not built with modern ideas of ventilation in mind, and after 100 yrs, presumably there are no moisture problems as a result. Go figure. IMO your only concern should be adding R-value. Modern houses are usually built with ventilation in mind, while trying to retrofit that same style of ventilation into older houses can only be done by sacrificing r-value in areas where it is needed the most. Adding ventilation also backfires in Northern climates when that moving air carries away heat in winter months.

IMO, concentrate on air sealing any penetrations in the attic floor that might leak air... (do this asap before they get there) and forget adding ventilation. What you currently have is likely sufficient.
 
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Old 10-03-20, 07:32 AM
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Thanks for the feedback.
The attic seems to be dry without any moisture issues. It appears that gable vents were covered over at some point, perhaps replaced by the ridge vents.
My concern with not adding any ventilation and only sealing attic floor penetrations and adding insulation on top is how that will effect summer temperatures. Wouldn't summer temps would only increase in the house after improving the attic insulation, without any other changes?
As I understand it, intake vents should have at least as much airflow as the exhaust ridge vent. Perhaps the current original leaks and lack of any sealing are providing this amount of airflow. Is there anyway to measure this onsite, perhaps with a pressure gauge or comparing outside & attic temperatures?
 
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Old 10-03-20, 08:10 AM
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Ventilation does not measurably affect the temperature in the conditioned part of the house provided there is adequate insulation in the attic, and insulation alone will not make your upstairs cooler. Its main affect will be to keep it warmer in the winter. If you do nothing to cool the upstairs in the summer, insulation may keep it comfortable longer each summer morning, but it will also remain hotter every evening, since insulation only slows heat loss/gain, it does not stop it.

If you are trying to heat or air conditioning the upstairs, the insulation is what will help you climate control that space... not the ventilation.

Bill Rose: "Attic ventilation is the Britney Spears of Building Science: no newscast would be complete without it, but it contains absolutely no substance."
 
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