Understanding the concerns with vented to unvented retrofit of attic


I am working within a generally hot and humid location that might become quite cold, but only briefly in the winter (zone 3 below the humid line). I recently inherited an A-frame house that has blown in over-ceiling cellulose insulation in the attic, and is otherwise completely vented and uninsulated with no radiant or vapor barrier. This is an all electric house, with no gas lines.

The venting is provided by a continuous ridge vent and soffit vents (which may show signs of rodent entry, but no other signs were discovered). My issue is that the air handler/HVAC system is within the attic, and my understanding is that often justifies the expense of attempting an unvented retrofit. It is certainly apparent that the attic is routinely a more extreme temperature than general exterior temperatures (compared exterior thermometers in shade, in direct sunlight, and 2 in the attic, but not one placed on the exterior of the roof).

My understanding is that moisture becomes the biggest concern when making the retrofit (that and it's an attic half filled with junk). Spray on foams seem to be the standard solution to this, but I wanted ask a question so I understood the issue with obvious/easy first thought, why not just screw in polyiso to the interior side of the rafters/bottom of the chord:
Simple schematic of polyiso rafters

Meaning the pink layer in the drawing, and would have ~4in of dead space before the board. I'm assuming this would cause condensation to form around the trusses (exterior to the polyiso), leading to rot? And then any gaps or venting sufficient to prevent the condensation are probably going to require enough air exchange that the value of insulating is largely mitigated?

I note there are also no insulation baffles, and part of me wonders if it wouldn't be possible to intentional condensate moisture (on a small pipe or large exposed wire for example) and have it drain out the soffit? Presumably even if such a system would help, it wouldn't be perfect, and there would still be rot risk, just over a longer period.

The roof itself is fine and doesn't need replacing, or else I thought this was a particularly interesting idea.

Is there a reasonable to retrofit an attic to be unvented without the use of spray foams? I won't lie, a large reason for my motivation is a desire to move all the "in-law's significantly sentimental storage" (read junk) to one half of the attic, insolate the other half, move it the other half and repeat. Using a spray foam in such a situation seems more likely to get on everything.

Edit in response to comments:

It should be noted this is perhaps not making a true unvented attic, but instead just expanding the size of the insulated space, such that the rafter cavities are where the ventilation occurs.

This still brings up my concern on moisture, where the question would be: if the ventilation is only at the soffit and ridge vent, will that be enough circulation to avoid a condensation issue? Presumably would also be concern that the insulation was provided with double polyiso boards with non-overlapping seams? The distance along the rafter cavities is ~7.1m/23.3ft. Would such a plan require something like a fan at the ridge side to encourage airflow?

Best Answer

Your proposal of insulating with solid poly panels is completely feasible with very little to no chance of condensation.

It is common in much of Florida where the weather is hot with a few months of cooler weather, ( more in northern fla.) for there to be homes with blown in insulation with no vapor barrier. This is old style construction and not common in modern new builds, however the situation remains the same. It has been proven that soffit and ridge venting does provide enough air flow to prevent condensation.

That said, I will say that in your drawing the poly is very possibly too low. That may impede airflow to the main area of the attic. That should not happen. Blocking air flow and completely sealing off the attic could result in damage from trapped moisture. (Something you don't want.)

The attached drawing shows a blue line where the poly should end before blocking airflow. Of course this should be modified in the field to achieve the result of unrestricted air flow.

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