Making roof felt airtight in garage.

[toontoonarmy123]

How to make this airtight? I want to stop air entering garage through the bottom of the plastic sheet under roof tiles in my garage as I want to convert it into a home gym. There are soffets in the fascia overhand that ventilate in the roof space. I am also going to put insulated wrap as a makeshift ceiling under this

 

[noseall]

How to make this airtight? I want to stop air entering garage through the bottom of the plastic sheet under roof tiles in my garage as I want to convert it into a home gym. There are soffets in the fascia overhand that ventilate in the roof space. I am also going to put insulated wrap as a makeshift ceiling under this

Why do you want to prevent ventilation in your roof space?

If you are insulating conventionally (ceiling level), then the vented bit will be above the insulation in any case.