Closed cell spray foam is often useful to solve the type of problem you describe.
Duct closure for attic air conditioner during winter.
Insulated flexible pipes about 7 inches inside diameter connect from the central duct to ceiling vents in each room.
I have a dedicated ac system with the air handler in unconditioned attic heat is provided by hydronic baseboard.
I was wondering is there any figures on the expected energy temp lose.
You don t close the ac vents in winter leave them open in summer or winter.
If the space is too tight to do the work from the attic side you ll have to remove the drywall ceiling or a portion of the drywall ceiling to do the work.
The hot or cool air is coming out from the same air ducts it depends on how high or low temperatures and the hot or cold switch lever that you set the central air condition unit will automatically switch to hot or cold as you have controlled it.
In the winter the attic is very cold because the roof and gables are not insulated.
When outside temperatures drop down below about 40 pretty much nov thru march condensation starts forming in the ducts and drips from some of the ceiling registers.
The air is re introduced as icy drafts through lower vents.
The heat exchanger is connected to a long duct running down the center of the attic from end to end.
Next time use high quality tape to get a good air seal.
Your attempt to plug the ducts with rigid foam was well meaning but ineffective almost certainly because your plugs weren t airtight.
An air conditioner is designed to be used in all weather conditions.
To do the required work one would normally access the area from the attic.
First winter after moving.
If that happens your unit will rust and deteriorate quickly.
Winterizing your home saves money hot air produced by heating systems in fall winter lose thermal values of 30 or more through the central air duct system.
You shouldn t install ducts in a vented unconditioned attic.
When outside temps drop down below about 40 pretty much nov thru march condensation starts forming in the ducts and drips from some of the ceiling registers.
Note that you can t use air permeable insulation like fiberglass to solve this problem unless the fiberglass insulation includes a polyethylene jacket.
What i ve observed attic temp 50c vent temp 21c attic temp 37c vent temp 18c attic temp 25c vent temp 13c not sure what the r value off flexible duct or main metal duct is.
I live in montreal and had a lennox air conditioner and ducts installed in the attic.
The second possible remedy to your condensation problem is to prevent indoor air from entering the ducts during the winter.
Condensation problem in the ductwork in wintertime question i have a dedicated ductwork system with the air handler in unconditioned attic.
Burying ducts in attic insulation.