Where do I start. We recently upgraded windows, installed a new roof, and resided our entire house. While ripping off the old cedar slat and installing house wrap I discovered that our hvac "plenum (I'll call it)" passed through the block foundation and whomever installed it has simply stuffed fiberglass pink insulation between the sheet metal and the concrete blocks on both the return and supply ducts, all of which now is a soggy mess of green slime under the ductwork leaving a 5 or 6 inch gap all the way around each duct and in between. (Entire unit is ouside, air conditioning and oil burner) I start looking things over and I figure this thing is about 15 ish years old. Tonight the blower has decided to take a dump. So I'm laying here wondering do I dump a blower motor into this thing and keep rocking, and if I do I have to find a better way to seal the ducts to the blocks and insulate the crap out of them before it rusts completely through. Or do I pull it out for scrap metal and start fresh with a decent heat pump and get rid of this lousy oil burner. We primarily heat with the IS but at the beginning and end of the seasons it's always nice to hit the thermostat and forget about it. Or if we are away from home knowing it won't be frosty when we get home is always a plus. While I'm on my soap box is there a particular reason batts of insulation are no longer used up in between the floor joists? I know all the rage is spray foam and conditioned crawls, but this is an old home. I'd be forever digging a french drain and dehumidifying and sump pits and blahhhhh. Help.....please lol
Depends on how much you want to spend, the cheaper option is replace blower motor. Get thru the winter,buy some time to research your options. Somewhat cheap option for the gaps, I think I'd get some thick foam board (XPS) glue that to block (inside would be better) some spray can foam insulation to seal up any gaps the foam can't. Buy some marine grade plywood or pressure treated lumber and glue (or tapcon) that to the outside block. Run a piece of wood next to the plenum like a stud so you have some support and whatever option you choose to use isn't flapping in the breeze.. Or if you don't want to glue/tapcon to the block, Frame the gap in the best you can, I'd probably just friction fit the studs. If it's unconditioned space where the joists are, it could be due to mold issues. At least that is my uneducated guess. Or maybe they just didn't do it in your place?
In the Pines .. has the best idea, just be careful on the foam board meaning get the right one.. the fumes on it are toxic if they melt. And get the spray foam that's design for heat. I believe it's a red can.
Get the foil backed foam board. The foil reflects heat so it takes longer before meltage/fumes begins. Same idea as what I did in my basement with rigid foam boards and Reflectix. Code may, or may not, require that the foam be covered by class 1 fire protectant (which is the foil/Reflectix). Probably easier and faster to use the foil backed foam for you application.
