It took a crew of five working diligently 8+hours a day six days to do ours, but that included ripping out the old ducts, including duct boots. Unlike
@2manyrocks, I'm going to suggest that this one is for the pros, at least until you've experienced a few. There's a lot to go wrong.
Like what?
Well the basic issue is that if you have dead space, you run the risk of humidity and mold. Then there is the potential issue of separate radon abatement below the membrane.
So before the vents are sealed, you have to get the under house drainage sorted out, perimeter drains if needed, barrier walls, sump pump, radon abatement, etc., then you need to put a water
and vapor proof layer down. Standard LDPE is quite porous to water vapor, and won't cut it. There's inside of the foundation needs to be sealed, and then the membrane brought up to beyond the height of the perimeter foundation, and sealed to the house perimeter joist above the sill. Any obstacles, like piers and footings need to be sealed in the same way, and all of the floor seams need to be sealed with vapor barrier tape. Insulation can be added after the membrane is up, and then sealed both to prevent air intrusion, and to prevent cold spots that might be condensation points.
With that done, the vents are sealed, but the space needs to stay dry going forward. That means either a dehumidifier, and all that goes with it (power, drain pump, remote humidity monitor, overflow alarm, and noise isolation), or you need to divert some airflow from the house to keep the humidity down, and have a method for return air. Pros and cons to both. If one has a dehumidifier, a circulation fan is needed, but often not installed. I have never found dehumidifiers to be reliable, and constantly in need of maintenance. If I could have installed it in the space above, I might have considered it, but not in my crawlspace, but it depends on the access. If it is like
@ultrarunner's 5' high space, with a drain, I think having a dehumidifier might be a different story. I don't, so I didn't put a dehumidifier in, but I did put multiple humidity and temperature monitors and we will see how it goes. So far it tracks the house closely, reinforcing the idea that there is a fair amount of air exchange between the inhabited space and the crawlspace.
@dieselscout80 there are arguments both ways on the insulation, but if it were me I would, and did, leave the insulation on. I wanted some control over how much heat or cooling was going into the space. I would bear in mind that most ducts have pitifully little insulation on them (R2-3.5 in many installations), so they are going to leak and absorb a fair amount of heat. If the crawlspace gets humid and you have uninsulated ducts, you run the risk of condensation, mold, and corrosion.
I have found one, repeat one, HVAC professional in my general area, which is not small, who can actually do an accurate Manual J. Seriously. After striking out, I gave up trying to find one, and then found an energy specialist who was also an architect, who actually knew how to do one. Most of the better HVAC folks load up a number of assumptions about "average homes" for your house and call it done. Might be close, but probably won't be. The typical person just eyeballs it. (X thousand sq.ft. mean Y tons of cooling and NN thousand BTUs of heating.) I would not consider doing one until after you have an accurate energy audit, and fixing all of the identified issues. So, by all means consider doing a Manual J, but GIGO. There's a great YouTube video by a couple of HVAC professionals that gets into why one should not trust a Manual J without getting quite a bit of specific data on the home from a series of other tests.
All the best,
Peter
