I don't have any trouble taking apart duct that's got
mastic on it. (then again... the only time I ever have to
take it apart again is if the homeowner remodels their
home or I have to replace the hvac equipment.. no biggy
either way) We use a brand called Air Seal... it works
real good and the ducts come back apart without a ton of
effort. I have seen some mastic that sets up like a hard
epoxy coating or something, maybe thats the kind you've
seen.
I am very picky when it comes to air leaks in duct work.
As I said earlier, the house sucks in the same amount of
air your duct system leaks out. This not only leads to
comfort issues (ask all the people around here that
condemn heat pumps due to air leaks in their duct system),
it also leads to mold/mildew/growth problems.
ALL air leaks are important.
I'm pretty sure someone is going to wonder about the 'new'
addage... "They make houses so much airtite these days"
In my opinion, the 'airtite' homes really need economizers
put on the equipment to allow for fresh air to be brought
into the house in a controlled / filtered manner.
Albeit, selling a customer on this is kinda tough though...
i.e. Customer spends thousands of dollars to get his house
energy effecient... ala airtight
HVAC contractor comes along and says...
since you have such an airtight house, we need to install
a fresh air intake on your system to bring in outside air.
One of my duties at work is to do Heat Pump setup and
preinspection. (trying to give short version of it)
I had one very recent that measured 15% UNDER rated
capacity. Not only did it reduce the unit from a 3.5 ton
to a 3 ton.... it also introduced more moisture into the
house, more dirt into the duct system, and so on)
It had about 30ft of duct with the slips and drives not
sealed.
I had our installers seal it up with Air Seal.
Now I have Full Capacity.
Just by looking at the duct, it didn't look like it was
leaking much. Especially being a tad bit oversized and
the static pressure being on the low side.
So the duct wasn't 'sucking that hard' to pull the air in.
The air temp under the house was only about 50degree's too.
HEre's another reason to seal your supply ducts...
IN the summer, those air leaks are going to be blowing
cold air into the insulation. The insulation has a
foilback covering that helps prevent the air from leaking
out there, so the air migrates to the stapled seams and
ends where the insulation buts up to one another.
Surface temp of insulation drops below dew point of
environment its put in.
Surface of duct wrap starts sweating.
After an extended period of time, the moisture barrier
(foil back) starts to become saturated. Then the
insulation acts as a sponge absorbing the water. Then the
insulation gets all soggy. Then it stops resisting heat
transfer. and then and then and then muahahahahha
Plumber crawls under house and comes out telling homeowner
about the duct wrap is completely filled with water and
falling off the pipes.
Homeowner gets mad at hvac contractor cuz they spent
thousands of dollars to get the job done and now its
falling apart.
Extend to others the grace that God has given you.