Running ductwork in cathedral ceiling
Doing a renovation of an attic that will have semi-cathedral ceilings (they will be flat at the top). My HVAC guy wants to run the ductwork up the bays of the roof, and then have the register at the top.
My concern is that in the bays where the ductwork is, we’ll have to take out most of the insulation. So, we’ll have hot (or cool) air running within a couple inches of the exterior of the house. Seems inefficient to me, let alone potentially causing problems with ice dams.
He says they do it all the time (which I know doesn’t make it right). Anyone have any experience with this?
Replies
I wouldn't do it. IMHO, your concerns are justified. Come winter, you'll have a neat little stripe down the roof right where those ducts run.
Even if you spray foam around the the ducts, you still will have very little R value left in the bay.
When I did my attic reno with the requisite cathedral ceilings we placed all registers in knee walls or floors with a couple returns high in gable end walls. You might run a return in the bays, but I wouldn't run supplies there.
From a design POV, why would you want a register in the ceiling anyway?
The idea is then you don't have to worry about furniture placement - which I understand. It's a real pain if the floor register is where you want to put something.
What kind of wall do you need for a wall register? Seems like a pretty thin space.
What kind of wall do you need for a wall register? Seems like a pretty thin space.
Wall register size varies based on the design of course, but they tend to be wider. In our case, they were knee walls, so we had lots of open attic space behind the wall and could use round ducts bigger than could fit in a stud bay.
In a normal wall, you have to worry about plate penetrations and joists to run the duct work, but rectangular ducting is not that hard.
Here's an interesting site that gives equivalent duct sizing info between round and rectangular: http://www.engineeringtoolbox.com/equivalent-diameter-d_443.html
Yeah, why in the ceiling?
If you are forced to put it in the ceiling, consider making the roof rafters deeper to enable insulating behind the duct ... switch to rigid insulation to maximize R-value in a small depth. Also make sure you seal with liquid mastic duct sealant all joints that occur in this type of area!! Above all NO DUCT TAPE!! Duct tape ... good for EVERYTHING .... EXCEPT sealing ductwork!!!!
Also maximize the width of the duct to minimize the depth in the rafter ... not the best, but better than nothing. Then maybe you can get 3-4 inches of rigid insulation behind it.
There are various ways to increase rafter depth w/ little effort/cost ... but it does start to eat into your headroom/space. Pay your money, take your choice.
Supply registers should be low. Makes no sense to me to run the heats the way your guy suggests.
Couldn't you run bot the return and supply up the gable end walls? If they are too thin, add strapping or another 2x4 wall to allow the duct work to go through.
"It is what it is."