Insulating Kitchen Exhaust Hood Ducting

JustinS

Hi everyone

I'd like to get thoughts on how best to insulate the ducting that's used for my kitchen exhaust hood...

The exhaust hood is located on the wall shared with my garage - unfortunately, the ducting was too big (8") to go up into the space between the 1st and 2nd floors (joists were only ~7" at this point) so the only option was to go out into the garage before turning and heading out the back of hte house...

To do this, the builder made a low-hanging box in the garage to enclose the ducting (see attached pic)... unfortunately, since this ducting is exposed to the unfinished space above the garage, the attached hood gets very cold in the winter...

I am trying to figure out how best to remedy this... one thought that I have had is to wrap the duct itself in insulation but I think that this is only an initial step since the most I can find in terms of R-value is like R-8... I'm also thinking that I should line the box with rigid foam board insulation (the polyiso stuff)... since the box is made of 2x4, I should be able to get around R-20 or so on the sides... one concern is that there's not much space between the duct and the bottom of the box (won't be able to fit much beyond the wrapping insulation below it)...

What do you all think?

Bob Bona_4

I'm liking the rigid foam board in between the 2x4 and slipping R8 over the duct. I would seal the joints and even the elbow joints and snap lock seams with Hardcast rubber membrane foil tape or at least mastic them before wrapping.

JustinS

What would you recommend in the 2" or so that I have below the ducting? Can't put any rigid foam board there, at least directly below it... I can line the rest of the box to better insulate the entire space but there will be a weak point directly below the ducting...

JustinS

How's this?

http://www.homedepot.com/p/Master-Flow-8-in-x-25-ft-Insulated-Flexible-Duct-R8-Silver-Jacket-F8IFD8X300/202562741?N=5yc1vZc4nk

25 ft is a little long for me - total duct length is around 10 ft... but I can't find R8 in less than 25 ft for 8-in ducting...

Bob Bona_4

You can stuff the board in between the cats on the bottom, no? Or, can you drop the soffit a little more? I would buy the box of R8 flex and would have slipped the outer part over the pipe as it was assembled and taped, leaving the membrane behind.

JustinS

Drop the soffit a little more? Not sure what you mean? Do you mean the box itself? If so, I don't think so cause it's sheet-rocked...

Leaving the membrane behind? What membrane?

Bob Bona_4

Your soffit is open in the picture still. Cut your foam board to fit in between the framing.

We typically sleeve the pipe with r8 and use flex as the material. The membrane of the flex gets scrapped. But we do this as the pipe gets put together, in sections, so the foil/insulation is seamless except for the section joints which get taped.

Rich_49

After you do all this be prepared to still experience some cold (not as much) through the hood . Heat will still escape through the vent and cold will still be conducted by the metal . Sort of a thermal bridge effect just like when framers use too much lumber on exterior walls thinking they are doing something real good in an energy efficient envelope when all they are doing is connecting the outdoors with the indoors and leaving you with cold stripes on your drywall when you thought you were getting an R30 wall assembly

don_9

Off topic.Does your kitchen hood bring in make up air in as well? Just thinking to myself, in a tight house how long of run time on the hood would it take before my house goes in to a negative pressure.

icesailor

Smart builder and ventilator.

Pipes increase as do their squares. One 8" round duct has an area of 6.28" Sq. Inches. A 8' square duct has 64 square inches in area. A 4" square duct has 16 Square Inches in area.

Where I worked, most used a transition to square duct and used a square termination like 3.5" X10" (35 Sq. Inches) that fit the hood exhaust.

Its fortunate for you that codes now (in most Jurisdictions) require vents to the outside for kitchen hoods. They are a great source of cold air infiltration into a home. But you've got to have them. For when you burn up all that bacon and other things on the stove. Then, you run the bathroom fan to get rid of that hot moist air from a shower. Where do you think the make up air comes from? Why, right through the kitchen hood fan exhaust. Especially in the North East where the prevailing cold winter winds come from the North West. (Hint: Use a Microwave. You won't hardly ever use the stove, and you won't need a fan.) When you run the kitchen fan, it sucks fresh cold air through the bathroom fan.

Where I worked, I never saw a kitchen hood fan vented with 8" round. It was always transitioned to square. My wholesaler even stocked special copper square terminations and 4" round ones for dryers with dampers. For those that money wasn't an object unless it was a choice between the copper termination and a 30' transplanted tree to block some neighbor's view.

I once went on a "Cold House" call to a new house. They had a Jenn-Aire cooktop with the brill on the top, vented down and out on the NW side with 8" round pipe. There was so much cold air coming back (even with a damper) that it froze water in a pot left on the stove overnight with water.

IME

icesailor

don said:

Off topic. Just thinking to myself, in a tight house how long of run time on the hood would it take before my house goes in to a negative pressure.

In less time than it took you to write and ask the question. In my experience.

Bob Bona_4

Lack of makeup air has always been my pet peeve too. When I regularly rough in 7, 8, 10" round venting..that's mucho air movement!

Rich_49

Nobody ever takes into account how many BTUs need to be replaced when running those monsters .

don_9

I know in my area any domestic kitchen exhaust hood capable of exhausting in excess of 400 cfm require the same amount of make up air.i went in a house acouple months ago with one of these hoods pulling the house in a negative.ok well let me be fair n say that the hood was not the only reason.i also found the cap on supply trunk in the attic off so that too was pulling the house in to a negative.supply duct in attic,equipment and return inside the building envelope.

JustinS

Bob Bona said:

Your soffit is open in the picture still. Cut your foam board to fit in between the framing.



We typically sleeve the pipe with r8 and use flex as the material. The membrane of the flex gets scrapped. But we do this as the pipe gets put together, in sections, so the foil/insulation is seamless except for the section joints which get taped.

Sorry, I guess I forgot to mention that this was when the house was under construction... I wanted to be able to show the ducting in relation to the surrounding wood...

JustinS

don said:

Off topic.Does your kitchen hood bring in make up air in as well? Just thinking to myself, in a tight house how long of run time on the hood would it take before my house goes in to a negative pressure.

No, I don't believe it has any sort of intake...

JustinS

Rich said:

After you do all this be prepared to still experience some cold (not as much) through the hood . Heat will still escape through the vent and cold will still be conducted by the metal . Sort of a thermal bridge effect just like when framers use too much lumber on exterior walls thinking they are doing something real good in an energy efficient envelope when all they are doing is connecting the outdoors with the indoors and leaving you with cold stripes on your drywall when you thought you were getting an R30 wall assembly

Right now, my issue is that the hood is extremely cold and cold air drops down from under the hood (even with the outside damper taped shut)... I assume that this cold air movement is mostly due to convection as the air around the hood cools when in contact with it?

Do you think that wrapping the duct in R-8 and then insulating the box will help a noticeable amount? I am OK with the hood being cool to the touch but would very much like to "eliminate" the noticeably cold air draft as well as not have the damper taped shut during the winter...

If not, how do people deal with this in the Northeast (Massachusetts)? I find it hard to believe that people just accept cold air circulating around the hood...

Thanks!

icesailor

Starlings (an invasive pest bird species) love those exhaust vents. They have figured out ways to flip the dampers open and move in for communal living.

JustinS

icesailor said:

Starlings (an invasive pest bird species) love those exhaust vents. They have figured out ways to flip the dampers open and move in for communal living.

Hmmmm, I really don't like the vent I have because it's just a piece of metal that flaps open way too easily
in the wind but I wasn't (even now) able to find another type (it's a Broan)... selection seems very limited for 8" round duct damper...

Bob Bona_4

Yes, the vent terms available leave a lot to be desired. It's a very unsatisfying facet of the trade.I suppose Southwark wall caps are the better choice- aluminum, spring loaded damper..and oh so fragile. Try getting one undamaged out of the box. The galv steel brand's dampers are cheesy and can oscillate open and closed from wind harmonics- I've had to weight them to keep them closed (watch the hinges wear out next). I've had coppersmiths make them for me (read almost 3 digits)...meh..

That and the complete disregard for balanced air exchange by commercial hoods posing as residential appliances all makes for part of HVAC that everyone sees as a necessary evil.

JustinS

Wondering if I might be able to insulate the ducting with R-6 with a heat tracing wire beneath it... thoughts?

Bob Bona_4

Why do R6 if you can do R8? R8 is code for ducting in unconditioned spaces. Forget the heat tape. Not designed for that and would be ineffective. Just make sure your hood and exterior dampers are as tight as possible.

JustinS

I haven't been able to find R8 - the insulation I linked above is actually with flexible ducting, as opposed to a sleeve like I thought...

icesailor

R-8 may be code in unconditioned spaces, but you have to look long and hard to find it. It may be Code in Florida, but if you request it, it is a special order. No one uses it.

"Code" may require the hood to be vented. How often do you need to run the fan? Do you burn up a lot of food in the kitchen?

I've built four houses from scratch and renovated the Condo I a, in in Florida. I've never had a vented hood in my life. If cold weather infiltration was an issue, I's be making sure that there was some access panel so I could disconnect it. "F" the cold air. If I use the stove top 10 times in a month, I'm overdoing it. I use the Microwave. If I burn toast, it sets off the SD's.

When I lived in Massachusetts in a high wind location, I walked into far too many cold kitchens with hood infiltration.

Do what you want.

PS: Don't EVER put heat tape in an unconditioned, confined space and cover it with insulation. The wrong type of heat tape can cause a fire and burn your house down. Hopefully, not with you in it.

You'll save far more money by keeping the tires on your car properly inflated on the high side than you ever will by what you are trying to do to save energy.

Bob Bona_4

Are you kidding me ice? Since about 2000 R8 is code for all duct in unconditioned space in CT. I can't walk in a supply house without tripping over R8 wrap or flex.

You'd think FL of all places would want to double down on duct loss..

icesailor

Why would I jest? When I asked for R-8 for the new replacement flex duct in the unconditioned attic space, I had the same looks that an alien, fresh off the space ship down here in Florida gets. I saw something around that time about the R-8 being Code. That the REAL HVAC Professionals used R-8. The Hackaroos who use Home Depot and Lowes as their supply source only use R-6 because that's all they stock. When I asked for R-8, I was told that the suppliers don't stock it and the installers don't use it. The Duct Board is slightly thicker.

My old R-6 Flex Duct was on top of the old insulation. I covered it with more fiberglass batts in 2006. Florida Code requires that all duct in unconditioned spaces must be held up in the air and not in the floor. After the change, and the new duct was completely exposed to 130 degree attic air, I could use the air for winter heating when the blower starts. I cut it all down, covered it again with Fiberglass, and had an equivalent of R-30 blown in on top of it on the whole entire attic space. My energy bill went down.

Florida? What else would you expect when the copper potable water tube is Type "M" Copper. Buried in the ground.

Bob Bona_4

Wow! What's going on down there that R8 is so exotic? You're right about Home Dopeys with the R6 though, noticed that on the way to the caulk isle.

JustinS

icesailor said:

You'll save far more money by keeping the tires on your car properly inflated on the high side than you ever will by what you are trying to do to save energy.

I'm not really doing this to save money, I'm trying to make my kitchen not so cold because of the crazy cold air around my hood...

Can someone recommend some insulation for my duct?

JustinS

Here are some pictures of what access to the ducting box looks like from the unfinished space above...

I'm re-thinking using the rigid foam board because I'd have to cut it into many pieces to get it through the small openings between the joists...

Can I just fill the box with fiberglass insulation? I could just use unfaced insulation in the box since there's already a vapor barrier in the existing wall insulation and the ducting is sealed with foil tape or mastic... it'd be resistant to vapors, right?

I had thought of removing the insulation between the house and the box and then insulating the box itself, effectively making the box part of the house's heating envelope... but now, as I think about it, that's probably not advisable since the box is 2x4 construction which would cut down my R-value... and if I did this, I couldn't fill the box with insulation because I need the vapor barrier between the house and the insulation, right?

Other thoughts? Many thanks for suggestions...

Canucker

From the looks of the pics, you have an air infiltration issue that will still be there after you insulate the duct work. Insulating it will probably alleviate that to a level that's acceptable to you(or not) but I bet you'll still have some from the ineffective damper.
I can't make any recommendations on the vapour barrier, as I'm not sure where you're located(In my area it's required. Necessary? That's debatable)

JustinS

Canucker said:

From the looks of the pics, you have an air infiltration issue that will still be there after you insulate the duct work. Insulating it will probably alleviate that to a level that's acceptable to you(or not) but I bet you'll still have some from the ineffective damper.
I can't make any recommendations on the vapour barrier, as I'm not sure where you're located(In my area it's required. Necessary? That's debatable)

I live in Massachusetts...

I assume that you mean the air infiltration from the uncovered box that the ductwork is in? If so, I am thinking of covering it with rigid foam board insulation...

Canucker

No, I mean the air leaking in through the pipe past the damper. I'm assuming from the pics that the box is in your garage? I can think of a few ways to insulate it but I think the best bang for your buck would be to sleeve it in the R8 that was suggested and see if there is an improvement. IMO, I think you'll still have cold coming in, it just won't be from the walls of the duct work cooling the air.

JustinS

Canucker said:

No, I mean the air leaking in through the pipe past the damper. I'm assuming from the pics that the box is in your garage? I can think of a few ways to insulate it but I think the best bang for your buck would be to sleeve it in the R8 that was suggested and see if there is an improvement. IMO, I think you'll still have cold coming in, it just won't be from the walls of the duct work cooling the air.

So through the damper itself? Yes, this is above my garage and the box hangs down into the garage...

I've looked for R8 and am having trouble finding some - can you recommend any in particular?

What other ways would you insulate?

Canucker

If you're handy, I would take down the drywall covering that box, attach 2" of foam to the outside. Foam board over the inside of the wall that the outlet is on(In the box), then make sure all the gaps are sealed.(Foam or caulking is fine) Lay insulation over the top, then get some long screws and drywall over the exposed foam in the garage. Oh, and make sure the joints on the duct work is sealed before you do all of this. It sounds like a lot but you should be able to finish in a couple of hours, not counting the re-mudding process.

JustinS

What other ways would you insulate?

Canucker said:

If you're handy, I would take down the drywall covering that box, attach 2" of foam to the outside. Foam board over the inside of the wall that the outlet is on(In the box), then make sure all the gaps are sealed.(Foam or caulking is fine) Lay insulation over the top, then get some long screws and drywall over the exposed foam in the garage. Oh, and make sure the joints on the duct work is sealed before you do all of this. It sounds like a lot but you should be able to finish in a couple of hours, not counting the re-mudding process.

Something to think about... I spoke to my town's building inspector yesterday and he recommended as simple as just filling the box inside with unfaced fiberglass insulation and then covering it up with foam board on top... with the fiberglass, I'll be able to "stuff" it into various nooks and such...

Canucker

JustinS said:

What other ways would you insulate?

Canucker said:

If you're handy, I would take down the drywall covering that box, attach 2" of foam to the outside. Foam board over the inside of the wall that the outlet is on(In the box), then make sure all the gaps are sealed.(Foam or caulking is fine) Lay insulation over the top, then get some long screws and drywall over the exposed foam in the garage. Oh, and make sure the joints on the duct work is sealed before you do all of this. It sounds like a lot but you should be able to finish in a couple of hours, not counting the re-mudding process.

Something to think about... I spoke to my town's building inspector yesterday and he recommended as simple as just filling the box inside with unfaced fiberglass insulation and then covering it up with foam board on top... with the fiberglass, I'll be able to "stuff" it into various nooks and such...

I like your inspector's idea of putting foam board on top and covering with the fibreglass(I see I missed that in my earlier description, sorry), not a fan of just stuffing loose fill in there with nothing as a vapour barrier and here's why.(Unless he meant to build the foam box AND fill it with loose fill? That I like)
I assume that Mass. has a cooling season as well as a heating season? I'll bet it gets just a little humid outside during the summer, no?(Haven't had a chance to visit yet, it's on my list though) During the A/C part of the year, the cold air in the house may be escaping through that duct, as well. If it drops its surfaces below the dew point, moisture won't stop with the loose fill and could condense on the piping, making it a little moist. The loose fill will stop the heat transfer though, which means it won't dry out anymore, could start to mold. Not good. If you use the 2" foam board and pay attention to air sealing, you won't have the same issue, as the foam board will act as a vapour barrier at that thickness.

Rich_49

This is absolutely remarkable . You will accomplish nothing except to stay out of your wife's crosshairs for a couple hours by doing any of this . That could be beneficial too though . Hot goes to cold and high goes to low , doesn't matter where you are , hot or cold , humid or dry . The 2 points of this vent that make any difference are the ends that directly connect the indoors to the outdoors . That's it , end of story . If you were to perform a blower door test on the house before and after all this effort , nothing would change , the infiltration rate would remain identical

Bob Bona_4

That's what I've been saying. Air movement is going to come from leaky dampers on the hood inside and or the outdoor term. Sleeving the pipe with R8 will just help reduce condensation. Sleeve it and move on to foam or rubber sealing/weatherstripping the flap outside would be a starting point.

JustinS

I have no doubt that there is air leaking through the dampers but it's not the only problem... last winter, I taped the outside damper shut and still had cold air dropping down from under and around the hood... since the ducting is sealed properly, my thinking is that this is the result of the ducting being extremely cold by exposure to the unconditioned space above the garage...

If it's not contributing, that would mean that anyone in my area who has an externally vented exhaust hood would have this kind of cold draft and I find that rather difficult to accept... can't imagine people would be OK with that...

JustinS

Bob Bona said:

That's what I've been saying. Air movement is going to come from leaky dampers on the hood inside and or the outdoor term. Sleeving the pipe with R8 will just help reduce condensation. Sleeve it and move on to foam or rubber sealing/weatherstripping the flap outside would be a starting point.

There's already a small amount of foam on the outside of the damper that the flap rests against when "closed"... I can't imagine it does much of anything...

Bob Bona_4

Yes, I've yet to see a quality exterior hood mass produced.

You can stuff a bale of insulation in that cavity and all it would do is slow down the enevitable cold transfer. Unless you did this: The end of the cavity abutting the interior space is open to that space so that conditioned air is IN the cavity. Like a closet tipped on its side. The framing of the cavity is insulated like an exterior wall and wrapped in drywall.

Not very practical, is it? But the ultimate solution to keeping the pipe temperate.

Then, as I said before, before sleeving the pipe with R8 (which would be unnessary if that fantasy scenario above existed) seal the pipe joints with mastic or Hardcast tape. Even the Pittsburgh seams on the straight pipe and the adjustable seams on the 90s. Stop any infiltration into the pipe. That includes the weatherstrip of at least the exterior damper.

Make sense?