More Energy-Smart Details
Editor’s note: This is an updated take on a GBA Q&A Spotlight that was published in a recent issue of Fine Homebuilding. With some additional info and new illustrations, we thought it was worth sharing here, even if it is a bit redundant.
Q: I’m planning a new one-story, 1400-sq.-ft. slab-on-grade house with a vented roof and loose-fill insulation on top of the drywall ceiling. A high water table prevents building on a crawlspace or basement. The design includes a main section and a guest suite under the same roof separated by a large covered porch serving both spaces. The attic is unconditioned. I know it’s bad practice to put ductwork and air handlers in an unconditioned space, but where do you put them when you don’t have a basement or crawlspace?
A: As you suggest, the challenge with a slab-on-grade house is keeping the equipment and ductwork out of the unconditioned attic and inside the conditioned enclosure where they will operate most efficiently. Some contractors might suggest heating and cooling exclusively with ductless minisplits, but I’d stay far away from this option. Bedrooms and other smaller spaces built to modern standards in all but the most extreme climates only need a few hundred Btus per hour of heating or cooling. The smallest ductless units are 6000 Btu and turn down to a minimum of about 1500 Btu per hour. If you install this type of equipment and it’s oversized, the rooms may feel uncomfortably warm or cold because of on/off cycling and clammy because of insufficient dehumidification.
One way to create a conditioned space for ductwork without a crawlspace is to build a framed floor above the slab, creating a conditioned “mini basement” for a conventional HVAC system that runs the…
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19 Comments
The drawing shows an area with very little insulation where the lower part of the truss meets the plenum area. The blown insulation should be sloped up next to the plenum area to give a continuous R-value for the ceiling assembly. -24F here this morning so this weak point is even more noticeable.
Doug
Doug,
Another part of the detail I'd modify is the attachment of the ceiling framing in the plenum. You are much better off drywalling the entire service enclosure prior to framing the ceiling so you don’t have to seal around each ceiling joist, then attaching a header through the drywall for the framing.
Excellent suggestions from both Doug and Malcolm.
For the insulation, just mounding it up in that area probably works but you might add a second baffle, parallel to the first baffle, but two feet away from the plenum, so there's a channel to fill all the way up to the level of the insulation on top of the plenum. Actually, in that case, you could omit the first baffle.
I would also consider using metal 3x8 oval duct supplies laid right on top of the ceiling drywall. This type of duct can be made airtight and essentially is on the warm side of the room. Have done this before in reverse with a room over a garage. Ran the ductwork right under the top web of the 2x4 web trusses and filled the space (16") with blown insulation. The rim joist is very well insulated and air sealed with R-30 polyiso. This superinsulated art studio is 20' from the furnace room but is the most comfortable space in the house.
Doug
i don't have that particular type of framing, but i do have a long knee wall where the ceiling changes from 8' to 10'.
spent way too long googling solutions, but this is what i came up with. https://imgur.com/a/vgCqqFQ
basically just used some cheap garden netting + 2x4s to build a basic frame along the entirety of the knee wall. the netting was placed a little over 15" from the osb and i sprayed about 12-15" of cellulose above the top plate of the 10' wall.
its not the prettiest, but it was super cheap, provides continuous insulation that you were describing... and since i couldn't find a better solution, i went with that.
If the air handler (or for that matter erv or hrv was In a closet ) you could use exposed ductwork elsewhere. Not to everyone’ s taste but are there practical issues with this approach.
Noise comes to mind but that could be managed.
Has anyone on the forum done this?
Yes. We did this in my most recent build. In addition to providing space for the mechanicals, the electricians were happy not drilling holes for the ceiling lights. We strapped most of the other ceilings for wiring, but the plenum made it faster and easier. Noise is no more of an issue than it would be if the equipment were in a closet. Probably a bit less, as the air handler is hanging from the truss above. The biggest advantage of this approach is that you don't give up any closet space, which is always at a premium. The biggest challenge is making sure that the plenum space is tall enough for everything that goes in there, plus room to work. That can be difficult with lower sloped roofs. Access for equipment maintenance is generally through a scuttle hole, and its location must be considered early in the build because there's no room to crawl around up there once it's done.
I did this on a spec house. It isn't something I plan to do again and although I think there's some inherent issues with it, my struggles might have been more to do with my implementation.
My house design had the bedrooms on opposite sides of the house with the dreaded vaulted great room in the middle. The master side of the house had the air handler in a mechanical room and I soffited down for the trunk line on that side of the house. To get across the vaulted area the trunk line went vertical then ran in the peak of the scissor trusses to the other side of the house where plenum trusses allowed for a flat eight foot ceiling. Getting the trunk line across the house was the trouble, round duct ran without issue in the truss bays to the registers. There was only a central return in the house as there was barely enough room for the supply ductwork. I hadn't discussed my plans with the HVAC contractor prior to him to showing up on site so I'm very lucky we were able to accomplish this. Once the he understood what I was after he was on board as I think it made his job a little easier because it was effectively a basement installation. There were of course many ands, buts and what ifs along the way as we figured it out. Something I found out was it was no problem for the truss company to add a bump up for the trunk line. I ordered from Menards and it seemed to me they charge by span and slope so I'm not sure this costed me any more.
I used GP Forcefield to line out the conditioned areas of the ceiling before drywall. The overhead work combined with the inside corner and 3D taping wasn't fun but the real trouble was transitioning the air barrier from the face of drywall to the face of the Forcefield. It can't be taped of course as the materials are different, the drywall is an aesthetic material and because of the order of operations. I resulted to a smorgasbord of acoustical caulking on bottom cords before drywall and air sealing from the attic side with tape, caulking and can foam after drywall. The convoluted air barrier made me nervous but a blower door test confirmed it was sufficient.
A couple more things that made this difficult were that the HVAC Contractor wanted to go to the edges of the house with his registers. Although the real work was to do with the trunk line, this added to the labor and materials for the air sealed inverted soffits. Additionally, because I was doing the framing with whatever help I could scrounge up, I didn't have all the chase materials in place before the trades came through. This meant some wires that had to be notched around. I apparently didn't do a great job of explaining to the electrician the concept as I also had a few wires running in and out of the air sealed chases. This included the middle of the house where my only solution was to transition from the Forcefield peak to the drywall with a housewrap skirt.
If I were to do this again I might pick an easier layout, make sure everyone is on board about the purpose and location of the air sealed chases and use drywall for the air sealing. I'm sure drywall is cheaper than Forcefield or Zip and I think the inside corners and 3D corners would be easier but most importantly I think the face of the chase drywall could be taped to the face of the regular ceiling drywall and then the final (aesthetic only) drywall could also be taped to where the two meet for an air tight finished look.
Continuing to think more about this...
If drywall were used to air seal the chases it must be in before the HVAC is but the electrician needs the attic open to run wires.
Perhaps the plumber and electrician come in first, then the chase and standard ceiling drywall, then the HVAC, then the aesthetic-only drywall is applied.
If one were subbing out the framing and drywall but willing to do some work they could slip in between the hanging and finishing of the drywall to fire tape the chase drywall and tape it to the regular ceiling drywall before the HVAC is installed and then apply more framing and aesthetic-only drywall. I believe this would make for standard operating procedure for the framer, plumber, electrician and drywaller. Coordination with the HVAC contractor at design would be critical. I haven't thought about this enough and there might be some more coordination required with those trades.
What we've done is line the chase with a class II responsive (smart) vapor retarder (also acting as the air barrier), then install a plywood or OSB sheathing throughout the chase to protect the vapor retarder. The sheathing also gives the MEP installers a place to fasten anywhere inside the chase. Carpenter friendly with no drywall taping required.
I see you are -38F this morning, design temperature plus !! We are warmer today than yesterday at -20F. I feel a chill being next to the 100 sf of glass by my computer, inside glass temperature is 51F and this is low E.
Doug
Hi Doug, yes, the past few days have been a bit chilly. I've been on the road a lot; glad I have heated seats and steering wheel in my truck.
Sounds wimpy, luxury here is a working heater fan in the truck. Are you making any observations with these extended cold temperatures? The one thing I notice is the cold saturation of the exterior walls. The 2x4 walls are insulated here with a full fiberglass batt but the inside of the wall temperature is currently 62F. Thermostat setting is 72F and outside temperature is -26F. Cold climate builders rejoice, it is cold !!
Doug
I was in a 20-year-old house yesterday performing a blower door test. RH was upper 20% and condensation was present on the dual-pane windows. There was evidence of past water damage from this condition. The exterior surface wall temps in this home were in the 50's. Mean radiant temperatures are causing comfort complaints, but because of the lower RH, no evidence of water damage on the walls. Continuous exterior insulation and tri-pane windows do make a difference.
Randy, I'm sure you've covered this elsewhere, how is the Intello integrated with the drywall air barrier? Do you put acoustical caulk on it before drywall is hung?
We use membranes across our entire ceiling, but yes, using an acoustical sealant on the membrane should be effective. I would be sure there is a solid surface at that interface to make sure the sealant mushes into the drywall. You may need to install solid blocking between the rafters at that connection point.
Our approach is similar to Randy’s, using a robust smart air/vapour membrane to line the plenum bump in the truss. Running 2x4 straps perpendicular to the truss along the centre of the space allows for mechanical equipment attachment. Air sealed, three sided boxes made from 3/4” plywood or OSB create branch cavities are very easy to install between trusses and seal to the vertical sides of the plenum bump.
We also slope our loose fill generously along the sides of the plenum bump, and with extended vent baffles at the eaves there is no issue with wind washing.
Probably the nicest thing about this method that we have found, is that with an integrated service cavity at the ceiling, with the exception of through attic/roof plg venting and the odd electrical conduit which need to be roughed-in ahead of time, all of the air sealing and insulation work can be done prior to MEP.
To rephrase a question I asked unclearly earlier, has anyone on the forum used exposed erv ducts in a residential setting.
malady,
I see it in architectural periodicals, most commonly where the project is a reuse of a warehouse space. It presupposes an industrial aesthetic, which isn’t very common in new build residential. The ducting and supports are more expensive because they have to be chosen for their looks as well as their functionality.
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