HVAC as it relates to tight home practices.
Hello,
Looking to get input and professional advice on HVAC in a new tight home build. My preference is in favor of concealed cassette mini-splits and spray foaming all wall cavities and roof decking, resulting in the HVAC being inside the thermal envelope. I however have more than a few concerns, special features and natural forces working against me.
Details:
Location East Texas, an hour from Dallas / Tyler. Climate Zone 8. Average humidity level 55-70%.
The build is a 1400 sq. ft. 3 Bed, 2 bath slab on grade with an attached insulated & climate controlled 936 sq. ft. garage. 2x6x10′ framing with Zip wall sheathing and roof decking. In theory, I plan on spray foam insulation. Spray foam will be 5.5″ of open-cell in walls and 5.5″ on roof deck with a potential of 1.5″ of closed-cell going down first and then finished off by open-cell. That’s where the simple ends and special features begin.
The entire house and garage are covered by a simple 6/12 gable roofline running 72′ E-W in width. The house section has scissor trusses for vaulted ceilings adding 6.5′ in heigh to each room, except for the 142 sq. ft of master bath / closet and 54 sq. ft. for the 2nd bath. These rooms will have straight 10′ ceilings to provide small attic spaces for the hidden cassette mini splits. The garage will be covered by attic trusses for storage / potential HVAC equipment. I also plan on using an ERV system on the house side and tying that into the mini-split returns.
I need to know as much as possible in order to either DIY or manage subpar HVAC contractors to the best of my ability.
My concerns / questions.
1. Moisture build-up / condensation. What are the best ways / precautions to prevent it in my scenario?
2. Should I insulate my duct work? If left rigid and uninsulated my theory is that it would provide the necessary radiant heat / cooling effect for the scissor truss cavities, I am however more concerned about airflow in the roughly 4,212 cubic feet of air space in the garage attic causing problems. This is a large area to provide a separate air vent to for simple storage. Should I cut this attic off entirely from the house and garage, by insulating and providing natural ventilation? The garage and its attic in theory will already be isolated from the house with spray foam due to toxic fumes.
Thank you for your time.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Have a look at Matt Risingers Youtube channel about his new tight home build in Austin, Texas.
https://www.youtube.com/playlist?list=PLDYh81z-Rhxj4mulCrScAArjx7Db14t78
Are you sure you're in climate zone 8?
Thanks, @Rockies63.
I Love what Matt is doing and have watched many of his videos. I've got loads of green / passive books and have spent many hours doing research on the subject. There is a big difference between reading about it / watching it and actually implementing it. Some hesitation on my part as well.
You are in climate zone 3.
1) This is what a dehumidifier is for. This is especially important for air tight homes because your AC will run less often and won’t be able to dehumidify.
2) For conditioned attics and crawlspace, it is required to have 20 CFM of supply air for every 1000 sq. ft. of attic or crawlspace. You should at least insulate the supply ducts, you shouldn’t rely on them to cool down these ancillary spaces. You want them insulated to provide better heating and cooling to the zones they are designed to supply.
You should also try to design another insulation and air tightness strategy besides spray foam. Even Matt Risinger admits he doesn’t get a good air seal from spray foam alone, it takes a methodical air sealing strategy starting with framing and WRB application.
Thanks, matthew25.
I have planned and implemented strategies for better air sealing other than just relying on the spray foam. I left these out of the details to shorten the read. Below are the following steps and products.
1. Sill plates have been put down on top of a 1/4" solid EPDM gasket by Conservation Technologies.
2. In places where footing / ledger walls / foundation were a little rougher, sill plate and gasket were placed on top of fiber reinforced roofing cement.
3. Where the wall meets foundation is Zip liquid flashed.
4. Wall sheathing and roof decking are Zip will all seems tapped. Roof decking will additionally be covered by a solid EPDM peel and stick Ice dam.
5. I plan to use backer rod and Prosoco Air dam around windows and doors.
6. I will be applying simple caulk on the interior where top plate and studs are joined / butted, before drywall goes up.
I wonder if I could use a separate ERV for this? Seems to me that circulating in fresh outside air would be a bad idea as it won't be the same temperature. I could also circulate the climatized interior air from the garage up to the attic. Thinking this product from Panasonic, https://www.amazon.com/Panasonic-FV-04VE1-WhisperComfortTM-Ventilation.
Sounds like 20-40 CFM should be more than sufficient for this conjoined attic space, so I may be able to pull one HVAC vent from the garage system. I plan on using an 18,000 BTU slim duct mini split and will check manufactures specs for max CFM and specs. https://senville.com/18000-btu-concealed-duct-air-conditioner-heat-pump-sena-18hf-id/
Additionally, I am installing a heat pump water heater and plan to use that to pull out hot air and dump in cool air, intakes / outlets separated by 20'. Between the ERV and water heater, this might be close to enough air flow through the attic space. The Attic trusses and scissor trusses voids will be free flowing between each other.
Your plan for using ERV and water heater for air circulation in the attic can probably work.
But, back to question #1, an ERV is not a dehumidifier. And a HPWH does not provide enough latent cooling to act as a whole home dehumidifier. You still need a dehumidifier, and you can pipe the inlet or outlet of that into your attic space. Humid air is more buoyant and will rise to the highest part of your house.
Thanks again matthew25.
I will look into the specs and capabilities of dehumidifiers. Do you know of anything that I could use that would take the place of a dehumidifier and the ERV?
What about tying the dehumidifier into the return duct of the slim duct mini-split and pulling return air from the garage attic? All this on top of the Heat pump water heater.
Just get a simple plug in dehumidifier model for your attic and run a hose to a drain line with an air gap. Otherwise you’ll have to couple the mini split and the dehumidifier together.
I can’t think of a substitute for an ERV because it dilutes the air for CO2 and VOC concerns. A good pleated 4” media filter can help reduce some pollutants but not all.
Watch some of Scott True's you tube videos on sealing details; he is building amazingly high quality homes near you in Bastrop, Tx.
If you use cassettes, you don't have ducts. They are basically the same thing as a wall head, but a bit more concealed. They have the same problem in that they provide air to only the room they are in, and they don't make ones that are small enough to put one in each room.
Consider ducted minisplits if you can find a decent contractor. Otherwise, go with a ducted heat pump. If there is noone to work on it, it does not matter how good the system is.
Thanks!
I really want to do slim duct minisplit for the Seer and energy ratings. Hoping to get the daily household energy usage around 9KWH a day. LED everything, DC fans, tight envelope and as much insulation as I can possibly get in. It's my dream to finish it off with a 9KWH solar system putting out 20-30 KWH daily.
Climate zone 8 in Texas is for plant hardiness (https://www.pro1design.com/wp-content/uploads/2015/02/USDA-Cold-Hardiness-map.jpeg)
Map for HVAC: https://inspectapedia.com/aircond/U.S.-Climate-Zone-Map-US-IECC.jpg
Thanks, for catching that.