Humidifier with a flash-and-batt house?
SueVA2013
| Posted in Energy Efficiency and Durability on
Hello. We are building a house in Virginia and the construction of the exterior wall is: Hardiplank, Tyvek, OSB, 1 inch closed cell spray foam, kraft faced R19 batt, and Drywall.
Our HVAC contractor is not installing the humidifier as he says tight homes don’t need them. His comment was we might be calling him to install a Dehumidifier in all probability.
Is that true?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
I agree that tight homes are unlikely to need a humidifier, and much more likely to need a good ventilation system to maintain indoor air quality. What is he installing for ventilation?
I do not think that flash-and-batt insulation means the house will automatically be tight. If you want to build tight, there are quite a few things that need air-sealing attention in addition to the exterior walls.
Our house has 2 unit. One in the unconditioned attic that is a heat pump and ac and will be getting a ERV. This system serves our second floor that has all the bedrooms.
The second unit in the basement has a natural gas furnace. This will serve our basement and main level. He said there is no need for ventilation on this unit as our kitchen exhaust has an associated air make up intake that opens and closes a damper as our exhaust is turned on or off.
Susan,
Your HVAC contractor gets a big thumbs-up for giving you excellent advice about humidifiers. Humidifiers are dangerous. The use of humidifiers is associated with wall rot.
Your HVAC contractor gets a big thumbs-down for installing an air handler and other equipment in an unconditioned attic. That equipment should have been installed inside of your home's thermal envelope. For more information on this issue, see Creating a Conditioned Attic.
Martin, maybe a small thumbs down on the attic install, it is done almost 100% of the time.
What needs to be done is someone has to post the exact install cost difference to install the upper system in conditioned space. Then know the added savings in utility costs.
Guesses...
Conditioned install add $6,000
Savings annually, save $300-600
Risk of getting bad foam job.... 1%
Risk of roof leaks going undetected 1%
Homes are owned on average 5 years, payback may happen at sale with low monthly bills.
Biggest cost annually, mortgage, property taxes, cell phone, cable tv with 800 channels
least cost.... utilities
Tough sell IMO
There is a niche... for those like us... but it is a small niche...
AJ - You are close to the figure for the conditioned space install. It was like 4.5 K more to run extra duct work and other stuff to get it in our basement mechanical room. In all fairness to him he offered it to us as an option!
Are we good on ventilation?
"It was like 4.5 K more to run extra duct work and other stuff to get it in our basement mechanical room."
What other extremely low risk investment could you possibly find that would have annual after-tax payments in double-digit percentages (or mid-to high single-digits, worst case)?
If it's not too late, it's worth the up-charge to bring it all inside of conditioned space, which will limit the possibility of air-handler driven air infiltration. It's infiltration that makes wintertime air dry, and summertime air sticky. Leaks or imbalances between supplies & returns get amplified when the ducts penetrate the pressure boundary of the house- it's not just an insulation thing.
Too late :(.
While I might agree on return being good, I also have to consider capital allocation and priorities. Also since the designer did not design the house for such our HVAC contractor would have to cut into one of our living spaces.
Going back to original topic, are we good on ventilation by installing just one ERV with the unit in the unconditioned attic that serves our second floor that has all the bedrooms. The basement and first floor will not get ERV.
Susan, when these guys talk about bringing the HVAC inside the house, they mean insulate the roof of that attic space instead of the floor, not physically move the equipment elsewhere. It stays in the place it was originally intended, the insulation is what moves (and probably changes from fiberglass to spray foam).
If your house turns out truly tight, then you will want a comprehensive ERV system that serves both floors. Ask your builder if he has had blower door tests done on other houses and what the results were.
Also, if you install an ERV, it should have its own separate ductwork rather than being tied into the ductwork for the heating/cooling.
Susan,
Unless your house is very large, a single HRV or ERV should serve it well. However, the ERV should have dedicated ventilation ductwork; you shouldn't try to use ducts designed for delivering space heating or air conditioning for ventilation.
This ductwork usually supplies fresh outdoor air to your living room and bedrooms, while exhausting stale air from your bathrooms, laundry room, and sometimes your kitchen.
For more information on these topics, see:
Designing a Good Ventilation System
Ducting HRVs and ERVs
There are many ways to bring it all indoors in new construction,. but as a retrofit insulating at the roof deck is usually a more viable (but not super cheap) option. It's usually cheaper to frame the upper floor a bit taller to accommodate the ducts & air handlers above a finished ceiling, but below an air barrier & thick fiber insulation layer, or run the trunk ducts laterally in a conditioned crawlspace, serving second floor rooms with duct chases. Whether a roof deck insulation solution is at all viable here depends, but it's a long shot. (I'd expect it to cost a lot more than the $4500 the HVAC contractor quoted for the alternative installation.)