Helpful? 0

When is an HRV necessary?

I'm building a ranch home in northern Idaho with a walk out basement. There will be some crawl space areas at one end of the house. I intend to seal them from the exterior so it is conditioned space. The main roof will have scissor trusses with spray foam under the roof deck so the space in the middle of the scissor trusses will also be a conditioned space.
Trying to get as airtight a house as possible, I would think that an HRV system will be highly recommended or even required. I am not very familiar with these systems.
Will the truss and crawl conditioned spaces also need to be factored into this system?

Asked by James Mills
Posted Fri, 02/28/2014 - 18:46


11 Answers

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Helpful? -1

IMO, any room called "conditioned space" must have ventilation; depending on size, a supply vent should be installed, and it would be best to install a passive return vent to allow the "conditioned space" air to circulate to an open space in the house. In dry climate like Idaho, is less of an issue than in mix-humid or humid climate, but it's always good to be safe.

Answered by Armando Cobo
Posted Fri, 02/28/2014 - 19:53

Helpful? -1

Most designers of residential ventilation systems base their design on the ASHRAE 62.2 standard, which calculates the ventilation rate based on the "conditioned area" of the house. A common-sense interpretation would exclude the crawl space (and probably the attic, unless the attic is used as habitable space) from this calculation.

It's a good idea to include a mechanical ventilation system in any tight house, but there are several types of ventilation systems that don't include an HRV or ERV. For more information on ventilation options, see Designing a Good Ventilation System.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 03/01/2014 - 07:58

Helpful? -1

If your question is "Will the truss and crawl conditioned spaces also need to be factored into this system?" is what the code says:
2009 & 2012 IRC - R408.3 Unvented Crawl Space: A) Continuously operated mechanical exhaust ventilation at a rate equal to 1cfm per 50sf of crawl space area. or B) Conditioned air supply sized to 1cfm per 50sf of underfloor area, and a return air pathway to the common area with a duct or transfer grill.
2009 IRC R806.4 & 2012 IRC – R806.5 Unvented Attics: This code references insulation and does not talk about ventilation; however, good building science practice tells me to apply the same rule from the crawl space ventilation. There is no way to guarantee that your attic space will never have moisture or humidity issues in the next 100 years. Hence CONDITIONED ATTICS.

Answered by Armando Cobo
Posted Sat, 03/01/2014 - 10:23
Edited Sat, 03/01/2014 - 11:20.

Helpful? 1

HRV, ERV ?? How does the deciding criteria change if we are using radiant floor heat in the basement floor slab and in a lightweight concrete based first floor? We will, most likely put in ducts for AC.

Answered by James Mills
Posted Sat, 03/01/2014 - 10:41

Helpful? -1

This is a very interesting question. We are in the process of designing a house in southern Idaho. We are aiming to do in slab radiant heat and cooling using a geothermal ground loop heat pump. The upstairs will be the same. Upstairs cooling in the summer is the crux. We are looking at possibly routing the HRV(ERV?) outputs across some additional pex inside some rigid ducting so this air will be somewhat chilled by the cool water in the pex tubing. Not sure if HRV or ERV is the best solution.

Answered by Mike Baughman
Posted Sat, 03/01/2014 - 10:54

Helpful? -1

James - What type of HVAC system are you using in your house? Crawl space and attic ventilation can be achieved with a simple supply vent from the HVAC system or a small 50-75cfm fan.
Mike - I'm not familiar with your routing an HRV application inside ducts. In dry climates, I spec HRVs for balanced ventilation.

Answered by Armando Cobo
Posted Sat, 03/01/2014 - 11:16

Helpful? 1

The running of HRV fresh air through a duct containing pex with chilled water from the heat pump is an idea I've had to supplement the radiant cooling of the floor slab. Cooling with a chilled slab is not that effective and I've got concerns that it may not be able to keep up. I don't know of anyone else trying this, so maybe it's a crazy idea.

Answered by Mike Baughman
Posted Sat, 03/01/2014 - 12:12

Helpful? 0

Armando, I'm looking at an exterior wood boiler with propane back up for in floor radiant heat in basement slab and light weight concrete on first floor.

Answered by James Mills
Posted Mon, 03/03/2014 - 09:47

Helpful? 0

James - Sorry, but I have no clue; its not the type of systems we use in the SW. Maybe somebody else is familiar with your requirements and give you advise.

Answered by Armando Cobo
Posted Mon, 03/03/2014 - 15:14

Helpful? 0

The heating system & radiation types have no bearing on the ventilation requirement/type. Heating systems neither add or remove moisture from the conditioned space air, it only heats it. Raising the temperature lowers the relative humidity, but that's only because it's RELATIVE (to the temperature).

In an ID climate there's no point to an ERV- HRV is fine. Summertime dew points in ID are fairly low, and the average latent loads are even NEGATIVE in that area, unlike most of the eastern US. The scant amount of moisture recovery you'd get out of it in winter just isn't worth the trouble and shorter core-life of an ERV.

There's also no point to (and possibly reasons NOT to) running the ventilation at the full ASHRAE 62.2 rates, since that would overly dry your house in winter. Unless you smoke and use lots of toxic cleaners/spray-aerosols the air quality inside your house can usually remain pretty high in winter just running the ventilation under dehumidistat control. High humidity isn't a functional way of measuring air quality, but overventilating to the point that the air is below 30% RH is rarely necessary.

Answered by Dana Dorsett
Posted Tue, 03/04/2014 - 16:21

Helpful? 0

Dana, I'm glad you sound familiar with the area,( just east of Coeur d'Alene, ID). I am a carpenter and that is my field of expertise. After 35 years, I am familiar enough with the other trades, but smart enough to get advice from people with more experience than I do in other areas. I've done a lot of high end work, but never built a "passive house." It means a lot more today than it did years ago.
This house in ID will be our last. I don't expect it to meet the high passive house criteria, but it will be the best sealed and insulated house I have built. A happy medium. With all the passive home information out there and all the varying opinions, it's sometimes tough to figure out where that medium is going to fall.
We don't use a lot of toxic cleaners and the like. No smokers. For most of the time it will be two of us and a dog until family & friends show up. My wife likes to cook & can stuff. I'd say fairly normal activities.
I appreciate the advice, thank you.

Answered by James Mills
Posted Tue, 03/04/2014 - 18:08

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