Need for a HRV
I am building a reasonably well insulated tight house (R 27.5 walls, R40 roof, R22 ICF crawl space walls, R10 slab) in Victoria, British Columbia (similar climate to Seattle). The walls will have 1 1/2″ EPS on the outside, taped to form an air barrier and vapor retarder paint on the interior drywall (no poly air/vapor barrier). Target air exchange rate is 1 to 1.5 air changes per hour. The house will be heated with an electric air to air heat pump. My heating contractor tells me that I do not need a HRV and that at an added cost of $3,000 to $4,000 my pay back period will be forever. My designer tells me that I should install a HRV and that they are an efficient energy saving device with an attractive pay back period. I am at a loss what to do. How important is a HRV for comfort, health and energy efficiency for the house I am planning to build and how cost effective are they in my climate?
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Replies
Target air exchange rate of 1 to 1.5 per hour... at 50 pascals, or natural conditions?
Sorry, David, at 50PA
The least cost option would work well for you, which is to use your bathroom fans to do the work of an HRV.
I typed Panasonic in the search box... More info than you could ever ask for....
https://www.greenbuildingadvisor.com/search/node/panasonic
But.... what about ventilation when the fans are not running; what about energy loss by not recovering heat of exhaust air; what about unequal distribution of incoming fresh air? Also, if my house actually delivers 1.0 to 1.5 ACH at 50Pa as designed will the house be too tight to deliver adequate make up air for the fans via leaks?
Ross,
Your house will be much too tight for adequate air exchange to maintain a healthy indoor environment and evacuate excess moisture. What your house requires is mechanical ventilation at a rate of 1 cfm per 100 sf of conditioned floor area plus 7.5 cfm/person, continuously 24/7 (typically in the 60 cfm range).
In your climate, the payback for an HRV would indeed be quite long. The least costly whole house ventilation system is an exhaust-only system, using quiet efficient bath fans on programmable 24 hour timers (in addition to the point of use switches) and passive make up air inlets located where fresh air is needed - in bedrooms and living areas. I prefer American Aldes Airlet 100s.
By the way, EPS foam does not come close to meeting air barrier material standards. I would recommend using the Air-Tight Drywall Approach as well.
Ross, what type of range hood are you putting in the kitchen?
Unknown at this time, David, but we are planning on a 36" Blue Star gas range which will have a large CFM requirement. My building code might require me to supply back-up air for the range hood.
You don't need more than 100-200 cfm for a range hood.
Lsitburek, in Build Guide, Cold Climates, pg 333, says any kitchen fan > 100 cfm should be "carefully integrated into the design of the entire building", or words to that effect. To me, that says to keep them about 100 cfm. FWIW....
Ross,
You definitely need a mechanical ventilation system.
An HRV is an expensive option. I agree that it will take many years, probably decades, before the added cost of installing an HRV would be recouped in anticipated energy savings.
Here is an article that lays out your ventilation options: Designing a Good Ventilation System.
Robert, I am curious about your statement that properly taped EPS outside of the wall sheathing will not meet air barrier standards. My understanding is that a 1 1/2" layer of EPS foam on the outside is an excellent air barrier. After all you cannot such air through this material.
Martin,
I have read the article, thanks, but am still confused. I do not doubt the superiority of a properly designed dedicated ventilation system but am not sure the benefits outweigh the cost. My planned home is 2,150 sq ft with an expected leakage rate, @50Pa, of 1.0 to 1.5 ACH. This is 190 to 290 cfm. On the other hand, the ASHRAE standard for mechanical ventilation calls for 36 cfm in my house (two occupants) and many experts think this is too high. Further, my gas range will require ventilation of about 400 cfm or higher (gas ranges require high ventilation rates to expel combustion pollutants). This requires make up air which might have to be heated at source. Then there are other ventilation sources such as clothes dryer, bath fans and vacuum. All of these have ventilation rates that make the ASHRAE standard appear miniscule. I know that these devices ventilate intermittently and that the make up air is not distributed evenly throughout the building but still......
Ross,
Do you understand how tight 1 ACH 50 is?
This is an extremely tight home!
You MUST have whole home ventilation strategy for this home.
This is building 101.
A typo on my part, Robert, I meant XSP - extruded polystyrene. I keep thinking The "E" stands for extruded not expanded.
Ross,
I'll repeat myself: your house needs a mechanical ventilation system.
There are many options. It's certainly possible to use your bath exhaust fan(s) to provide mechanical ventilation for your house.
If you choose to install a powerful range hood fan, you'll have to come up with a makeup air solution -- anything from opening a window, to a dedicated passive makeup air vent with a motorized damper, to a makeup air system with a supply fan and electric resistance heat.
Ross,
ASHRAE requires 100 cfm kitchen exhaust fan. A gas range does not require any more than that, and I would never install a hood in a tight house that moved more than 200 cfm on high speed.
ASHRAE used to have a standard of 0.35 ACH (which for your house, assuming 8' ceilings, would be 100 cfm) and recently reduced the required air exchange rate based on floor area and occupancy load. 36 cfm continuous is NOT too much if you want to control indoor humidity and maintain a healthy indoor environment.
Yes, there is an energy penalty for fresh air, but what's the cost of poor health and a rotting house?
If you do a lot of laundry, I would recommend installing the dryer behind a weatherstripped laundry room door and provide dedicated make-up air for that room (that's how I build my super-insulated houses).
XPS is an air barrier material. EPS doesn' t even come close. The Air Barrier Association of America (ABAA) has set 0.02 l/s-m²@75 Pa as the standard for an air barrier material. In various tests, EPS foam allows between 4.7 and 12.24 l/s-m²@75 Pa of air to flow.
I'm not sure how your house system will meet this almost-Passive House standard, but you might want to check your numbers.
If the conditioned space is 2150 SF and assuming 8' ceilings, that's a house volume of 17,200 CF. So 1 ACH would be 17200/60 = 257 CFM50 and 1.5 ACH would be 430 CFM50.
And the ASHRAE 62.2 standard of 7.5 cfm/person plus 1 cfm/100 ft² of conditioned space assumes a baseline natural leakage of 2 cfm/100 ft² , which is the equivalent of 0.15 ACHnat (or about 2.25 ACH50).
0.15 ACHnat would create a baseline infiltration of 43 cfm for your house, for a total air exchange of 79 cfm. So, if your house will really be that tight, then you should go beyond the minimum air change requirement.