Energy and Building Code Math for Ventilation
Hello GBA Community,
You have helped me make important decisions regarding my new house which is scheduled to break ground in March. But now I have a curiosity question for the energy experts out there.
My 2,000 sq ft single level house is being built in Zone 6. Energy Code seems to require that the blower door test produce results of less than 3 ACH. That is great! I believe in that! I guess because I spend a lot of time outside and I sleep with my window open, so I get plenty of fresh air.
But now I have just learned (if I have this correct) that the IRC requires that houses of that size with 3 bedrooms requires 60 cfm of mechanical ventilation. So it sounds like I need to have a fan on in my house. I really dislike fans (mechanical anything). I designed my bathrooms so they had opening windows to comply with county code of having either a fan or an opening window. Same with the kitchen, no fan for me, just opening windows for venting. So I am pretty disappointed that I have to have a mechanical ventilation system at all!
But I have to ask the experts…how does the math work between these two code requirements? On the one hand I have to make my house tight enough for 3 ACH…..but then, because I comply with that energy code I have to put in (for me, exhaust ventilation) to “force” my house to take in outside air.
Can someone enlighten me that the math somehow works? What is that math if the ventilation fan is “forcing” my house to leak inward to the tune of 60 cfm. If someone could provide the math for a 2,000 sq ft house with flat 9 foot ceilings, I would like to know if, after I “force” the house to take in outside air at 60 CFM, am I still ahead of the energy game?
This is just nerdy curiosity. I love nerd stuff. I just don’t know how to do the math.
Thanks all!
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Replies
I had the same question a while back.
There is no math. They are two separate requirements. You can't adjust the leakiness of your house to meet the ventilation requirement.
The difference between them is infiltration is uncontrolled. It's highest when the temperature difference between inside and outside is greatest, and when it's windy outside. The number they use in a Manual J is the maximum. On a mild day with no wind there might not be much at all. Ventilation, on the other hand, is controlled -- you specify when and where it happens.
A couple things you can do to make it more palatable: you can use a heat-recovery ventilator or energy-recovery ventilator to mitigate the energy loss. You can also have the ventilator triggered by humidity, C02 or occupancy.
Blackfeet,
The two aims are:
- To control where the air in your house comes from. That is to limit the amount of air moving both in and out through your building assemblies - which isn't a desirable thing both from an energy standpoint, or for the longevity of your house, as moisture piggybacks on that air.
- To control the amount of ventilation air in the house for human health and to reduce indoor pollutants, and humidity if necessary
Leaky houses leak more the colder it is. Tight houses with a heat recovery ventilator can have consistent ventilation all the time, and most of the heat in the outgoing air is transferred to the incoming air. Energy modeling usually in climate zones 5 and 6 shows that it costs less to operate a tight house with an ERV or HRV than it does to have a naturally drafty house.
There is the initial cost of the ventilation equipment, which is significant and you wouldn't want to see the payback period. But fresh, filtered air is important.
You can do exhaust-only ventilation, which will cost more to operate than heat recovery ventilation, the incoming air is not filtered and your house will get very dry when it's cold out, but the initial cost is relatively low.
Note that in the Winter, exhaust only ventilation will make the building pressure more negative and this is good for the building. It provides drying (infiltration) in places that would otherwise see wetting (ex-filtration). But a well built house doesn't need such drying.
A downside to exhaust only is that you have no idea where the fresh air will enter - probably not where you need it, leaving some rooms severely under ventilated.
Check with your building official on acceptable ventilation strategies. I believe exhaust only no longer meets code in Minnesota.
Thanks for the replies. I still really want to know the math. So I’m going to take a stab at it and then maybe you can correct me.
My house is 2,000 sf with 9 ft ceilings. 2,000 x 9 is 18,000 cf of air. If I have 6 ACH that means I have 84,000 cf of air coming in every hour. Multiply by 24, I and have 2,592,000 cf of air coming in a day. If I get my ACH to 3 I can knock that down to 1,296,000. If I now have to use mechanical ventilation at 60 cfm....60x60x24 is 86,400. So I am blocking out 1.2 million of air a day with a tight envelope and bringing in 84,600 for health. I now get it. 93% of that blocked air is staying out of the building (in my example). That is a big savings.
Is this correct? Also, I need a system that keeps the house as humid as possible. I live in very dry climate in the high desert of Idaho. I have an eye condition that is complicated by dry air. Which system will keep the humidity in during the winter?
But don't confuse the ACH you get with the blower door with the ACH you get under normal conditions. It will usually be much less (eg, 1/20), but with high wind, could be more and with no wind, near zero.
You have to divide the ACH50 number by 20 to get real-world infiltration. So really you're cutting out 60,000 CFM and bringing in 84,000 CFM.
But here's what I don't get: Do you have a choice as to whether to follow the code? If not, you have to build tight and provide ventilation.
Even if you do have a choice, I would say you'll get the most comfortable and best-performing house if you build it as tight as you can and then ventilate to your comfort.
I'd look into ERV's for humidity management.
The ACH value of a house is measured during the blower door test (-50 pascals). Under normal conditions, the average actual natural infiltration rate is often estimated by dividing by 20.