Ventilation Rates

Conversations

Topic: Ventilation Rates

What should the ventilation rates be for buildings? But as our Industry Experts discuss, effective ventilation is not all about the rates.

 

Joseph Lstiburek  Principal, Building Science Corporation

 

Joseph Lstiburek

December 05, 2013: Are we good with current ASHRAE Standard 62.2 ventilation rates? Are they too high or low? Are there any concerns or issues? I’m curious about what other people think.

 

 

 

David Bell  Director of Building Science, Masco Home Services

 

David Bell

December 05, 2013: The BSC ventilation rates are what we cite in the Environments for Living standard. But other questions come up, not just the rates. What are the impacts of where the ventilation air is brought into a home? Should it be hard ducted to the air handler? If so, how close to the air handler should the hard duct connection be made? Many different kinds of devices and controls are available. There does not appear to be a uniform method of installation. There are many questions we get asked that are not about the rates specifically. We’ve had those rates in place for a number of years, with a one third duty cycle. I can’t justify a need to increase them – if anything, I’d decrease them. We’ve seen no issues overall over the last 10 years that would raise a flag to us to suggest that they’re too low or not enough. We’ve seen some incidents related more to dehumidification or high humidity levels in the southeast, which don’t appear to correlate necessarily to the flow of fresh air ventilation, but appear to be a by-product of high performance homes in an extremely humid climate – low sensible load homes. Many people are attributing high humidity levels to the ventilation alone, shutting their ventilation off completely. Ventilation becomes the culprit, even at rates that are 1/2 to 1/3 the ASHRAE Standard 62.2 rates – people think that the ventilation rate is why they’re having these problems and don’t appreciate the effect the huge energy efficiency improvements over the past decade have on interior moisture levels. We’ve actually had builders’ warranty people telling their homeowners to shut the ventilation off and just have no ventilation. So we really want to educate and be able to point people in the right direction.

 

John Straube  Principal, Building Science Laboratories / Building Science Consulting Inc.

 

John Straube

December 05, 2013: Well, we know that zero ventilation is not the right number. It’s a worthwhile challenge to answer all those questions – all those lists of what devices are available and how they should be used. How do you control ventilation, where does the air get injected – those are a whole bunch of practical questions not connected to rate, and they’re really important. But the one thing we know for sure is that ventilation rates should never be zero.

 

David Bell  Director of Building Science, Masco Home Services

 

David Bell

December 05, 2013: Of course. We’ve required fresh air ventilation for over 10 years and would not say in any way, shape or form that there shouldn’t be fresh air ventilation. But what is the right rate? We’ve also had some issues come up with the new ENERGY STAR rates, with Honeywell systems, where customers feel that the things are just running continuously, and the rates are way too high. I think that’s also helping drive the blame on the ventilation system and having the builders’ field personnel just shut it off.

 

Joseph Lstiburek  Principal, Building Science Corporation

 

Joseph Lstiburek

December 05, 2013: We’ve got a series of things going on at the same time. As we’ve gone to the 2012 IECC or we go to a Challenge Home or high performance homes that are smallish, in the 2000 ft 2 conditioned floor area range or smaller, even the older, lower ventilation rates lead to elevated humidities. In other words, it’s not ventilation alone that’s driving the elevated humidities, it’s higher performance. There’s this question that perhaps supplemental dehumidification is necessary in a high performance building, period – especially one that’s relatively small or that is a townhouse or a condominium. I suspect the difference between the 2010 62.2 and the 2013 62.2 is merely exacerbating a trend that was already in place. My intuition is that the difference between the 2010 and 2013 62.2 is bumping up operating costs on heating and cooling 10 to 15% and also elevating RH 10% to 15%. But the trend on elevated RH was already there because of high performance. Low load houses lead to high interior RH in humid air conditioning climates. There is no such thing as a free thermodynamic lunch.

 

David Bell  Director of Building Science, Masco Home Services

 

David Bell

December 05, 2013: I agree with that. And it’s interesting – it goes back to what we’re dealing with – I would raise my hands high for “please don’t give us any more moisture or latent load to deal with in the house” because we’re edging towards a situation where a giant tiled shower adjacent to the master bedroom will affect RH. But homeowners don’t want to stop taking showers in their big, luxurious showers. Who would have thought that interior moisture loads in houses in the southeast could be an issue? Air change and air leakage – exterior moisture – were always the main culprit. I don’t think that is the case anymore.

 

Joseph Lstiburek  Principal, Building Science Corporation

 

Joseph Lstiburek

December 05, 2013: Here’s another thing: what interior relative humidity is too high? I’m concerned that the 60% “thou shalt not exceed” number that people are mentioning is rather low. I think excursions above 60% are acceptable. The question is, what kind of an excursion, for how long? Now that people are actually measuring things, they’re saying “oh my god, I’m at 70% humidity” and I’m thinking “that’s actually quite normal and quite common in many houses.”

 

John Straube  Principal, Building Science Laboratories / Building Science Consulting Inc.

 

John Straube

December 05, 2013: I agree, Joe. As you know, I’ve been measuring my own house, which is airtight and well-insulated, and I’ll tell you that this morning it was 70% and it has been for about 4 days. And it’s not because it’s hot and humid out, it’s because it’s neither hot nor cold, so it’s just sitting there. There’s no thermal drive, there’s no equipment running. And so is that a problem? I know the 80% number, “thou shalt not exceed 80% for more than half an hour” might be a real number, but we can survive at 70% for weeks. We need some kind of more sophisticated measure than “never over 60%”.

 

Joseph Lstiburek  Principal, Building Science Corporation

 

Joseph Lstiburek

December 05, 2013: And John, you are in southwestern Ontario not in southeastern Georgia.

 

 

 

 

David Bell  Director of Building Science, Masco Home Services

 

David Bell

December 05, 2013: Homeowners come to us with indoor air quality documents from the EPA that say between 30 and 50% RH is where you want to be, and they are focusing on that and not knowing the whole story. So we need to help them know more of the story so they don’t think that their builders are just trying to make excuses.

 

John Straube  Principal, Building Science Laboratories / Building Science Consulting Inc.

 

John Straube

December 05, 2013: In terms of practical advice, I think we do know that 50% is unreasonable anywhere east of the Mississippi. The only way to achieve it would be with a specialized dehumidifying ventilator or some such thing. Maybe the EPA has to say that: if you want to hit these targets, you can’t buy a normal house.

 

Joseph Lstiburek  Principal, Building Science Corporation

 

Joseph Lstiburek

December 05, 2013: I’m thinking that 60% might be unreasonable.

 

 

 

 

John Straube  Principal, Building Science Laboratories / Building Science Consulting Inc.

 

John Straube

December 05, 2013: Well, it’s a tough number to pick. It’s even tougher to pick the range and the allowable excursions. But we certainly can discuss “is it physically possible.” And I think if you’re lucky, 60% may be possible. It doesn’t mean it’s the right number. But I think 50% is literally not possible, on an hourly RH basis, in almost any kind of modern home in almost any climate east of the Mississippi.

 

Joseph Lstiburek  Principal, Building Science Corporation

 

Joseph Lstiburek

December 05, 2013: East of Interstate 35. I think someone moved the Mississippi RH fence farther west since I started practicing as an engineer. I remember the old guys telling me the Mississippi was the humidity “line”. I don’t buy that anymore. I have been spending too much time in Texas and Oklahoma dealing with part load humidity problems and can tell you that 50% is not an appropriate number there either.

 

David Bell  Director of Building Science, Masco Home Services

 

David Bell

December 05, 2013: I’ll just throw one more thing out there: one of the hardwood flooring providers who works with some of our builders has very stringent RH requirements for their engineered products. This particular provider requires that their materials have to be stored, installed and after installation should be in an environment that does not exceed 55% RH. And they say that they’re taking this from the national hardwood floor standard. So now manufacturers have drawn a line as well.

 

John Straube  Principal, Building Science Laboratories / Building Science Consulting Inc.

 

John Straube

December 05, 2013: So that’s important – that’s a very practical point. It’s a get-out-of-jail-free card for all those hardwood flooring manufacturers. It sounds to me like there needs to be some more work done, first around understanding what acceptable RH is and then talking to people such as cabinet makers, wood flooring and wood trim manufacturers. Let’s get some real science behind what it takes to operate buildings. And it’s not about RH, it’s about RH range – I mean, scientifically we know that to be the answer – there’s nothing right about 50 or 55 for wood products, it’s only the delta between the high and the low that matters. So some research could be done to inform the hardwood industry and help them set up more realistic targets and still protect themselves.

 

Mac Sheldon  National Accounts Manager, Demilec USA, LLC

 

Mac Sheldon

December 05, 2013: I’m concerned about the energy penalty related to over-ventilation. I live in a heating climate and I’m tuned to the North. Comparing the BSC method to the 2013 62.2, with an example of a 2000 ft 2 house, from the very worst to the very best, there’s a 53 cfm difference. In my climate that is over a $100 per year hit. I’m very concerned that people are going to react by turning their ventilation systems off completely as a reaction to the excessive ventilation rates cited in 62.2 2013.