air-tight vs. draft-free
Last Post 28 Aug 2015 02:18 PM by craigtoo. 43 Replies.
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walleygirlUser is Offline
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24 Jul 2015 11:21 PM
Conventional green wisdom calls for sealing up houses air-tight, and then using air exchangers to keep the inside environment healthy. This concept has never sat well with me. It is basically substituting expensive technology, machines that require inputs and maintenance, for something that is free and right outside the walls. And, I simply do not trust things like tape and plastic wraps to survive a lifetime. So I feel like we are creating a false sense of security by relying on these things to never puncture or fail, rather than just designing the house so that it doesn't need to be airtight to function well. Cob and strawbale homes "breathe" and don't require air exchangers. For a number of reasons I'm not interested in doing either, but surely there must be some conventional way to achieve the same results? Someone once said (online discussion elsewhere) "there's a difference between air-tight and draft-free", suggesting that drafts are the real culprit in homes with uneven temps and high heating costs. I've often wondered, then, if one can have air exchange without drafts, and without compromising the ability to heat the home efficiently. Any thoughts?
jonrUser is Offline
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25 Jul 2015 10:35 AM
Conventional green wisdom is correct. If you want a simpler and reliable wall air sealing, consider ICFs or poured concrete. But you still have ceiling, window and door sealing and you still need a HRV/ERV. But don't worry, it won't create drafts (there is little flow compared to hot air heating).
walleygirlUser is Offline
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25 Jul 2015 01:01 PM
Keeping in mind here that I know little about the building science behind the conventional wisdom of sealing our houses up in plastic (I'm looking at this from a more simplistic/holistic viewpoint...which of course means I'm probably missing something!), is it possible that slowing the air exchange so that it isn't taking room temp air and dumping it outside immediately, or bringing cold air in fast, but perhaps losing or gaining heat slowly as the air passes through a wall or roof assembly, could allow for fresh air to enter the house (and stale to leave it) without mechanical ventilation?

Seems to me the problem is not the air leaving the house through leaks, but the air taking the hard-won heat from the inside of the house with it. So if we could allow the air to leave slow enough that it cools on its way out...? I'm probably missing something important here...I'm just trying to wrap my head around this.
Bob IUser is Offline
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25 Jul 2015 02:10 PM
Walleygirl: this is the beauty of heat recovery ventilation (HRVs & ERVs). They have a gizmo - the exact type varies by manufacturer - which allows the cold air to pass on one side of a membrane - aluminum or some other material - while outgoing warm air passes by on the other side. Thru simple physics, the heat from the outgoing air is drawn to the cold incoming air and the heat is transferred so the incoming air is only slighly cooler than room air. It doesn't transfer all of it, but they are up to 75-90% efficient You will find a lot of information at GreenBuildingAdvisor.com - and you can search for information on the website for mini splits, HRVs, insulation and many other facets of good building practices.
Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
walleygirlUser is Offline
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25 Jul 2015 04:27 PM
I'm familiar with how they work, they seem to do the job very well and efficiently. And I think I've read pretty much every article on GBA at this point, lol! I don't doubt they work, I'm trying to understand the wisdom behind airtight homes that require mechanical ventilation.

My point (question) is this: cob and strawbale buildings don't need HVAC systems. They breathe. Not just vapour passes through, but air exchange happens too. And yet they stay warm and comfortable in winter, and cool in summer, with excellent air quality and are efficient to heat, especially when designed as passive solar (originally, we planned to build such a home, and there are several in our area and lots of expertise around the building of them here, but for numerous reasons we decided on a conventional stick-frame structure).

Having established that you can build a conventional stick-frame wall structure that, like cob and strawbale, does not need a vapour barrier (and thus can "breathe"), using (for example) Roxul insulation on the interior and exterior of the wall, what does that mean in terms of air exchange? In such a wall, is the vapour passing through but the air isn't? (let's pretend there is no wood sheathing for the sake of argument)...and if the air IS passing through but heat isn't, then why would one need an HVAC system?

I guess I don't understand the vector for heat transfer. In a thermal bridge, for example, like a stud in a wall, heat travels through the material, but air certainly doesn't travel through the wood. So heat is moving, air is not. In the case of insulation that breathes, like rock wool or maybe cellulose, is it the opposite (air can pass through, but heat does not)?
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25 Jul 2015 05:09 PM
Other things being equal, a building using a HRV/ERV will be more energy efficient than one with the same ventilation rate traveling through the walls. Because it exchanges the heat instead of losing it to the outdoors. Unless you get air to flow in both directions in the same little area, a wall doesn't do this.
gosolarUser is Offline
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25 Jul 2015 05:57 PM
air tight
walleygirlUser is Offline
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25 Jul 2015 06:36 PM
Posted By jonr on 25 Jul 2015 05:09 PM
Other things being equal, a building using a HRV/ERV will be more energy efficient than one with the same ventilation rate traveling through the walls.


Okay, but how much more efficient? And is the cost savings enough to offset the cost of making the house airtight and then purchasing, installing, maintaining, and operating an HVAC system? The people I know living in cob homes in my region are paying very little to heat their homes as it is. So if we could have a similar level of efficiency in a stick-frame home, even if it wasn't as good as airtight-and-HVAC, then why go the latter route? (btw, I hope I'm being clear here - consider this a philosophical argument to better understand the concepts; I'm not trying to be argumentative and I hope I don't come across that way!)
jonrUser is Offline
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25 Jul 2015 08:55 PM
Let's make some assumptions:

House 1: 1 ACH natural (quite leaky, might achieve a healthy .35 ACH in all rooms and most conditions with no HRV)
House 2: .1 ACH natural (OK but not state of the art)

2000 sq ft.
9 ft ceiling
20F outside, 70F inside
$.15/kwh electric heat

House 1 will cost about $.71 per hour for infiltration alone and house 2 will be $.07/hr (plus HRV costs/losses).

0.018 * 1 * 2000 * 9 * (70-20) * .15 / 3412

I'll ignore the damage caused by all that air depositing moisture in the walls and/or the lack of humidity in the interior. Edit: none of this has anything to do with conductive heat loss. That needs to be added to the above to get total heat cost.
walleygirlUser is Offline
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25 Jul 2015 11:04 PM
Insulative materials (fibreglass, rock wool, foam) keep heat from passing through, but they are not resistant to air flow right (well, I think rigid foam may be)? I mean, doesn't air pass through these, but heat doesn't? Otherwise, why is insulation not sufficient to make a home airtight?

So doesn't that mean that you can have air passing through without taking all the heat with it? And would materials with different insulative properties therefore have different ratios of air loss to heat loss?

So then if that is true, why is there be a linear relationship between ACH and heat cost?
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26 Jul 2015 07:10 AM
You seem to be using HVAC and ERV/HRV interchangeably. In some climates, you need HVAC. Where I lived in FL, the daytime highs are in the 90's, and the lows in the 80's, with very high humidity. That climate needs HVAC to be comfortable. As for ERV/HRV- it is easier to build a building, seal it, then introduce a 'leak' of known size and airflow.
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26 Jul 2015 08:54 AM
Straw bail are great and ventilation may be "perfect" until the window blows. It is ultimately a matter of control required to maintain a certain level of comfort. The lower your standards the lower the cost; of construction, maintenance and operation. I have lived in a pup tent, a wall tent and have often admired the ever simple teepee. Now that's a light footprint on the earth!
MA<br>www.badgerboilerservice.com
Bob IUser is Offline
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26 Jul 2015 10:29 AM
good article on strawbale houses & moisture: http://www.buildingscience.com/documents/digests/bsd-112-building-science-for-strawbale-buildings/?searchterm=straw%20bale%20houses

"you can have air passing through without taking all the heat with it?" Depends entirely on the amount of air; as you increase the airflow, you increase the heat loss. Different insulations have different rates of air flow - fiberglass batts being pretty high; dense pack cellulose being on the low end. Heat passes through all insulations - the R value per inch can be used as an approximation of the amount of heat, but these measurements are not perfect. Most fiberous insulations like fiberglass batts & cellulose, are around R-3.5-4, but the actual insulation value is related to the amount of air moving through the wall, so that a slapdash fiberglass batt installation may be "rated" as R-19 but actually perform at a far lower rate. (Not accounting for thermal bridging which can be 20-25% of the R value).

The bottom line is that with a tight house, the actual R value of the assembly has a major effect on the heat use and comfort. When you change the air tightness you will affect that use, whether it's a straw bale house or a foam house. If you choose to build a "leaky" house, you have no real control of the amount of heat loss; with a tight, mechanically ventilated house you do. I understand your reluctance; I built a large addition to my home many years ago with 9" thick walls, fiberglass batt insulation and pine sheathing (so it could breathe). Never worked as well as I had hoped although it was "ok". It wasn't until I started air sealing the house in ernest that the it became much more comfortable and much easier to heat. So I can tell you from experience that it's far easier to built it right at the start than to correct it later on. We know tight houses work extremely well; we know that making the house air tight (i.e.: less than 1.0 ACH50) is a major factor in that. We also know that better air quality is better for our health, and that normal contanimenents and off gassing of so many common materials decreases air quality. Seems to me if you want a house with no mechanical ventilation you should aim for 3ACH50 or more, and accept the indoor air contaminants and the resulting lower air quality and the fuel use penalty. But it's so easy to do it right, why would you?



Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
walleygirlUser is Offline
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26 Jul 2015 12:42 PM
Okay, I think I'm starting to get it.

A bit of Googling showed me that cob actually has a fairly low R-value, but because the entire thick wall acts as thermal mass, the heat it brings in due to that function outweighs the loss of heat as air passes through the walls. Stick frame walls don't have thermal mass (not enough anyway), so now I think I understand how the former can be breathable and warm, but the latter would be breathable and result in net heat loss.

By the way, when I read that a material is "breathable" (like Roxul, for example) do they mean that air can actually flow through it, or do they mean just water vapour?

jdebree, you are right I get confused about heat pumps vs. HVAC (and by HVAC, I mean something that brings in fresh outside air and removes stale indoor air). If I have a heat pump, am I exchanging the air in my home too? Or do I need a separate system? Is a heat pump an example of an HVAC system? And does that mean if I had radiant in-floor heating, for example, that I would need a separate air exchange system for my home?

Bob, I used cob and strawbale as interchangeable examples of "breathable" wall materials, but you are correct the latter is definitely not something to use in a wet, humid climate like ours. Nobody here builds with strawbale, a couple of folks used it for North-facing walls but sandwiched between thick layers of cob for that reason. Still risky, IMO.

Okay, so the more leaks you have, the lower the actual R-value of the building. That makes sense. But does that depend on the RATE of leakage? For example, is there a difference between a slow leak and a direct open hole in the wall? And could one, in theory, have a slow enough rate of air leakage that you maintain a healthy indoor air environment but without significant heat loss?
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26 Jul 2015 02:31 PM
R-Value is a heat transfer number based on material resistance to temperature differential.

Uncontrolled air loss in infiltration; a number that could easily double the heat loss of your structure. No serious builder builds any house today without controlling air by mechanical means, either an ERV or a HRV depending on climate.

Thermal mass is over rated, again depending on general construction, climate and lifestyle.

There is no magic bullet and you can't design a new home via Google. You need an Architect and/or an heating, ventilating and air conditioning (HVAC) designer to get the options right. First the floor plan, then windows, insulation and ACCA Manual 'J'. Only then can you way your options logically.
MA<br>www.badgerboilerservice.com
jonrUser is Offline
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26 Jul 2015 02:57 PM
Thermal mass is over rated, again depending on general construction, climate and lifestyle.


I'll second that. And as buildings become better insulated, it's even more true. Wallboard alone offers significant thermal mass effect (as I sit here quite comfortably with no AC and 84F outside).

walleygirl: Heat loss due to infiltration and heat loss due to conduction (relates to R value) are separate things. You need to be clear which one you are referring to.
Bob IUser is Offline
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26 Jul 2015 03:28 PM
All fiberous materials including Roxul allow some air to pass through; only solid foam is completely air tight - but not at the joints. Roux does allow water vapor to pass through so it is "permeable". (and since it is made of rock, it does not support the growth of mold.)
A heat pump has no part in air exchange. Electrical wires and refrigerant are the only things that pass from the interior to the exterior & vice versa. The air that is actually heated/cooled is air that is already in the house.
Slow vs "fast" leaks - the determining factor is the amount of air leakage over time - how long it takes for the air you have paid to heat to leave the house. So a high rate of leakage means that there are either a lot of "slow" leaks or fewer "fast" leaks. It's like a picnic cooler - the length of time ice will last, is determined, in part, by how often the lid is open. Proping it open 1/8" will keep the ice frozen longer than if the cover is wide open for the same length of time. It will last the longest if the lid is closed.
Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
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26 Jul 2015 07:46 PM
Thanks, everyone, for participating in this discussion. I've taken your comments, done some more reading both here and on Green Building Advisor, and I've come to the conclusion that I needn't devote so much cognitive energy to this issue.

Most of the year, our windows are open - it's mild here and we like the fresh air. The house is only sealed up for about 3 months of the year, during the winter, so that is the only time when mechanical ventilation would be necessary. Given our relatively small house and other factors, I don't think we'll need an HRV, but can get by with exhaust-only ventilation. Of course I'll discuss this further with our builders, but it makes my whole line of questioning a bit moot.
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27 Jul 2015 06:31 AM
Most HVAC systems do NOT bring in outside air at all. A heat pump is simply a type of A/C unit that heats as well as cools. The only connection my heat pumps have to the outside is the refrigeration lines. The indoor half just recirculates the indoor air, doing nothing for air quality except adjusting the temperature and some dehumidifying. An HRV or ERV brings in fresh air from outside, and exhausts the stale air. In some cases, the HVAC system is tied in with the HRV (alphabet soup, I know!). An HRV has a heat exchanger to scavenge some of the energy in the indoor air so it is not all lost when bringing in fresh outdoor air.
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27 Jul 2015 10:30 AM
I would not live there without an HRV. You will find the rH quite high in winter and want to control it continually instead of intermittently.
MA<br>www.badgerboilerservice.com
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