When is air sealing too much?
About a year ago I had an energy audit and a company air sealed our attic and added cellulose. Our house is a 1950s house.
Our building airflow was 1836. After they did the work it rose to 1976. They had added additional ventilation in the roof to bring it up to code which they said now the building is responding the way it should. There is no mechanical ventilation in the home and we still have single pane windows that are a bit leaky. I have insulated the rim joists I can get to in the parts of the based that are unsealed.
One of likely airflow culprit is our pet door to the garage. I was going to move it within the building envelope it’s for cats gaining access to a kitty litter box.
I am also thinking of weatherstriping the windows and add a new door with insulation and weatherstriping. Also the walls are uninsulated and I want to add densely packed cellulose to those (2×4 framing).
Will this make the house too tight and I’ll need to added mechanical ventilation?
Thanks for the great info and site.
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Replies
From what you've told us John, the kitty door is the least of your issues & you'd have to do much more to ever stop air leakage and losses. We'd love to know your climate zone and style of house. As for windows, if you can't afford new, try interior storms (inserts) like: http://www.energywisemfg.com/
As for "Our building airflow was 1836. After they did the work it rose to 1976"
If those are blower door CFM readings, we'd need to know the cubic footage of your house to make any sense of it. Most use ACH (air changes per hour) at 50 pascals.
"There is no mechanical ventilation in the home"
That is very unsafe especially if you burn fossil fuel for heating or cooking. At least get a good bath fan.
"I have insulated the rim joists I can get to in the parts of the based that are unsealed."
You'll need to explain in detail. Was it sealed around each sill box before insulating? What type of insulation?
At least you're on the right track!
Hi Paul, thanks for asking for more details:
Our climate zone is 4 and our house is one story ranch style house with a finished basement.
The square footage is 2712 with 8ft tall ceilings so 21,696 cubic feet. Yes those were blower door readings Pre-CFM @ 50 Pa.
We do have fans for all the bathrooms and the gas oven has a range. I meant a central ventilation system. I'd love to get rid of the gas range but my wife loves it.
I used spray foam and rigid insulation (1.5" thick) to seal the rim joists. The sill box weren't sealed before I added the rigid insulation.
John,
Q. "When is air sealing too much?"
A. You can never perform "too much" air sealing work. Sealing leaks is always good. There is no such thing as "too tight."
Q. "Will this make the house too tight and I’ll need to added mechanical ventilation?"
A. Don't be afraid of mechanical ventilation. Mechanical ventilation is your friend. The point at which an old leaky house becomes tight enough to begin worrying about indoor air quality varies from house to house. A small house is more likely to need mechanical ventilation than a big house. The higher the number of occupants -- especially if the house is small -- the more mechanical ventilation matters. So there is no "magic number" (in terms of ach50) at which mechanical ventilation is needed.
Every tight home needs mechanical ventilation, and every green home should be a tight home.
If you smell odors in bedrooms, or if you see condensation on the interior of your window panes in winter, you probably need mechanical ventilation.
A mechanical ventilation system can be as simple as a high-quality bathroom exhaust fan connected to a 24-hour timer.
"Our building airflow was 1836. After they did the work it rose to 1976. "
Am I understanding correctly that leakage rates increased? If so, is there a chance that the increased roof ventilation is a culprit (what else changed?)?
If that is the case, it would seem to suggest the barrier between conditioned space and the roof venting space is not tight, and the increased roof venting has essentially increased conditioned space venting (they should be decoupled).
Sorry if I've misunderstood.
Tyler,
I had the same impression that you had. If improvements in attic ventilation make a house leakier, it means that there are serious air leaks in the ceiling that separates the conditioned space from the attic.
My reaction was a bit stronger. If the company air sealed the attic and added cellulose, the leakage should go down, not up, even if the attic is wide open to the elements. If the airflow went up, they made the holes between the house and attic bigger. That's going in the wrong direction. There is no way that these results can be explained as "the building responding as it should."
You probably wasted your money on the air sealing and insulation. You should think about making them tear it out and do it right.
I agree with Peter. The increase in attic ventilation doesn't explain an increase in the house leakage, unless the attic previously had no ventilation, and also was somehow better air sealed than the house. The only explanation is more leakage points in the conditioned envelope. So either they made the ceiling to attic leakage worse, or they messed something up with their testing (forgot to close something, etc). I think the latter is the most likely. The fact that they tried to explain the bigger number away as a good byproduct of the attic ventilation tells me they either don't know what they're doing or they're unethical, or both.
"One of likely airflow culprit is our pet door to the garage."
Not sure this would account for much change, but did you check to see if the pressure of the test actually opened the door? That would change the effective leakage area from the gaps in the seals to nearly the entire (kitty) door.
It’s possible that after the air sealing and insulating work was completed, there was then enough pressure created by the blower door (I’m assuming that’s how those “airflow” numbers were measured) to open the kitty door. Once the kitty door opened, it may have provided enough extra leakage to result in the higher “airflow” numbers for the second test. The pet doors I’ve seen take a bit of force to overcome their seal, especially the magnetic types, and once that’s happened it takes much less force to hold the door open than it did to initially open it.
Try taping the pet door shut if you have another blower door test performed. Regular painters tape is probably sufficient, and won’t leave marks. You’re not dealing with very high forces here.
Bill
Even now you should install mechanical ventilation and use it. Who knows what you are breathing in a bedroom on a calm night.
> added additional ventilation in the roof
Depending on details, this can increase leakage. For example, a worse soffit:ridge ratio will decrease attic pressure (increasing ex-filtration).
Jon R,
Unbalanced low & high ventilation can result in increased depressurization of the attic and increased exfiltration from the conditioned space. But that is a very low pressure (<1Pa) behavior. That wouldn't have any effect on blower door tests at 50 Pa. The testers either didn't do the tests in the same conditions, or somebody did something bad to the attic floor.
When adding mechanical ventilation for a home what is the best way to add intakes for fresh air? Is there a way to add dampers that only open when the system is on?
John,
Not all mechanical ventilation systems have air intakes. (Exhaust-only systems depend on random envelope cracks.)
Manufacturers of balanced ventilation systems (manufacturers of HRVs and ERVs) provide duct termination fittings for exhaust and intake. These termination fittings include backdraft dampers.
21696 / (60 * 1976) = 5.46 ACH 50
At 5.46 I see no need for more ventilation unless you have 20+ occupants.
If I were you, I would be very upset with this contractor. They made bigger holes in your house than ones they plugged up. I would demand they come back and seal until you are below 1836, no fast talking excuses should be accepted. At the very least you should talk to the supervisor.
Walta
Hi John -
It's sounds to me as though BOTH contractors may have let you down.
1. For your weatherization contractor to try and explain away the increased CFM of leakage the way your report is complete hogwash. And leaks at the lid of the building can be the most important (in terms of stack effect).
2. I hear quite often that professionals conducting blower door tests feel their job is complete once the quantitative portion is done (getting the CFM50). But the QUALITATIVE part of a blower door test is to me just as important: identify the location and the relative SIZE of the various leaks. If that were the case, you would know what sort of contribution the pet door made: probably significant if it was "sucked" open; probably not if it remained closed during the test.
Finally, venting your gas range is important for all of the combustion biproducts: CO2, any less-than-completely combusted fuel, and water. For each therm or about 90k Btus, you dump about a pound or a pint of water into the air from combusted propane or methane. It can add up.
Peter
Wouldn't increasing attic ventilation enhance the stack effect, accounting for increased air leakage?
Increasing soffit ventilation lowers the stack effect, increasing ridge ventilation increases it.
Air sealing the upper floor ceiling plane is the highestin importance, air sealing the basement is the other highest importance. The stack effect drive is a function of the distance from the lowest leak in the building to the highest. Seal the bottom AND the top and you've reduced the stack effect drive.