Negative House Pressure

[taverty]

My house built in 2004 is suffering from some pretty bad stack effect. And I'm trying to figure out how to alleviate it and hoping I could get some advice here. If I open a window or front door I get a decent gush of cold air coming into the house. My direct vent gas fireplace feels like an open window if it's off. When off, my down draft stove top just dumps in cold air and the damper is fairly well closed.

I already had an energy audit and they determined that my house was too tight. At the time of the audit they said my house was exchanging ~5.06 volumes/day. And I needed ~5.9-8.4 volumes/day.

It's funny, because I thought my house was too leaky exacerbating the stack effect. The energy audit company told me not to seal the house any further until I got mechanical ventilation.

Following their advice I got a whole home ventilation system professionally installed, an Aprilaire 8126. The stack effect hasn’t changed at all. I still have a good solid stream of air trying to get in the house whenever I open a window or door.

I've sealed all the major mechanical penetrations in the attic hoping to slow down the loss of air to the attic and there still doesn’t seem to be much of a change. I also sealed the heavy leaks identified on my audit report. I’m at a loss of what to try next.

I need additional insulation in my attic, but I’m hesitant to add more until I get the stack effect solved. If there’s a problem in the attic, sifting through more attic insulation will just make it harder.

Could it be that I just need to add the attic insulation to slow down the conductive loss?

I don’t have enough insulation in the attic and it’s poorly distributed (this is stated in my energy audit). In some spots I’m as low as 2”, average of 6" range of 2"-10". I’d really like some insight before I have the contractor out to beef up the insulation. The insulation company doesn't think sealing all the top plates is cost effect since my house is already too tight. Is this correct? As I said, I'd hate to have them dump in a load of insulation to only have to shovel it out of the way to seal the top plates.

If it helps, I'm located in Montgomery County Maryland.

Thanks in advance!

[energy_rater_La]

when you put in the whole house system, this doesn't address the
stack effect, this is a fresh air exchange system.
this allows you to continue to tighten the house, with fresh air
being provided by whsystem.

do local contractors recommend foam sealing attic with foam at
the roofline? comparing this cost to air sealing top plates etc
& adding insulation on attic floor would be where I'd start if this
install is an option for your climate.

this is a starting point for discussion, not a solution.
more info will be needed as we learn what you know
and what your house requires. as every house is different
there isn't a fixed answer, for every house.

best of luck.

[firecontrol]

The only way you're going to be able to accurately identify where the air in the home is going and creating the negative pressure is with a blower door test.

[Shophound]

I think you need some diagnostics beyond a blower door. I'd want to start with a simple test...a manometer with a hose inserted underneath your front door on a cold day when you would expect pronounced stack effect in the house. If it reads greater than 3 Pascals I think you have more investigation regarding what is driving stack effect in the house.

And along that line, first place I look to when I want to narrow down a primary driver of stack effect is the ceiling on the highest level of the house adjoining the attic. The usual offenders are recessed light fixtures, unsealed HVAC supply air boots, unsealed HVAC ducting and air handlers in the attic, pull-down stairs to the attic and/or attic hatch covers. From there it becomes a little more difficult in that you want to find thermal bypasses, which can exist in chases open to the attic from lower in the house.

One thing missing from your original post is any expression of comfort issues. You say you get cold air entering the house whenever you open the door. That will happen in an airtight house. Cold air is heavier than hot air. In winter, outdoor barometric presssures average higher than in summer, meaning the weight of the atmosphere is pressing down on your house harder in winter than in summer. When you open the door, you have a density difference, and in so many things dealing with physics, whenever a difference exists, some form of movement or exchange occurs.

If the house has uncomfortable temperature differences even when it's been closed up awhile, you can eye stack effect with caution. If you're just trying to stop cold air from entering the house every time the doors open, but otherwise the house is comfortable, you may need to reassess your understanding of stack effect.

[taverty]

Thanks for the input!

The company that quoted the additional insulation said it was very, very expensive to foam seal the attic. I didn't push for a price because she flat out said it wasn't worth it. Meaning, it would take many years to recoup costs. I kinda wish I at least asked for a ballpark.

I understand the ventilation system wasn't supposed to stop the stack effect. They said it should stop the constant flow of air from pushing in the down draft and gas fireplace. They said my house has to make up the air somewhere so it was coming in the easiest place it could find. So the ventilation system was supposed to be easier than the down draft and fp. But when the system is on it really doesn't change the flow of air from the two weak spots. However, if I open a window or door in the house the air stops pushing in from the down draft and fireplace. I also get a decent push of air from my electrical sockets on outside walls. I've installed gaskets, and that's helped but the air still comes through the actual plug.

The problem is, it does make the house uncomfortable. The family room and the kitchen by the cooktop is always colder than the other rooms. And of course, those are the two rooms we send most of our time it. The family room just feels drafty when you sit there on a cold day. I've resorted to putting a sheet over the fp and that has helped quite a bit.

The basement and main floor are heated with propane and the second with an attic air handler and heat pump. The heat pump is cycling quite a bit because I think we're losing heat too fast. I already had an HVAC tech confirm the heat pump system was sized properly. I even put in an Ecobee thermostat so I could track how often my heat pump comes on. Ecobee gave our house a one star rating for heat retention!

The energy audit company did a blower door test to help me find the weak spots. They found that the bathroom ventilation fans, fan boxes, attic hatch and recessed lights were leaky. They also determined my top plates were leaky. So I went into the attic and spray foamed and caulked all electrical/fans boxes I could. I weather stripped the attic hatch and even put foam backing and fiberglass on top of it. I got most of what they identified. I tried doing the top plates but stopped because that became a nightmare real quick. It was just way too hard to shovel the fiberglass and caulk.

The insulation company is saying to add the insulation and it's probably conductive heat loss. But the word "probably" has me concerned.

One thing I didn't look into is the connections on the HVAC system. Maybe they are leaky, the HVAC tech looked at the system in the attic and said it was done well but non of the connections from the main trunk have sealant. They are insulated, and the insulation butts up to the main trunk well.

I'll ask the energy audit company if they do testing with a mamometer. Do you happen to know the name of the test you're referring to?

Thanks again!

[motoguy128]

Sounds like air leaks upstairs are the main culprit.

If you unit is cycling often, good chance it's oversized for the current conditons, in part because your downstairs is heating your upstairs. The Ecobee probably is loooking at rate of change overall, so it the temp is changing quickly, you heat retention is poor.

Recessed lights are evil in terms of air sealing. The top sill plate can be a nightmare. I don't know why you were told the house was too tight. Leaking ductwork could be a huge soruce of heat loss.

A simple inrared camera will find all your leaks in the attic. It's the primary tool for a home energy audit. Without one, they are flying blind.

[Shophound]

Your latest post gives a more clear picture of what you're experiencing in the house. I reread some things and now see by "down draft", you must mean a stovetop that ventilates downward, like a Jennair range.

That you're seeing cold air enter at the stove and through the direct vent fireplace...may I assume both of these items are downstairs?

Have you had anything done with the recessed lights? I would not try any DIY remedies with them, but rather seek professional solutions, if they indeed are the unsealed type. If you can reach an HVAC supply grill in the upstairs ceiling, remove the grill and check for a gap between the sheet metal "boot" of the duct and the drywall of the ceiling. If you have any returns in the ceiling, do the same thing, if they can be safely reached. This gap, wherever it is found, must be sealed. I would do everything possible to make the accessible ceiling on the top story as airtight as possible before concerning myself with top plates, which, as you've seen, are more trouble than it's worth to do anything about.

With what you've described concerning discomfort, drafts, and detectable air movement, I'd say whoever did your blower door test may have missed something. It could be some who conduct such tests do not have a full grasp on how a house actually behaves in extreme weather conditions...hard to say in your case. But I find it hard to accept that your house is "tight", even before the forced ventilation was added. Typical trigger points for adding mechanical ventilation to a house is 0.35 air changes per hour, or ACH. Your numbers above don't sound familiar, but if you were never given a figure such as 0.35, your mechanical ventilation may be adding insult to injury in an already leaky house.

One tell-tale sign that a house is too "tight" is that it suffers from high indoor humidity levels in winter. If you never had that problem, you were never too "tight".

[taverty]

Motoguy, I thought the same thing about the upstairs system maybe being oversized. But I had a HVAC tech take a look, he said even with the downstairs unit that the system was the correct size. He said I'd want it barely on the oversize side because it was in the attic and that's not an efficient place to have an airhandler. Yeah, my rate of change is pretty drastic. That Ecobee thermostat is great for seeing problems like mine. As soon as the unit is off I lose heat almost immediately. The graph is a bunch of mini spikes.

Shophound, yes you are correct, the downdraft is the ventilation system for my cooktop. And the gas fireplace is a direct vent type. They are both on my first floor.

I only have one recessed light on my second floor. It is one of those really large direct contact type. The energy audit company suggested I only put a bead of caulk between the metal case and the drywall. The leak from it was minor compared to the bathroom ventilation fans. They were REALLY bad, I mean all blue on their infrared camera.

I will look into the HVAC boots. When I was up there last, I looked at a few of them and they looked great. Insulated boots with flanges so you couldn't see a raw drywall edge. I'll go back up and caulk the flanges.

I agree something is really amiss. I never suffer from high humidity, actually I have an Aprilaire humidifier running on my propane furnace, and it's on quite a bit.

If it helps the data sheet on my audit report has CFMs. The sentences read:

For good ventilation you need between 2791 and 3897 CFMs of air passing through the fan, which is between 5.9 and 8.4 volumes of air per day. The blower door test indicates that your home is exchanging ~5.6 volumes, or 2400 CFMs. You are slightly below the ventilation parameters and should get mechanical ventilation installed. Once that is done you can further seal the house.

Also here are the airflow numbers from their data sheet:

sqfeet: 4523.84
Volume: 39743.90
CFMs @ 50: 2400

Airflow building: 231.84
Airflow people: 75
BAS at CFMnat: 231.84
BAS at CFM50: 3987.64
2791.34
70% of BAS: 6

Actual
Volumes/day: 5.06
Volumes needed: 8.40

I don't really know what all that means, but that's what came off my report.

Even the woman running the test, she didn't understand how my home could be so "tight", and have such high energy bills. I was on their low efficiency line, sometimes even worse!

I'm at a loss, I don't mind spending the money to get my home more efficient. But I don't want to spend it and have a marginal improvement, or hardly any because I have this stack effect problem.

Since it doesn't seem to add up, I'll probably hire another energy audit company just to get a second look at all of this.

Thanks again for all of your input. I really do appreciate it.

[energy_rater_La]

note that homeowner has pav in attic.
not in this thread..but in thread on another forum.

[tipsrfine]

Good catch ERLA. I love detective work.

[mbarson]

The numbers I come up with making some assumptions is about .24 ACH. 2400 cfm50 for that size house is not bad. With mechanical ventilation you do not need to worry about too tight of a house except for being concerned with natural draft appliances. I do see it is a supply only system which are not recommended in heating climates. I would go with an HRV. I hope your water heater is not a natural draft unit. If it is you need a worst case depressurization test. Your auditor should have known this. At a minimum make sure you have a CO alarm that is less than 5 years old for some sort of protection. If it is an electric water heater disregard, but still have a CO alarm.

My thought is leaky ducts in the attic. If you have leaky supply ducts it will depressurize your house when the unit is running. An attic ventilator will make this worse. Have the ducts tested for leaks and make needed repairs to seal them. Turn off the attic ventilator, they are not needed. You are correct about adding insulation and not air sealing. If I were you I would seal every crack I could find including the top plates. I did my attic last year and spent at least 9 hours with caulk and can foam. Another tip I give home owners is to replace old bath fans with a quality low sone unit so you won't have to go back up at a later date and replace when they fail. Moving the insulation if part of the fun of air sealing, if you don't want to do it hire it done... it is very important. Adding insulation is the best investment you can make in an attic with low levels. Again I would focus on the ducts, to be that negative seems like it is mechanically induced.

[taverty]

Would the powered attic fan make a difference in the winter? When that was asked in the other forum I almost had an aha moment. But in the winter the fan is never on. It's not even on that much in the summer, the house is on a fairly heavily treed lot and the roof is shaded for all but mid day.

I can't thank you all enough for the help.

[mbarson]

If the fan is off it won't make a difference. I would unplug it if it were my house, they are not needed. Leaky ducts will cause pressure problems. What kind of water heater do you have?

[energy_rater_La]

from other forum:
oh...from other forum:
The fan is a large attic ventilation fan near the ridge of the roof. The roof has two gable vents on either end and lots of small round soffit vents.

pav probably doesn't come on in winter..depends on what it is set to.
but it will...when operating... depressurize the attic. exacerbates duct/return
leaks and leaks from attic into house...the one (yay!) recessed light, the
oversized cuts at bath fans..and top plate leaks.
disable the pav & when you replace it, use passive vents.

and remember..this is a hole in the roof...air in...air out.

I hear you about the gaskets...but the way to stop the leak comming out
of the slots for plugs. baby proof things you put in outlets to keep kids from
sticking stuff in the outlets. put the piece of foam gasket that you usually
throw away on the baby proof plug & put it in the outlet.
thats a Zero Draft tip btw. (credit where it is due)

interesting reading between both forums.
questions are really flowing now!!

as for foam...call some local companies for pricing. if you can measure roof sq ft
(or even if you can't) they can give you a sq ft pricing. estimates are usually free.

best of luck.

[energy_rater_La]

oops, sorry bout that!

[motoguy128]

Reading all this again, id focus on the ductwork. Leaky ducts in the attic could be pushing a lot of air out the house and causing a big jump in bills. Many new construction have flex run by the lowest bidder. that means slap it in quick, usually by a helper wirh little training or experience.


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[taverty]

mbarson, my hot water heater is electric. The only drafting appliance I have is my basement propane furnace. It's a Trane XB80. I think it has a powered flue.

I'll call around and see if I can find a company that will do duct testing. The energy audit company didn't recommend it at the time because the ducting in my attic looks a lot like my basement. Large main rectangular insulated trunk line down the center of the attic with short round insulated flex ducts leading to the boots in the ceiling. The audit lady said she's never seen an attic installation like mine. She said she almost always sees small boxes with round flex ducts all over the attic. So she thought it looked like a very tight installation. But we all know looks can be deceiving!

At this point I'll try anything though! I even went around the attic fluffing the insulation trying to find black insulation that would indicate an air leak. I couldn't really find any black insulation in places other than the leaks the audit company pointed out.

I'll also call around and get some foam estimates. I'm afraid the foam will be in the 8K or up range. The insulation company wanted $1,800 for the extra blown in fiberglass. I have an unfinished basement and they gave me a quote to foam seal my rim joist and that was $1,300! So I couldn't even imagine a whole attic!

energy_rater_La, it's funny you mention the baby safety plugs: My 16 month old just the other day decided he liked plugging things in like mom and dad. So I just started plugging in the safety plugs all over the house. Great idea on using the round throw out piece with the safety plug! You'll def get credit for that one!

The one thing I'm curious with on the attic duct system leaking: Would there be a difference between the air coming in when the system is on vs off? Since it got cold and somewhat windy this evening, I've got my nice cold draft coming up through my cooktop downdraft vent. The amount of air coming up doesn't seem any different with the attic furnace on or off. Could it just be I have a constant static loss on the duct system, or should the stack effect be amplified with the attic furnace on?


Thanks again!

[taverty]

I may have found a possible problem.

I just went to do a quick gap check on the two returns and when I removed the filter from the smaller of the two returns a lot of cold air came pouring out of the return. The inside of the large return was warm to the touch, the small return was freezing cold. I'm talking about the area of the return boot right past where the filter sits.

I remember checking both returns when I was in the attic two weekends ago and both are connected and there are no breaks. Also, when I turn the attic air handler on, the air sucks nice and strong from the smaller return. I'm wondering if I have a strange heat loop going on. The two returns connect to the air handler via a little box on end of the unit. The large return that sits right next to the handler has a very short round insulated run to the side of the box. The smaller return is connected to the top of the box and runs up into the top of the rafters and down to the return boot about 10' away. It's connected like a "L".

I wonder if when the air handler is off the hot air from the house is going up into the larger return into the little box on the end of the handler. Then up into the smaller return duct where the air cools then falls back into the living space. It's like a little convection circuit. The two returns aren't that far apart on the ceiling. The large return is in the hallway right outside the bedroom the smaller return is located.

However, this doesn't really explain the stack effect because the air should be balanced since it should be a closed system. Is that a right assumption? Hey I'm grabbing at anything now!

I'll definitely see if I can find an energy auditor who can look at the duct system. Either way, I probably shouldn't have a blast of cold air coming out of one of the returns and not the other.

Thanks!

[Shophound]

Some photos of what you're seeing may help this conversation a lot.

If the ducts and air handler in the attic are leaky, it will affect the house pressurization/depressurization when the unit runs and when it is off. Return leakage from the attic greater than supply leakage into the attic will pressurize the house when the system runs. Supply leakage to the attic greater than return leakage from the attic will depressurize the house when the system runs. If the ducts and air handler are tight, or leakage on both sides is about equal, the net pressurization on the house is about neutral.

I'm reminded of an article written by Dr. Joe Lstiburek at buildingscience dot com. In it he says, "A blower door measures the characteristics of the house, not the leakage rate of the house in service". What I think he means is that a blower door test delivers a number, such as "When we depressurize the house 50 pascals, it is leaking at this rate per minute". That shows you how leaky your house is (a characteristic), but it does not tell you where the leaks are, or how the house will leak in the real world. In the real world, you can have a cold norther blasting against the north and west sides of your house, with the east and south sides in a leeward state. Meaning the north and west walls are leaking air into the house, while the east and south walls are leaking air from the house to the outdoors. What that also means is that the high winds can cause the house to leak at a rate more or less severely than the blower door test revealed. Combine this with stack effect, and you have a house leaking in multiple directions. A "complex three dimensional movement of air in buildings", as it's been called.

All a blower door test does is state "When we depressurized/pressurized your house to a determined level (commonly 50 pascals), it leaked at "x" cubic feet per minute. Real world conditions can exceed that leak rate, because nature knows no limits on what pressures it may apply to your house.

The article referenced above, while a bit technical, can be found here:

http://www.buildingscience.com/docum...t-and-airtight

[mbarson]

I'm glad you don't have a gas water heater. You could peel back some duct insulation to see what it looks like on the joints. I would be surprised if it was sealed with mastic. Without being there it is difficult to access the whole house. There are many dynamic things going on that should be checked. Is there a Home Performance company in your area? Probably someone with BPI.ORG training. You need someone with the experience and equipment to make an accurate determination.

On the returns the cold air is coming from somewhere, I would try to track that down. The unit should be isolated from the outside. And as requested pictures could be helpful. Perhaps the negative pressure is not from stack effect or the ducts. Cold air should not be coming in from the return. It is hard to say from here! I would make sure the ducts are sealed or have them sealed if not.

As far as insulation goes you don't need spray foam. I only recommend spray foam for difficult areas that blown insulation would be too hard to install. Also I do not like fiberglass blown insulation. The best performance is from cellulose. Keep looking for a good contractor. The better ones have diagnostic equipment to quantify their results.

You could also try finding the leaks yourself. An inexpensive theatrical smoke machine you can purchase from party stores will allow you to see the air moving in your house. That may lead to the direction the air is leaving the house.I don't mean to fill the house completely with smoke, just enough to watch it move.

Keep giving us more info.