Negative House Pressure

tavertyMarch 19, 2013

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.

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, 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 started sealing 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’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. In some spots I’m as low as 6”. I’d really like some insight before I have the contractor out to beef up the insulation.

Thanks in advance!

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"I already had an energy audit and they determined that my house was too tight.... I still have a good solid stream of air trying to get in the house whenever I open a window or door."

These two statements contradict each other. In order for air to enter your house, an equal amount of air must leave to make room for the new air. What you are describing is a leaky house!

Did the energy audit person sell and install the ventilation system? Did you notice the air infiltration through the window prior to installing the ventilation system?

Is this a one or two story home? Do you have ceiling light fixture and other penetrations in the ceiling below the attic? Where is the HVAC system and ventilation system located in the house?

    Bookmark   March 19, 2013 at 11:24AM
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I replied on thread on other forum.

lots of contradictions in homes, it is often
more than one thing going on.

stack effect is air entering low and exiting higher.

more information is always a good thing. answering
questions will give us more info.

best of luck.

    Bookmark   March 19, 2013 at 12:43PM
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Yeah, I agree, my statements definitely contradict each other. That's why I'm so at a loss as to what I should do next.

The energy audit company did not install the ventilation system. The lady who owned the energy audit company said they don't sell any services to keep from any conflicts of interest.

I didn't noticed any difference between the air flow after they installed the ventilation system. No increase or decrease. So there may be a decrease, but it's so minor I really can't tell.

It's a two story with a basement. The ventilation system is installed on the basement unit. The contractor said it needed to be on the lowest floor of the house.

I have a good number of 2nd floor ceiling penetrations that the blower door test identified were leaky. I sealed pretty much all of those. The test also identified that my top plates were leaky. I sealed very little of the top plates because it became such a chore to do it.

The thing is even after sealing the leaky ceiling penetrations, I didn't feel any difference in the air flow on the first floor. I just can't tell how the air is escaping. I fluffed a lot of insulation looking for black insulation and I didn't find one smoking gun!

I have a propane furnace in the basement for the basement and 1st floor. And a heat pump and air handler in the attic for the second floor. I had an HVAC tech look at the attic unit and he said the unit was sized correctly and it was installed properly. However, the lines off the trunk don't have any sealant, but the branch pipes butt up against the trunk well and are insulated.

Thanks for responding on the other site. I wasn't sure if what I was posting on the HVAC site was OK since it seemed like it may only be for pros.

Thanks for the help!

    Bookmark   March 19, 2013 at 4:42PM
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Do you have a powered ventilation fan in the attic? If you do then that is the source of the problem.

    Bookmark   March 19, 2013 at 5:04PM
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I do have a powered ventilation fan in the attic. However, it is thermostat controlled. It only turns on during very hot summer days. It's off during the winter. My house doesn't have a ridge vent. The builder said a powered attic fan was better than a ridge vent.

Would this attic fan cause these issues in the winter?


    Bookmark   March 19, 2013 at 5:17PM
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Is the air coming in your door when the furnace is running or off?

    Bookmark   March 19, 2013 at 6:02PM
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Both running and off, I'd say there is slightly less air when the furnace is on.

But the ventilation system is connected to the furnace so it brings in fresh air from the outside when the furnace is running.

    Bookmark   March 19, 2013 at 6:18PM
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"Would this attic fan cause these issues in the winter?"

Just out of curiosity, is this an thru the End gable fan? How is it closed off when not operating?

    Bookmark   March 19, 2013 at 6:38PM
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Can you verify the fan attic is off? It is possible the thermostat control is not working properly.

Where are you located? On a sunny day in the winter it is possible for the attic to become warm enough for the fan to turn on.

    Bookmark   March 19, 2013 at 6:40PM
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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.

I was in the attic two weekends ago sealing the penetrations and it wasn't on. It was working last summer. I would poke my head in the attic hatch and hear/see it run. I just checked again today and it wasn't running and I still have a decent amount of air coming through my cooktop downdraft and fireplace.

I live in the Montgomery County Maryland area.

Thanks again!

    Bookmark   March 19, 2013 at 6:54PM
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this is going to get a little confusing...but can you
post your video over here?
I can't diy you on hvac talk forum, but can here.

this is what I noticed in your video.
R/A to master (first one shown) goes up from
return plenm, then over, then down to ceiling
return @ attic floor.
a straighter run would work better.
up..across & down is a more turns than you need.
gentle sweeping turns are one thing...but any
90 degree turns in supply or return reduces air flow.

also on both returns, the duct is
attached to framing members in the attic
then sheetrock ceiling, with R/A grill
inside the house @ ceiling.

if you dig the return framing out of the
blow fg, you'll see that the framing isn't sealed.
each 2x should have been caulked together,
then sealed to attic floor. insulation may be dirty
in this area, and sheetrock dust will be around
botton 2x @ attic floor.

on the inside of the house, you'll want to
seal it also..better two methods of air sealing
to catch anything you missed than just one.

I'd use paint on mastic, paint all joints, corners
& 2x's to each other. make a good seal everywhere
but pay special attention to framing at attic floor.
do same from interior of both returns.

now...back to plenum connections to equipement.
I see foil tape...not the best product to use in
my experience. I only use mastic tape, or paint on
mastic. sealing plenums to equipment..both supply
& return..use Hardcast brand #1402 mastic tape.

take off foil tape. use hardcast to seal top and sides
of plenums to equipment. the tricky part is sealing
the bottom joint. its fairly easy if you have someone
on the opposite side of plenum. you tear off the tape
from the roll, peel off the plastic backing & slowly
seal as far as you can reach...the other person
reaches from their side to continue the seal.

mastic tape needs to be pressed tightly to surfaces
to stick. (you can actually buy a little credit card
sized squeegie for this). surfaces need to be both
clean and dry.

the reason I'm telling you about this product
is that this will also be used at each of the
takeoffs from the sheet metal trunk line
for the flex ducts. these areas have no sealant.
paint on mastic is going to be difficult..bottom of
the duct usually isn't well sealed, unless it is something
you do often.

Hardcast is 3" wide, so it gives you room to position
it and covers well.
pop that lock tie on the flex. fold the insulation &
reflective liner back...and scoot it down the duct.
now you'll see that they used another lock tie
or maybe even duct tape (prolly lock tie/pundit..
its quicker). if it is a lock tie...leave it...but use
the mastic tape to seal the inner liner to the duct
take off. at the plenum you'll now have room
to work.

here, you have to tear the mastic tape into
pieces about 4-5" long to make the seal.
overlap each piece by 1". press it well to seal.
start at top of duct...and contine around until all
is covered.
pull insulation IN CONTACT with metal take off
and then pull outer liner to cover insulation.
I don't use lock ties here..again...mastic tape.
you can put a lock tie for added support..but
tightening it too much compresses insulation
& reduces R-value (condensation in my hot humid
seal outer liner of flex to fsk duct wrap paper
on hard pipe.

now rather than just covering up the leakage..
you are sealing it, and keeping insulation
in contact with metal of take off collars.

once you get the hang of will go
faster..and you might want to re-do the return
ducts to return plenum.

I'll let the guys on hvac talk discuss sizing of supply
plenum & ducts...I'm not good at gauging sizes of

now the other leakage...luckily you'll go inside the house
for this. take off supply grills. see that oversized gap
around supply box that the supply grill covers.
mastic tape. split the 3" width in half. keep the tape
within the area the supply grill will cover. tape from
sheetrock ceiling into metal lip of supply box.
but carefully. once mastic touches sticks.
and it will remove paint..and paper.
see picture attached.

while you have the ladder out...take off cover of
bath vent fan..the nut is behind the light bulb.
you'll prolly see the same oversized cut as at
supply boxes. nicely hidden behind cover.
use mastic tape to seal this area also.

when you're in the attic...take a look at damper on
bath fan. should be a back draft damper installed
so that damper hangs closed when fan isn't in use.
about 1/2 the time..the are installed incorrectly.
sometimes there isn't a damper at all. this is a
4" opening open all the time when no damper
is installed.
see...hidden holes in air barrier @ attic floor...
I use the mastic tape to connect the back draft damper
to the bath fan & to connect the venting to the damper.
once you decide which way the damper correctly installed
goes...put a piece of tape to show which side goes up.
pita to have venting connected & accidently put damper
the wrong way..ask me how I know! LOL.

this is how I seal ductwork & the bath fan penetrations etc.
all these areas..plenum to equipment..duct take offs
to box to hole in attic floor and
returns...these areas leak. low man on the job
runs duct..30% duct leakage isn't uncommon at all.

only one recessed light...correct?
you can use an ice chest (foam ice chest)
to seal this from in the attic. you have to have
4" clearance on all sides & to top of recessed can.
press the ice chest firmly in place. the foam will
show where you'll need to trim it to fit around 'legs'
of recessed light. notch for the legs, then run a bead
of caulk on both ice chest & attic floor (after moving
fg back). caulk at notches for legs of can light.
don't use foam to seal, as it doesn't seal well at all.

what are the batts of insulation on the attic floor?
is there a fireplace opening hiding under them?
can you take a picture of the fireplace from inside
the attic?
attic stair case? I have a diagram on how to best
seal this also..weatherstripping doesn't really last
but building a box around stair case with a removable great.. lasts. unfortunately I only have it in
pdf form..not a link. if you drop me an email..I can attach it
in my reply.

ok...that's my duct/return/bath vent/recessed light
attic seal advice.
post the video so I don't look like I'm tripping here...

here is the sealing of the supply pic
& maybe the better filter ( media filter) to fit in
existing returns. if I can attach both.
once ducts/returns are sealed..better filters are
great. keeps coil really clean & picks up a lot
of particluate without being restrictive.
I might have to do that on another post...

best of luck

    Bookmark   March 21, 2013 at 11:06PM
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here you go.

    Bookmark   March 21, 2013 at 11:11PM
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WOW thanks so much for the information! I'll definitely try everything you outlined. It's funny, both companies that bid the insulation job had an optional air sealing line. Both companies said it wasn't worth it and they weren't sure if it would solve the negative pressure. It wasn't cheap and neither company would touch the HVAC ducts. It's funny, it was like they offered the service but they totally didn't expect me to select it. That's why I decided to tackle air sealing on my own. I figured once I get the negative pressure under control I could have them back out to add the insulation.

That filter look really good. Where do you purchase a filter like that? My local Home Depot and Lowes doesn't carry those kind of filters.

The fireplace is on the first floor and sits in it's own cantilever. Which is why I think it's so leaky. That cantilever overhangs the foundation and the wind just whips up underneath and into the fireplace doghouse. My basement is unfinished so I can see into the cantilever, it's insulated with a few batts of FG. The insulation company is going to remove the batts and spray foam the underneath of the bump out when they're out to do the attic.

When I caulked up the bath fans two weekends ago I checked that all three bath fans had backdraft dampers and that they worked. They aren't the greatest, just little plastic flaps but they opened and closed correctly with the operation of the bath fan.

The extra batts in the attic are there just to beef up that area of the attic. It's the only way from the mechanical platform into the rest of the attic. I had been in the attic so many times in the past that I mashed down the blown in insulation in that area. So a few years back I purchased a bag of batts and threw them down in that area. See it's that experience that keeps me from letting the insulation company blow in additional insulation when I feel like I still need to work up there. Blown in insulation gets compacted SO easily!

Thanks again for the information. I'll drop you a line with my email.

Also here is the youtube link for everyone here:

    Bookmark   March 22, 2013 at 10:09AM
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most companies don't do duct sealing. it is too
time consuming, when they can be making more
money with easier work.
I do this work as a necessity..not being to
confidently recommend anyone to homeowners
in my area. I don't do the work on homes I do
energy ratings on...conflict of interest, but I do
a LOT of diagnostic work.
if you can't get someone like diy it
and I can walk you thru it via this forum or email
whichever you perfer.

you get media filters like the one I posted a pic
of thru hvac supply house or your hvac company.
I pay about $140 for 4 filters. these filters have a long
life as opposed to 30 day filters.
the same supply house would be the source for
lock ties,mastic, and hardcast 1402 mastic tape.

if you have difficulty in finding the tape..let me
know & I'll share my source.

as for the fireplace..I have a general idea of
what is going on. foam spray insulation would probably
be the way to seal around the fireplace..IF they
can get to it.
take a look at this link, and apply what he describes
to what is happening at your fireplace.
this is what I think is happening, its a common
mistake in building stages of homes.

can you take picture of the fireplace 'doghouse'
from inside the basement? there may be a solution
other than foam that will work better.

best of luck.

    Bookmark   March 22, 2013 at 1:03PM
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energy_rater_la, I think you're 100% right in terms of what's going on with my fireplace bump out. I attached a pic here and in the other HVAC forum of the fp dogshed. The insulation company was thinking spray foam was the best option. Originally she thought rigid foam would be best, but since we have I-Joists she said the spray foam would give a better seal. I open to any suggestions though!

I'll be going this Friday to my local HD to find the mastic tape and mastic. If they don't have it, I will def drop you a line if you didn't mind sharing your source.

Thanks again!

    Bookmark   March 26, 2013 at 11:04AM
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"Originally she thought rigid foam would be best, but since we have I-Joists she said the spray foam would give a better seal. "

it is because you have I joists that the foam sheathing
would work. if it were space joists then it would be
harder to seal. as it is, you have solid osb to seal to
on each side.

are you good at diy??

no problem if you need my hardcast source,
HD carries nashua brand, but it is not as strong
as hardcast and only comes in 2" width.
not a product I use because it isn't as robust,
& doesn't seal as well.

note hearthman's post on other thread.

best of luck

    Bookmark   March 26, 2013 at 1:15PM
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I've been thinking about "air in, air out" & stack effect
and how it relates to your house.

I have an idea to run by you, but deleted your
email by mistake.
could you drop me an email?


    Bookmark   March 30, 2013 at 11:04PM
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Don't be sharing any info with the op via private email ,unless there is a security factor to consider, that will help me in my HVAC education here. Please. Don't want to miss anything.

If there was an award for sharing information on this forum you win hands down. I might have to read it 50 times but have 50 times more education than when first read.

I am having duct work added to my small cottage this year and didn't even consider sealing lines before I came across this, just assumed they were good to go as delivered.

    Bookmark   March 31, 2013 at 1:37PM
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I'd be glad to answer any questions you have
southerncanuk, if I can be of help.
just start a thread with specific to your house.

glad to know that my posts are helpful to you.

    Bookmark   March 31, 2013 at 6:31PM
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Energy is an incredibly helpful individual and every time I hear the air handler I know dollars are staying in my pocket that otherwise would not be.

Thank you!

For others who browse this.. it makes a noticeable difference in allergies, comfort and dollars and energy saved.

    Bookmark   April 1, 2013 at 7:12PM
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anytime countyboymo.
you and I have trouble shot a few houses
together here!
recessed lights being the stand out in
my mind.

this house has me stumped.
I know the bumpouts are an issue, but
the blower door numbers & duct leakage
numbers really put up a red flag for me.

wish I could test it myself!

    Bookmark   April 2, 2013 at 9:53PM
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This is a tough one. Something is very wrong somewhere but where? It might be quite a few places that are being overlooked though.

    Bookmark   April 3, 2013 at 8:16PM
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