Old houses can be made super energy efficient as well!
Post: March 7, 2013
Deep Energy Retrofit in Jericho, Vt!
Part 6 - Air Tightness Layer
Window air sealing detail on the new window in the South wall.
Once the house was fully wrapped in 6" of polyisocyanurate board it was time to add a membrane to the outside to make it
airtight. Air tightness is one of the most important ways to make a house energy efficient.
Air tightness fights one of the most important heat losses a house can face - infiltration losses. To recap, infiltration is the air
that comes through the gaps in the wall. These occur mostly around major penetrations of the wall. Windows and doors are
the biggest holes we purposely put in our walls. It is often the place where gaps are at their worst. Other penetrations include
water and sewer as well as electrical service and cable coming through the wall into the house.
Why is infiltration such a problem? Infiltration is often described in air changes per hour at 50 Pascals (ACH50). This is the
amount of outside air that gets blown through gaps and holes into the house during a 22 mph wind. So say your house is like
mine was and has been shown, after a blower door test, to have and ACH50 of 8.25. This means that when there is a 22 mph
wind outside (not a very hard wind actually) the air in the house changes 8.25 times per hour or one time every 7.3 minutes.
This isn't a problem if the air outside is nice and warm (but not too warm). But if the temperature outside is cold then your
heating plant (boiler, furnace or heat pump, etc) must warm that air up about seven (7) times per hour. The colder the air, the
harder your boiler must work to heat the house up to the 68 degrees F you are hoping to keep it at.
Eco Houses of Vermont, LLC
Copyright 2013 Eco Houses of Vermont, LLC
Chris West is a Certified Passive House Consultant
Affiliated with the PHIUS and PHI 2013
The plan with my house is to bring the 8.25 ACH50 down to 2 ACH50.
We don't know yet if we will hit this number but that is our guess as to
where we will come out.
To achieve this goal we have wrapped the entire house with the variable
vapor open membranes I discussed in the last blog post. The brand we
used was Mento 1000 and the tape we used was Tescon tape. Both
products were purchased through 475 High Performance Building Supply
in Brooklyn, NY. Ken Levenson worked with us to decide on the best
product for our project and sent it up to us via UPS.
The window sealing detail was a bit tricky the first time, but Jim and his
crew soon got it down. Jim did say that this part of the job took much
more time than he was expecting due to the learning curve and the level
of detail required to do a quality job.
Here is a close up of how the did the taping in the corner. I
would have taken pictures of how they cut the Mentos 1000 to fit
the window but I was unfortunately out of town for this process.
I will check with Jim to see if they took any pics and post them
The bridge between the polyiso and the EPS will be bridged with
a 6" piece of the same Tescon tape ensuring that no air will be
getting between the two types of insulation and then into the
house. The polyiso insulation sticks out a half inch from the EPS
giving a good plane for a drip edge.
Southeast corner of the house. You can see the Mento 1000 variable
vapor membrane and the Tescon tape used to apply it and to seal it in
the window bays.
Transition between the Mentos 1000 membrane which is on top of the polyiso
insulation board and the EPS which, if you remember from previous post, covers
the cinder block wall from above grade to below grade.
It's all in the details
Other air sealing that still must be done before we can perform a
blower door test is at the chimney and the old access panel to
above the ceiling (under the roof).
So we will be keeping the chimney and the wood stove. Wood is a very common heating fuel in Vermont. If you have read
my earlier blogs I do have quite a problem with where the chimney is. It is on the North side of the house on the outside of
the house. This means that when the chimney gets warm/hot due to the hot gasses from the wood stove going through it,
this warmth is radiated out into the outside! I do have some friends who have their chimney in the middle of the house, a
much better situation.
We are keeping the chimney, as I said and we will need to do some work to make sure that the chimney doesn't form a huge
thermal bridge and also that the air sealing around the chimney is reasonable. We aren't trying to get the house to 1 ACH
50, but we would like to get to 2 ACH50.
Insulating the chimney. Is the first part of the job. You can see
from this picture the wonderful chimney on the North side of
the house. I have already insulated the part that is below
grade or near below grade. It is the white insulation (EPS) at
the bottom of the chimney.
The plan is to wrap the rest of the chimney in six inches (6")
of rockwool board. This is a rigid board, similar to the polyiso
but is made from rockwool. Rockwool is a byproduct of
making steel. The slag from the smelting process is spun into
a wool, similar to but much denser and resilliant than
The product we will be using is made by Roxul and has an
R-value of 4 per inch. The insulation will be brought on in two
three inch layers. My contractor, Jim Bradley, was unable to
find masonry screws that were 7" or 8" long so he came up
with a solution that should work. We are going to use 4"
masonry screws and use this to put the first layer of 3" of
insulation on. This will then be covered with a layer of 1/2"
plywood. The following layer will then be fastened to the
plywood using regular screws. This will then be air sealed
using the Mento 1000 like the rest of the above ground
portions of the house. Then strapping will be applied and
finally vinyl siding.
We decided to go with siding because it is more durable than
parging with cement and will allow the construction to
Jim and I were a little concerned about that plywood layer getting wet and not being able to dry. I called my moisture guru
Ken Levenson from 475 High Performance Building Products. After our talk we decided that any vapor that got into the
construction would be able to dry in two directions. Firstly, if the plywood got wet it would be able to dry towards the
outside where we have the Mento 1000, a rain screen and then vinyl siding. Secondly, the brick is porous and would also be
able to absorb moisture and then release that moisture into the chimney cavity itself. Thus it can dry to the inside as well as
the outside. It shouldn't be any problem.
Having decided on this Jim is going to get started on adding the rockwool to the chimney tomorrow and should be done in a
day or so.
North side of the house with chimney. Notice the deck in the background. You can
see the light between the house and the deck showing how the deck is not
attached to the house anymore.
The Chimney: Air Sealing
A rustic way to heat your home, if it is a leaky mess that is!
Wood stoves are great. Ok, maybe they aren't great. The efficiency
sucks (usually less than 75% for this older model) but the fuel is
cheap and readily available and renewable. I'm not saying that
burning wood doesn't produce CO2 and CO and NOx and SO2,
but it is a way to heat your house that is very popular here in
The cheap part is also part of fuel independence. I can go out into
the woods around my house and cut down enough wood to keep
my house warm for a whole season. Just saw, split, stack and use.
For a Passive House a wood stove is a deal breaker in most
instances. For starters a wood stove usually has a rated output that
is way more than any PH can use.
Secondly there are very few wood stoves that have the kind of air sealing you need to get your 0.6 ACH50.
My old Pendelton Avalon wood stove is from the 1990's. I bought it from the fellow I bought the house from. It is a good
little stove with lots of years of use still in it. My wife and kids love the fire and it gives off 55,000 Btu/h when it is running
hot. It is also an atmospheric combustion stove.
Atmospheric combustion is where the air that the stove (or other fire based heating system) uses for the fire comes from the
room where the stove is. This is fine if your house is leaky, but becomes a problem when you make a tight house. Once your
house is tight you need to bring fresh air in for the combustion process.
This stove has the ability to be fitted with an outside air feed. This air feed isn't very well sealed but it does work. The stove
will need to be lifted, feet put under it (it is sitting on it's belly right now) to raise it four inches or so to allow for this air feed
box to be installed.
Then we need to figure out where to get this fresh air from. A friend of mine suggested that there are plenty of people who
run the fresh air feed up the chimney and pull this down from the top of the chimney. This could work with my chimney
because it has two shafts. Just break through to the other chimney shaft and pull air from there. The only problem with this
scenario is the stack effect. Hot air rises. The warming of the chimney shaft next to the fresh air channel could cause an
updraft that could make the 'draw' of the fresh air difficult which could decrease the efficiency of the wood stove's burning of
My idea is to just cut a hole in the back of the chimney and run a pipe through there directly out to the outside. This would
give us a short and horizontal access to fresh air. A short run decreases the resistance caused by the inside surface of the
pipe and the horizontal run would eliminate the possible problems from the stack effect.
Sealing the chimney shaft
Ok. The biggest problem with a chimney is the shaft itself. This is a
big open hole in the house that needs to be dealt with. My chimney
shaft has a trapezoidal shape in the room and then gets smaller just
above this. The stove pipe coming out of the top of the wood stove
goes up through the shaft up to the top of the chimney. How do we
deal with this.
The guy who owned the house before me stuffed the hole with
fiberglass. Not the best solution, but as long as there were no sparks
not a big problem either. Now we want to seal the chimney shaft but
allow the stove pipe to go through it.
The solution is to make a piece of sheet metal that will fit the opening but has
flanges on the sides so that it can be attached to the sides of the chimney. This
then will be filled with a high temperature silicone caulk.
The sheet metal dam will have a hole in it to allow for the stove pipe to go
through it and on up the chimney.
I am still considering if I want to do this myself or if I want to hire someone to do
it. I have contacted a local chimney and stove expert to come and give me a
quote. More on this later!
Looking up the chimney at the wads of stuffed fiberglass that
were put there by the previous owner to 'air seal' the chimney.
The next blog will show the strapping being brought onto the outside of the
house for the rain screen as well as the building and mounting of the window