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Old 03-23-2013, 12:51 PM   #1
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Sealing around exterior furnace vent pipes


I had a brand new high efficiency furnace installed in my home. It's a 2 stage furnace and the installers put in 2 pvc pipes that go directly outdoors. There's a small gap of about 1/8" around each pipe.

They sealed around the pvc pipes outside with silicone. They just ran silicone around the pvc on the surface of the siding. Is this enough? I live in the Midwest with brutal winters/summers.

I just installed a sink a couple weeks ago and have some extra silicone left over. Can I put a bead of silicone around the pipes on the inside of my home (in my basement)? I thought this might be double the insurance for air/rain leaks. Also thought about buying some expanding foam. Any thoughts?

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Old 03-23-2013, 01:12 PM   #2
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Sealing around exterior furnace vent pipes


silicone is standard and what we use but unfortunately it can shrink eventually and dry out eventually and need replacing. low expanding foam inside is a good idea.

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Old 03-23-2013, 04:18 PM   #3
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Sealing around exterior furnace vent pipes


That is all I have around mine. I check it every year before fall, and fix any spots around the outside that the caulk has fallen off, or shrunk.
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Old 03-24-2013, 10:43 AM   #4
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Sealing around exterior furnace vent pipes


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Originally Posted by addfsd View Post
I had a brand new high efficiency furnace installed in my home. It's a 2 stage furnace and the installers put in 2 pvc pipes that go directly outdoors. There's a small gap of about 1/8" around each pipe.

They sealed around the pvc pipes outside with silicone. They just ran silicone around the pvc on the surface of the siding. Is this enough? I live in the Midwest with brutal winters/summers.

I just installed a sink a couple weeks ago and have some extra silicone left over. Can I put a bead of silicone around the pipes on the inside of my home (in my basement)? I thought this might be double the insurance for air/rain leaks. Also thought about buying some expanding foam. Any thoughts?



I would silicone them (space allowing) and add insulation around them.
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Old 03-24-2013, 11:33 AM   #5
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Sealing around exterior furnace vent pipes


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I would silicone them (space allowing) and add insulation around them.
If the hole was cut properly sized, there should not be any large gap around the piping. Silicone should be enough on the outside, inside, either batt insulation or XPS, or Closed Cell foam.

I preferred to use R-13 Batts in my basement when I did it, due to I still need some breathing of the structure, because we need makeup air for the water heater, even though power exhaust vent, and for the clothes dryer.

Of course I could use the old chimney for make-up, but she is coming down in April, and then I start getting ready for the walls to be insulated, and the house air sealed in the living area & attic.
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Old 03-24-2013, 01:02 PM   #6
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Sealing around exterior furnace vent pipes


You're right, I crawed into the space to take a closer look and there is no way expanding foam is going to get in that tiny perimeter space. I'll take all yall's advice to silicone then insulate.
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Old 03-24-2013, 01:13 PM   #7
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Sealing around exterior furnace vent pipes


Use Diversigum on the outside. Use the spray foam for the area between out and in.

Diversitech Diversigum Sealing Compound

Non-hardening. For plugging openings through foundations or around refrigeration tube entry.
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Old 03-24-2013, 01:36 PM   #8
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Sealing around exterior furnace vent pipes


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You're right, I crawed into the space to take a closer look and there is no way expanding foam is going to get in that tiny perimeter space. I'll take all yall's advice to silicone then insulate.
Is there anything in the sill & joist bays to insulate the plate from outside temp convection? It is quick and dirty to do the Kraft faced batt insulation, when you need to get 'er done. I have a spot that I could not tuck the batt into enough, so still have a blast of cold air coming out at that spot. Cannot wait until I can get the company in that I plan on using, to seal with Closed Cell around the perimeter of the basement.

Then I can go and place XPS on the walls below grade to help further insulate the basement, so that it is not a 6-12 degree difference. Humidity stays about the same as upstairs, which is right now 49%, which helps also.
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Old 03-24-2013, 01:38 PM   #9
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Sealing around exterior furnace vent pipes


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Use Diversigum on the outside. Use the spray foam for the area between out and in.

Diversitech Diversigum Sealing Compound

Non-hardening. For plugging openings through foundations or around refrigeration tube entry.
I just ended up using Crack foam from Big stuff around my a/c lines, where they come through the foundation. Found out that the Dap foam actually reacts with Copper, so had to clean it off and use the other.

BTW, we used to call Diversigum "Monkey ****" in the Navy.
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Old 03-24-2013, 01:42 PM   #10
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Sealing around exterior furnace vent pipes


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BTW, we used to call Diversigum "Monkey ****" in the Navy.
Monkey "excrement"??

Because of its look? Do explain.
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Old 03-24-2013, 02:08 PM   #11
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Sealing around exterior furnace vent pipes


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Monkey "excrement"??

Because of its look? Do explain.
Yep, but it was the whitish/gray colored in a can that we used to pack any openings for watertightness, and to stop any fire spread if a fire broke out in that space.

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