We just moved in a month ago and now this.  I would like to get as much help as possible for the SOLUTION and a permanent fix.  All help and insight is appreciated. Below the narrative is a pic i drew up showing a section of the chase in question.

PROBLEM: Upper level of the chimney chase (top) is completely saturated on the inside. Water had frozen due to recent COLD COLD temps and Yesterday was almost 40 so ice was melting. Sheathing is soaked, stainless cap has condensation, studs are all wet. Black spots starting on sheathing.

DISCOVERED BY: Water started dripping onto the brick from the OSB LID at the main level DETAILS ABOUT THE PARTS OF THE PUZZLE: Wood Stove is not installed and there is no pass through for the FLU at the top of the chimney chase. The Stainless cap that is installed at the top of the chase does not have a hole cut in it yet. Main level walls in alcove are all spray foam (open cell) walls with drywall and brick on the inside Flue passes through a CODE REQUIRED “Lid” which is ¾ OSB screwed to nailers I framed in. Second Story part of the Chimney Chase is also insulated the same as the main level with no drywall inside the chase. Same type “OSB Lid” installed at the second story to Attic pass through Attic level of the Chimney Chase is un-insulated and not vented anywhere, Sheathing separates it from the attic.

WHAT DID I DO WHEN I DISCOVERED THE DRIPPING WATER: I removed the lower level “3/4 OSB LID” and discovered water raining down from the Second story “LID” 9 ft above when I was. I went into the attic so I was above that second story “LID” and cut 2 holes about 2ft x 3ft through the sheathing separating the Chimney Chase from the Attic and discovered the damage and the saturated upper chimney chase cavity. I left those vents I cut OPEN in an effort to allow some air to flow I also left the main level LID out so the warm air from the house is flowing freely up that chase and hopefully it’ll dry out the upper level. I plan to put a fan in the attic blowing into the upper chase to dry it out quicker.

POSSIBLE CAUSES FOR CONDENSATION AND SATURSATION AT UPPER LEVEL: Lids were not AIRTIGHT, FLU pass through at main level was sealed with fire caulk but not second story pass through. Additionally, neither of the “3/4 OSB LIDS” were air sealed around the perimeter. Flu itself was not airsealed at the bottom. All of the above have been allowing WARM air to migrate up the Chimney chase consistently and with the upper level being un-insulated and trapping warm air inside the condensation began and continued.
The question NOW is. HOW DO I PUT IT BACK TOGETHER AND PREVENT THIS FROM HAPPENING AGAIN????

Question: Do I use the same design but ensure both lids are AIRSEALED on the perimeter as well as on the Flu?

Question: Do I only seal the main level LID to prevent air from escaping to the chase and then allow the other two levels of the Chase to vent together into the attic?

Question: Do I re-seal both lids and allow ONLY the upper portion of the chase to vent into the attic?

Sorry to hear about this Easyrider :-(

As you already know, these problems result from water vapor making its way to a building assembly condensation layer that is at or below the dew point temp. So in simple terms, for most building assemblies, there only three solutions to prevent this from occurring: 1) change the permeance of the assembly layers such that water vapor can’t take this journey, 2) change R-values of the assembly layers such that the condensation assembly layer is either eliminated or moved to a new location where condensation is acceptable, or 3) some combination of 1) and 2). We have a building assembly moisture analysis software that you may find useful in understanding and resolving this problem:

Borst Building Assembly Moisture Analysis Software

For this building assembly, there is even a fouth solution to prevent this, increase the temp in the cavity (e.g., by using the woodstove or via some other way) so as to increase the temp of the assembly layers to be higher than the dew point temp. I suspect that solution 1) would be challenging to successfully accomplish long-term.

Thanks Sail Away

Anyone Else?

Easy - If I understand what you say and read your drawing correctly, I would say that your assumptions are correct as to the cause.

For the solution, I would insulate the second storey "lid" (firestop) to the same level as the outside walls with Roxul and then fire caulk all joints to make it air tight. Now the complete chase is heated up to the attic level.

Leave the "holes" into the attic to vent the upper portion of the chase if the code in your area allows this. If not, install two small vents from the upper chase to the outside.
This will allow any moisture to dry out of the "attic" portion of the chase without causing any damage.

Incidentally in our area the code would require the fire stop to be insulated to attic levels and a vapour barrier placed on the warm side of the insulation.

Ok, so I have sealed off the second story LID or FIRESTOP with fire caulk and foam. Minimal leakage if any into the upper level of the chase. I also left the openings I cut into the Attic to get some air flow but the chase is STILL freezing up and has lots of moisture on the walls in that upper level!

Do I have to have that vapor barrier on the warm side of the insulation? How do I do that? Closed cell foam sprayed against the OSB to a thickness that matches the R-value of the walls in the chase? How close can it be to the flu? Will the area left un-insulated around the FLU cause problems still?

What if I were just to insulate the upper level of the chase the same as I did the lower two levels? Meaning insulate the outter walls and the floor....would that keep this from destroying the sheathing and making moisture if it were warmer in there? This is ruining the exterior sheathing of the house.

The amount of moisture moved by vapor diffusion is miniscule. In most homes south of US climate zone 6 vapor barriers create more problems than they solve.  Vapor diffusion is a distant third in terms of moisture migration.

Continue to concentrate on air sealing between the conditioned space and attic (the second most likely offender) but even more importantly take a hard look at sources of bulk-moisture intrusion, the #1 source of moisture problems.

Improperly lapped flashing , unsealed masonry above the roof deck, and masonry chimneys that do not have metal caps to divert the rain can all lead to high moisture levels in brick further below.



Insulating the portion of the chimney in side the attic with rock wool is safe, and needs no air barrier to be effective.  That raises the temperature of the brick which may keep it from building up ice, but it doesn't necessarily fix the problem if there is still lots of moisture coming in from above.

Thanks DANA. Trouble is I do not have a masonry chimney and the flu is not connected to a rain cap yet. Somehow the warm air is still getting up there and heating up that upper part of the chase and causing the problem. I am leaning more towards just insulating that upper part of the chase with ruxol like you said and sealing off the lower portion really well with the firestop.

If you have sealed the second floor ceiling to attic fire stop AND the flue itself has been sealed (tie a plastic bag over the bottom) then as Dana suggests, there has to be water leaking in. It might be blowing in as snow and melting.
If there is just minimal amounts of air infiltration from the second floor, any moisture should be diluted with the attic air and/or you should be seeing icing in the near attic area as well.

The chase is supposed to be sealed off from the attic so the only reason it's diluted is because I cut some access holes through the sheathing to allow that to happen. I am feeling air coming in through the cracks in the sheathing and seams in the chase framing so there is still plenty of air moving around in that upper chase area. I too am concerned that there is some moisture getting behind the siding on the outer corner so I will investigate that when it warms up. For now my plan is to minimize the moisture by sealing off the lower level again with another lid/firestop so that the moist warm air will be stopped even further away from the floor of the upper chase cavity.
Ultimately I will insulate those outer walls in the top level with ROXUL and hope it will control the temp better.

Easy - What I meant by diluted is that with the attic air circulating into the chase, it will move most of the moist infiltration air out of the chase before it can condense. This is a good thing! If you are feeling air move up there, there is no way this is a condensate problem unless the flue is wide open and pumping warm moist air against the tin chimney cap. If the flue is tightly closed, don't waste any more time air sealing the fire stops, you have bulk water or snow entering the top of the chase and you need to find it. Next snow storm or rain fall, crawl up there and watch. Look for tracks in the dust and try to find the highest occurrence of markings. Yes, with that amount of moisture, it will condensate on the tin cap because it is the coldest surface up there so you might be getting some tracks from that but the start is with the water entering from outside.
Is the chimney presenting a flat section of wall against a sloped roof? Is there a cricket missing to divert that water?

FBBP....sounds great and that's what I did yesterday. I think I have some wind driven rain/snow getting behind the vinyl siding on the outboard edge of the chase. The wind can get really strong where the house is and that corner is the North West Corner where the majority of the wind comes from this time of year.

The sheathing above the roof line is wet from the moisture as well but it's not wet at all below the seam so the flashing there is working well. The two outboard corners appear to be the issue right now but I will continue to troubleshoot.

I am not sure what a cricket missing is?

Posted By easyrider470 on 26 Feb 2015 07:57 AM
Thanks DANA. Trouble is I do not have a masonry chimney and the flu is not connected to a rain cap yet. Somehow the warm air is still getting up there and heating up that upper part of the chase and causing the problem. I am leaning more towards just insulating that upper part of the chase with ruxol like you said and sealing off the lower portion really well with the firestop.

Describe the chimney type, and how it is weather-flashed where it passes through the roof deck.  Odds are pretty good that this is a case of mis-installed flashing at the roof penetration allowing moisture to seep in.

A cricket is a large custom piece of flashing used for diverting bulk water around the flat up-slope side of masonry chimneys (like the chimney you DON'T have.)

Ok thanks I see it now.
My chimney chase is completely outside the roof line so I have a 10/12 pitch roof up against the flat side of the chimney. Does that make sense? When I framed it we built sections and installed them on top of each other to make the chase. These are completely exterior of the gable end of the roof trusses. I'd say roughly there is about 3 ft of flashing on both front and back slopes of the roof where the roofing touches the flat side of the chase.

Okay, if you are sure that it's not the roof intersection, the next most likely spot would be the chimney cap to siding intersection. Normally we put a 2x2 all the way around the top of the chase and then have the turn down of the cap drop one inch below this point. This allows the siding to tuck up under the cap and the drip hem of the cap would push the water out and over the siding. Sometime the turn down of the cap is not long enough and the water curls back to the chase and gets behind the siding. Also make sure the corner trim is tucked under the cap turn down.

If you are not sure if the water is marking the sheathing, take a roll of toilet tissue up with you and tack it up around the top. It will show any water contacting with it. Move it down till you detect water and that should lead you to the leak.

Thanks FBBP I actually shook a bunch of talk powder on the sheathing yesterday to do exactly what you are saying. TP sounds like a great idea as well!
I am going to inspect the cap as soon as possible as I believe that is the primary issue. I will also inspect the flashing just to be sure but it appears to be working correctly from the inside!

Hi there. I am wondering if you reached a conclusion on this issue? I am looking at something very similar sounding (although in my case it is almost certain to be condensation-driven since there is no moisture under warm weather conditions). Same scenario where there is a chase constructed above the roof deck that ends up very moist in winter, suspected due to ingress of interior air up the gaps around the flue pipe -- debate as to whether ventilation of the chase void is required/advisable (currently it is unvented). Thanks.