Chimney insulation

[jharkin]

Hi all...

I think this has been discussed before but I cant find too much with search so...

Anyway I posted in other threads about the massive insulation and air sealing upgrade we did this year - densepack cellulose in our walls, ceilings and sloped roof, rigid foam on the kneewalls etc. So far its made a big difference on the bills and judging by the lack of snow melt looks like its going to fix the ice damming in front.

Problem is, snow is still meting on the back addition roof over the small attic. Right now there is not enough snow to cause and issue but it tells me I'm still putting too much heat into that attic. It is well ventilated. Spending some time in there with the IR temp gun I think the issue is the chimney for the stove which passes through this attic. Its a full steel liner running inside the masonry flue but it still wams it. Temp gun reads the outside of the brick at 72F while the stove is cranking. Floor to the room below reads about 45-50.

So I think this warm chimney is the culprit.

I'm thinking I should wrap it in insulation, and see if that makes a difference. I believe its code legal to wrap it in Roxul, correct?

Just looking for thoughts from the experts before I do this...

 

[begreen]

Is the liner insulated?

 

[jharkin]

Good question BeGreen... Its an existing install that was here when we bought the place.. All I know for sure is that the blockoff plate is insulated and the install passed inspection (I have the FD report).

Most likely its not insulation, and being a 200 year old chimney who knows what the inside of that flue looks like... or how much space there is for insulation. When the weather gets better I could go up on the roof and see if I can take the top plate off to look down there. If its not insulated what are my options, can I pour perlite down the flue around the liner ?

Guess I need to educate myself on the codes and what is allowed in this case.

 

[Frozen Canuck]

In addition it would be wise to verify the manufacturer of the chimney & check with them as to whether or not it is rated to be site insulated (many were not), no point voiding your home fire ins policy.

 

[jharkin]

I wonder if I could even figure out the liner manufacturer... Inspection statement from when the current stove was put in (have to double check, think it was around '98) states that the liner was existing from the prior stove install... could be 20+ years old.

 

[semipro]

I agree that insulating the liner would be preferable but only if doing so won't cause other problems. However, if the flue size is close to that of the liner you may not be able to get much insulation (e.g., perlite) in there anyway so the effect may be limited.
Personally I'd be wary of applying any insulation directly to the masonry though Roxul sounds like a great application.
Another possible way to handle might be to build a sealed foam board (XPS or polyiso) box around, but not in contact with the chimney?

 

[Black Jaque Janaviac]

looking at that photo and reading that the construction is 200 years old you may want to put some thought into whether that roof can handle a snow-load.

It looks like the rafters are only 2x4 and spaced 24" apart or better. 200 years ago this may have been enough to handle whatever snowstorms mother nature could dish out because the snow would melt away in between snowstorms. Today we insulate the attic so well that the snow continues to accumulate from one blizzard to the next. The builders of the roof might not have anticipated that much snow.

Is the roof vented? Allowing some of that heat to escape through a ridge vent or other vent near the apex could be all you need to stop the ice damming. Of course in order to allow the heat to escape, you'll also need soffit vents to let cool air in.

 

[BrotherBart]

The first order of business is to find out what is actually inside that chimney and then proceed from there.

 

[valley ranch]

Good observation Spider. What type of roofing, metal?

I would put on a metal roof, than you can insulate the heck out of it.

 

[jharkin]

Appreciate the thoughts. Fr now I did wrap the chimney in $10 of Roxul as I had it available. It made a big difference. It is code approved for direct contact to chimneys - in fact the professional insulation crew that insulated this attic used it to dam around the chimney before they blew in cellulose. So far no issues, but I will definitely go up on the roof and pull the top plate to take a look come spring.

To the questions and comments posed....

- Draft is not a problem. If anything I get too much draft on cold days...

- I understand the creosote concern but this cat stove burns very clean with the setup as is. I inspect monthly and brush once a year and never get more than a coffee can of soot from 2 cords burning.

- This attic is smaller than it probably looks in the photos - this is over the back addition and its less than 5ft high at the ridge. The floor of the attic is sealed and insulated (7-8in of cellulose). It is ventilated - there is a continuous ridge vent, however the soffit vents aren't the best - just the small round ones every couple feet. Might add some more in the spring.

- Snow loads should not be an issue. Last winter we had 3 ft of snow on that roof without problem. Note that its got a good pitch to it. The idea that old houses are less study than modern construction is a common misconception. In fact the opposite is true. These old houses are built like tanks. Those rafters you see are actually 3.5x3in true (not nominal) dimension, cut from solid oak and step lap jointed into the top plates. Those top plates and the crossbeams trying them together are 6x7 (true) oak and sit on 6x6 oak vertical posts. Just one of those posts could carry 20 tons and there are 10 of them supporting the second floor of the main house and addition. The roof on the main house is even stronger, similar construction with 3x5 and a few 5x5 oak rafters, probably mortised together at the top. I'd bet real money this roof just as strong as a modern roof.

Its going a bit off on a tangent but its also a misconception about the snow melting off in the old days. In the Victorian era when they had a stove in every room that would indeed be the case but back in the 1700s with open fireplaces about the only room that would have had a fire going continuously is the kitchen. The rest of the house would have been an ice box excepting the occasional evening bedroom fire to heat up the bed warmers. The roof probably stayed ice cold all winter.

- To answer the last question - roofing is asphalt shingle. Id never put metal on, it would not look right at all. The only thing besides that shingles Id consider is real cedar shake but its just too expensive. NO need to insulate hte roof since I just put so much effort into insulating the attic floor.

 

[valley ranch]

Hope that answers your problem. As for me I love metal roofs they end many problems. Have a great day.

 

[dave_dj1]

I know this is a little old but when you say the attic is vented, how so?
If it's just the little ridge vent it should have matching soffet vents and even then they aren't enough! You need a gable vent or two, you want the attic the same temp as the outside air in winter and it will help keep your home cooler in the summer also.
dave