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Old 03-22-2018, 11:23 AM   #1
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How to Insulate an un-finished attic and prevent Ice-Dams in a 1920s 2-family MA Home


Hi everyone!

I have a 1920s home with an unfinished attic here in snowey Massachusetts. The attic ceiling is un-insulated, with exposed rafters and an asphalt tile roof. I was warned about ice-dams and how improper insulation will lead to a destroyed roof. The attic space is currently openly vented through two vents one at each end of the attic. The problem is I have no idea how to prevent ice-dams -- this is something new to me.

With the wisdom of the community, how can I set up my insulation propperly to prevent icedams?
  1. Is spray on closed-cell foam my best bet?
  2. Do i need to cut soffit into the attic roofline? Is there something else I should do
  3. Any other tips or tricks?
Thank you everyone in advance for the input!
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Old 03-22-2018, 11:24 AM   #2
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Re: How to Insulate an un-finished attic and prevent Ice-Dams in a 1920s 2-family MA


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Old 03-23-2018, 11:10 AM   #3
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Re: How to Insulate an un-finished attic and prevent Ice-Dams in a 1920s 2-family MA


Appreciate in advance the input.
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Old 03-23-2018, 11:19 AM   #4
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Re: How to Insulate an un-finished attic and prevent Ice-Dams in a 1920s 2-family MA


Ice dams are caused by heat leaking from the top of the wall warming the insulation and the roof right there. Soffet vents to an air gap between the insulation and the roof sheeting will allow air in but it has to have a way out to. box vents or ridge vents for that.
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Old 03-23-2018, 11:41 AM   #5
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Re: How to Insulate an un-finished attic and prevent Ice-Dams in a 1920s 2-family MA


What is it that you are trying to accomplish, in an overall sense?

Just more insulation for energy savings or do you want to use the attic as new living space sometime?

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Old 03-23-2018, 11:49 AM   #6
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Re: How to Insulate an un-finished attic and prevent Ice-Dams in a 1920s 2-family MA


Difficult question. I've been a few of those old attics up here in Maine and they don't always have the problems one might expect.

One disadvantage is we/you don't know exactly how the current roof is performing, or do you, first hand I mean?

With the floor covered as we see in the picture it tells me you don't have enough insulation in the attic floor and you don't have access to increase it and do the proper air sealing. I looked up the 09 energy codes for MA and they show R-38 for the ceiling. That may have increased depending upon which generation of codes your local authority is following. But at R-38 you would need about 11 inches of insulation and you wouldn't want the flooring over the top of it, a condensation issue.

Sounds like you would also want more ventilation.

Then there is the unvented attic approach, but that has its challenges as well.

Let's see what ideas others have.
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Old 03-25-2018, 01:14 PM   #7
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Re: How to Insulate an un-finished attic and prevent Ice-Dams in a 1920s 2-family MA


Air sealing is more important than insulation, if you have a lot of air leaking from the house, the insulation won't do it's job.

I see two potential options:

1. Remove the floor boards, remove existing insulation, air seal or better yet have a thin layer of foam sprayed to create a full thermal break.

Install baffles to not block the soffits vents, blow insulation over top to R50.

Install roof vents

2. Leave existing insulation and floor boards intact.

Close the soffit vents off, spray foam the roof deck.

My reservation about this option is that your roof deck boards don't appear to be in good shape; they have large gaps. When the roof is re-done, foam adhered to tar paper or whatever that black stuff is could come off as they remove shingles.
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Old 03-26-2018, 03:39 PM   #8
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Re: How to Insulate an un-finished attic and prevent Ice-Dams in a 1920s 2-family MA


Really appreciate everyone's input. I should have included more info in the original post!

Quote:
Originally Posted by AndyGump View Post
What is it that you are trying to accomplish, in an overall sense?
We are trying to add two bedrooms and a bathroom to the [currently] un-finished attic space.

Quote:
Originally Posted by Bud9051 View Post
we/you don't know exactly how the current roof is performing, or do you, first hand I mean?
Bud
The roof was re-done in 2015 -- new 30-year asphalt shingles etc. No leaks or evidence of leaks.

So it sounds like we should take up the current floor boards to re-insulate the ceiling. And then we should use spray on foam for the attic. Thoughts on this?
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Old 03-26-2018, 03:43 PM   #9
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Re: How to Insulate an un-finished attic and prevent Ice-Dams in a 1920s 2-family MA


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Originally Posted by user_12345a View Post
2. Leave existing insulation and floor boards intact.
Close the soffit vents off, spray foam the roof deck.
This is great input. I am leaning towards option #2. it is possible I do not know what the word "soffit" means, but to my understanding, I do not have any soffit vents?
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Old 03-26-2018, 04:09 PM   #10
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Re: How to Insulate an un-finished attic and prevent Ice-Dams in a 1920s 2-family MA


Quote:
Originally Posted by vr4 View Post

We are trying to add two bedrooms and a bathroom to the [currently] un-finished attic space.


Extremely important detail. What you will essentially be doing is creating short sections of cathedral ceilings. With your existing (looks like 2 x 6) rafters the depth is too shallow for a air space and fiberglass. Research cathedral ceilings and knee wall insulation. You are going to want a good ridge vent all the across and also a continuous low intake vent. If no soffits there are some other ways to get it done. A pic or two from the out side would help.
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Old 03-28-2018, 01:12 PM   #11
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Re: How to Insulate an un-finished attic and prevent Ice-Dams in a 1920s 2-family MA


Cathedral ceiling = good candidate for foam. Roofing material tends to be non-permeable so there's no drying potential. (which may be why they still try to ventilate cathedral ceilings)

Foam is a complete thermal and moisture break.

This is soffit vent:


It's needed to allow fresh air in from outside to replace the air expelled by roof vents.
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