insulating a finished attic
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rostou



Joined: 16 Feb 2015
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Location: Portland, Maine

PostPosted: Thu Feb 19, 2015 6:31 pm    Post subject: insulating a finished attic Reply with quote

I am finishing a gutted room in what was previously a finished attic. I realize that ideally the attic is not used as living space, but there is not really an option for us at this point. I have posted a few pictures of the room (I apologize for the low quality of the images). The house was built in approximately 1886 and is a balloon-framed two story (not including the attic). I have some questions related to insulation, which I am realizing is a complex topic in a finished attic.

1. I intend to build a free-standing inner frame that will support stone wool insulation the wall finish (wood). This will not only allow the rafter bays to remain open, but will even allow for a few inches of space below the rafters. Also, the area above the collar ties will all be open and uninsulated (collar-ties.jpg). Given this amount of air space, is it necessary to attempt to achieve some sort of outside venting? Venting is problematic as the rafters do not end at the soffits, but rather rest on a board laid across the end of the floor joists (top-plate.jpg). There is therefore no direct communication with the eaves and no simple way to ventilate through soffit vents (which I would have to install). I was considering introducing ventilation by drilling some holes down through the board that the rafters rest on, although I am concerned that this could in some way weaken or disturb the board. Do I need to be concerned with adding some sort of ventilation to this system? If so, would a few one inch holes at the base of each rafter bay be sufficient? Would this help keep the roof system dry, or would it have no effect, or could it even have a detrimental effect by keeping the area at the edge of the roof colder than the area nearer the top in the winter (since there are not vents up high), thus exacerbating the temperature differential and increasing the likelihood of ice dams? My feeling is that I would be fine without introducing direct venting if I build in the air space described above. I don't want to fall prey to the ventillation frenzy, which is probably mostly driven by industry propaganda, but at the same time I realize that I am not using the attic for its intended function and want to do everything I can to protect the structure of the building given our constraints.

2. Even though there is a stairway connecting the second floor of the house with the finished attic and therefore a large corridor for heat to rise, I am considering insulating the space between the ceiling of the second floor and the floor of the finished attic with stone wool insulation. Due to the limitations of installing insulation under roof the in the finished attic, I thought that this might help slow the heat rising up through the house at least. I have seen the recommendation on this forum to insulate horizontally but not vertically, that insulating wall cavities can increase the possibility of moisture buildup. Is moisture in insulated spaces between floors less of an issue than in the case of walls because, unlike walls, the surfaces of the wood are not cooled by the outside air, so that condensation is less of a factor? Can I freely insulate between floor joists without being concerned with moisture being trapped by the insulation? Would there not be a possibliity of condensation near the point where the joists meet the top of the wall?

Many thanks for any thoughts on any of my many questions!



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johnleeke
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Joined: 20 Aug 2004
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Location: Portland, Maine, USA

PostPosted: Wed Feb 25, 2015 9:45 am    Post subject: Reply with quote

Welcome to the Forum!

1.
Constructing your attic room with inches of space above and around is good.

Just from the photo it's difficult to know enough details of how the wall, attic floor and roof meet. It looks like you might be able to remove one more floor board and see more details. Can you provide a cross-section sketch of this? Here is an example of a cross-section sketch (upper left):


2.
Quote:
Is moisture in insulated spaces between floors less of an issue than in the case of walls because, unlike walls, the surfaces of the wood are not cooled by the outside air, so that condensation is less of a factor?


Yes.

The two key points on insulating between the floor joists:

--You have to do something to effective limit air flow through this floor structure. I have used "dense pack" cellulose insulation. "Dense pack" means at least 3 pounds of insulation per cubic foot. I do not have any experience with stone wool insulation and do not know if it can be dense packed.
There may be other ways to get an effective air retarder in your situation, but it seems unlikely in a house of this era.

--The insulation must be detailed effectively where it meets the wall and the roof structure meet. Here again, we need more details on how this area is constructed.

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rostou



Joined: 16 Feb 2015
Posts: 4
Location: Portland, Maine

PostPosted: Thu Feb 26, 2015 7:33 pm    Post subject: Reply with quote

Thanks John. I'm attaching a cross section of the eaves. Let me know if it's still not clear.

I was actually in the process of putting boards between the joists and on top of the wall to close up the large space that leads directly out the the soffits, visible in the picture in my initial post.

The Roxul site claims that stone wool insulation "effectively reduces airflow." I was planning to use batts between the joists that are thicker than the joists themselves so that they would be slightly compressed. I am thinking that this may not reduce airflow to the degree that you are suggesting is necessary? What if I placed a board or perhaps foil insulation at regular intervals between the joists and possibly foil insulation under the floor above the joists and over the ceiling below them? One reason I like the stone wool is that flame retardants are not applied to it whereas I believe they are in the case of cellulose. Also I think the stone wool batts would be much cheaper and easier to install.

If my approach would not effectively limit air flow, I suppose it would be better to not insulate between the joists at all? Also, does this same guideline of restricting airflow also apply to the insulation I would install under the roof, in my secondary framing?

I can post additional images if that would be helpful. The board in the picture cannot be removed because it extends under the board that the rafters rest on.



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johnleeke
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PostPosted: Sat Feb 28, 2015 3:47 pm    Post subject: Reply with quote

Quote:
Let me know if it's still not clear.


Your sketch is a start. We need to know about the hollow spaces inside the wall and above the soffit board. In particular, is there a connection between the spaces above the soffit board, within the wall and the space between the joists.

Can you post a photo of the exterior, like in the drawing above?

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rostou



Joined: 16 Feb 2015
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Location: Portland, Maine

PostPosted: Sat Feb 28, 2015 4:44 pm    Post subject: Reply with quote

Attached is a view of the eaves from the exterior and a labeled view from inside.

The joists rest on the top plate of the wall, and there is therefore no connection between the space inside the wall and the space between the joists or above the soffit. However, because the joists extend beyond the top plate, the space between the joists does connect with the space above the soffit, as seen in the picture.



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johnleeke
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PostPosted: Sun Mar 01, 2015 12:08 pm    Post subject: Reply with quote

OK, now I've more of situation in mind. Here are some thoughts for you to consider. I'm listing them in the order they might be done.

1. Examine all of the wood from inside to determine if there is any wood decay or deterioration of fasteners. Make a measured cross-section drawing accurately showing the position and thickness of all the boards.

2. Treat all of the surfaces of the soffit, facia, rafter tails, joist ends, roof deck boards, and new cleats, with a migrating borate preservative (such as BoraCare).

3. Continue with your plan to install a solid wood board to separate the space above the soffit and the space between the joists. Give the board a tight fit along it's top and bottom edges. It looks like your new cleat is installed in the right place to position the board correctly, it should probably be in the same plane as the exterior wall sheathing. This will continue your insulation over the top of the wall, and keep it out of the space above the soffit.

4. Treat the new board with borate preservative after it is installed, along with the top 2x4 of the plate and the lower two or three roof deck boards.

5. Now, or at any time in the future, you could ventilate the space above the soffit. You could add a standard continuous soffit vent product at the outer edge of the soffit, or create a custom vent by cutting off the outer 3/4" edge of the board and installing a roll of insect screening. This would provide at lease some ventilation, more would be better.
A typical attic ventilation scheme is to vent low and high so warming air picks up moisture and carries it up and outside. A common low vent is a continuous soffit vent, along the outer edge of the soffit board. A high vent could be gable end vents in the peak of the attic walls, or a continuous ridge vent. It would be necessary to connect the space above the soffit and the large attic space, probably by drilling holes in the attic floor board, right where the second floor board steps up.
This gets to your original question of whether this would weaken the boards too much. To actually answer this question I'd have to be there in person, but here are the issues: a. Is it possible to drill holes through the floor boards to connect the spaces? (examine the construction and consider your skills, tools, and the detailed measured cross-section drawing) b. Is the top floor board actually a part of the roof structural system? If so you would not want to drill holes in it. c. What is the nailing scheme and condition of the nails and wood in this whole area?

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rostou



Joined: 16 Feb 2015
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PostPosted: Sun Mar 01, 2015 7:24 pm    Post subject: Reply with quote

Many thanks for your detailed recommendations John. I will follow through and update this thread with any progress, which is inevitably slow in my case! I have also considered installing the board closing off the space above the soffit at a 45 degree angle so that the top of it would extend beyond the edge of the lower floorboard, thus allowing for a gap that would ventilate the underside of the roof. I am thinking that this would not be the best approach, however, as it would eat into the space for insulation between the joists and would greatly increase the possibility of leakage of warm air from the living area below into the area under the roof. I believe it would be better to drill about four one-inch holes between each set of rafters in the lower floor board, the one that extends under the board that the rafters actually rest on. There is enough room for this between the edge of the top board and the point where the board I've installed is. I can then affix screen over the top of these holes. Now if only I can get my hands on a right-angle drill...

When it comes time to add some vents to the soffit, I may drill one inch holes at regular intervals in the board and put screen on the inside while still on the ground. This should make installation up on the ladder easy. I've never seen such a "shotgun" soffit but I don't see why it wouldn't work.

Gable vents may have to wait a while as the peak of our roof is very high and close to power lines, so installation may be tricky.
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BenS



Joined: 08 May 2018
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Location: Victoria, British Columbia

PostPosted: Thu Oct 25, 2018 6:23 pm    Post subject: Ventilation along frieze boards? Reply with quote

John, you mentioned that ventilation from a slot cut in the soffit frieze board would help this situation. I have a frieze board that leaves a 1/4” to 1/2” gap where it meets the underside of the roof deck (exposed eaves) and I’m not sure if this was designed ventilation, sloppy work, or movement over 100 years of the house’s life.

Was this a design consideration 100 years ago and, if so, given that I have no gable vents, should some ridge or roof deck vents be added?

Finally, how is insect screening approached with this design? It will be very difficult to access the interior but I imagine adding a screen to the exterior of the frieze/eave intersection would require additional trim to secure it and hide the fasteners, thus changing the trim appearance.



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