Mansard Roof Insulation

[CR500]

Anyone think this would be a bad idea?

My house has a Mansard type roof. For those unfamiliar, the roof extends down (nearly vertical) completely covering the second floor walls.

This house is very cold and on the rare occasion that I heat to a normal temp (68 - 70), the furnace runs almost continuously. It is an old furnace which will be replaced next year. I added R30 unfaced to the existing R13 in the attic. While in the attic, I noticed there is no opening between the attic space and what would be eve's on a house with a normal roof.

From the outside of the house when looking up at the soffits (just above the first floor because of the Mansard) there are not any vents around the whole house. The attic space is vented with passive "mushroom" vents and one power vent. During the summer, the power vent runs all day.

I'm also putting on a new roof next year (probably something like Certainteed Grand Manor lifetime shingles) and will have ridge vents installed.

I cut of a section of the roof for some construction and can see part of the problem. The back side of the wall is completely open. There isn't any sheathing in direct contact with the back of the wall. Looks like they framed the exterior walls then pinched the wall insulation between the sheetrock and framing at the top and bottom of the wall. Then there is an air gap, then the roof sheathing. In the section of roof I cut open, if I look up at the second floor, I can see the insulation has billowed away from the wall allowing air to get around it.

Now here's my idea, what if I completely fill the area between the wall and vertical sections of roof with cellulose insulation (blown in). I could easily do it from the exterior when the new roof is going on. Since the house currently doesn't have any venting for this area I'd only be eliminating any naturally occurring air currents that might be running horizontally around the house within the space. Since this is outside the walls (rather than above the house), I don't think there's an issue with moisture.

The house (built in 1971) currently has two layers of shingles and the existing sheathing looks in good shape. I'm guessing that means it isn't getting too hot in this area and cooking the sheathing.

Attached is a sketch that shows the roof / attic. Also attached a photo of the roof (yes it is an ugly house).

What do you think?

Thanks

Rob

[AllanJ]

I would say yes, fill the space behind the mansard sections with insulation.

This produces the equivalent of:

1. A regular wall where the mansard is the exterior siding,
2. An attic with living space and insulation in the rafters instead of the floor.

Refinement: You do need attic venting. You could run ducts from the soffits up to the attic here and there, with soffit vents serving those ducts.

[CR500]

Hi Allan,

So there isn't an issue with packing in the insulation directly to the back of the sheathing?

Yup, know I need more venting up in the attic. One of these days I'll get up there and see if I can cut anything back to make room for a duct.

Thanks

Rob

[Wgoodrich]

The wording insulation backing is just what it sounds like. Draw the box of your home. Insulation is installed below grade on the crawl floor, then up the walls then across the ceiling making a complete box leaving no space exposed to outside ambient air to leak into the home. If you have 3 1/2" friction style insulation between the studs making the wall behind the mansord roof than your blanket is complete.

Picture your mansford roof being only an odd shaped soffit. You don't insulate the soffit area because the insulation blanket is installed between the room inside that the soffit being inside that outside framed wall. Same is with a mansford roof being only a glorified soffit.

Now you need more venting because a manford roof is designed to be uninsulaed same as a soffit with vents put in that horizontal area between the house wall and the bottom edge of the mansford roof then an air gap is to be maintained at the top of the wall between the top plate and the mansford roof / attic transition area to maintain a natural air flow for ventilation. Without the power vent but counting the bubble vents you should calcluate 1 square foot of roof vent for each 150 square feet of attic area. If you have a power vent then all you would need to do it install vents in teh horizontal soffit area at the bottom of the mansford roof. That mansford roof area is considered non heated same as an attic with the insulation blanket installed complete between the living area and the manford roof soffit area.

Do not insulate the manford roof, create intake venting instead as required per code.

Good Luck

Wg

[AllanJ]

It appears that the cross section halfway up the mansard, inside to outside, goes: paint, sheetrock, studs&insulation, empty space, roof sheathing, shingles. Unfortunately the existing insulation appears not to be firmly fastened to the studs.

Short of breaking down the sheetrock to restretch the insulation, there are only two choices, fill the empty space or don't fill the empty space (leave it as-is). If the empty space is filled, the R-value of the mansard roof area goes desirably way above the original R-13. Not filling the empty space leaves the R-value well below R-13 due to the billowing of the existing insulation.

At the points where ducts, perhaps 3x8 inches oval, are run vertically from the soffit to the attic, the cross section will go: paint, sheetrock, studs&insulation, sheet metal, empty space, sheet metal, perhaps more insulation, sheathing, shingles. If you use this method of ventilation there must be openings from behind the mansard sheathing at attic floor level, to the outside of the wall stud top plate, up into the attic, say between the rafters that frame the mansard. Should the roof sheathing come so close to the top plate (suggested by your drawing) leaving but a slit if any opening to the attic, a vent method not using the soffits may be better.

[CR500]

Thanks Allan / WG,

I haven't had the chance to get back up in the attic and examine the intersection of the top roof / walls / mansard side. If I remember correctly, back when I added the insulation in the attic there was no opening at all at the intersection.

I follow your explanation WG and it makes sense. The only problem is the poor construction of this house and the insulation bulging away from the studs creating an opening for the air to get around.

Allan, you also have it exactly correct!

[Wgoodrich]

You said you plan to re-roof. That is the time you should correct any flaws. When you strip the shingles also strip the decking and reach through to install such product as tyvek that can breath to hold the insulation inside the studs. Do it right or you will be doing it over. If you fill that mansford roof full of insulation you now have insulation against the back of the roof decking. This will hold moisture and rot your roof decking. Also it will close of the intake part of your venting needed in your attic. That power vent wasn't installed just for the fun of it. Your house has had a venting problem in the past inviting the power vent to help address that problem due to the improper design without the intake air vents under the mansford roof so air can enter the attic at the lowest point and exit through the bubble roof vents you spoke of. Air can not leave the attic unless air can get into the attic. Hot air rises. That is the principle of air intakes in teh soffit and air vents in teh roof as outlets of that air acting with the chimney affect.

Do your research before you proceed

good luck

Wg

[CR500]

Good point WG.

When the roof is off I can remove some of the decking around the house and install pipes for the soffit vents (just need to make sure I can get it into the attic). Don't think I want to remove enough to let me wrap the wall behind the mansard with Tyvec, but I may be able to put in some strapping to hold the insulation in the stud bays.

[AllanJ]

You might want to investigate now to plan on how to get vertical vent pipes through to the attic if they don't fit between the roof deck and the stud top plate. You probably don't want to be hurriedly figuring out how to reinforce the framing while the roofer is in the middle of his job if you end up cutting into the top plate every here and there to poke a pipe through.

If the original construction had just a slit into the attic going all the way around, that requires that the underside of the mansard down to the soffit be open all the way around also to achieve the needed square inches of venting area.

Would Tyvek (tm) and no roofing felt under the mansard's shingles provide adequate moisture escape in a situation with vertical ducts here and there and the rest of the space behind filled with insulation? (Perhaps use Grace Water Shield (tm) on the mansard deck on the lowest 3 feet to combat ice dams.)

[CR500]

I watched some of the houses in the neighborhood get new roofs. The speed at which it gets done is amazing. I don't think I can stop the crew for a week while I play around.

If I can't get vent pipes installed quickly (and if there aren't openings in the attic), I'll probably just do nothing. If the majority of mansard sheathing has to come off to fix the insulation and install Tyvec, then I'll convert the house to a standard colonial and brick the second floor. A contractor buddy laughs at the roof line when ever he sees it and he wanted to rip it off from day 1.

With the detached insulation in the mansard (I know there are even some empty stud bays), I don't want to let additional air blow up the walls. I'll follow WGs concerns about packing the mansard with insulation and rotted sheathing and I won't do anything. I'm going to save a bundle on the roof install and I'll just put that $ to running the funace.

I plan on installing as much ridge vent as possible. That won't let the air in at the low points, but combined with the power vent maybe it will help some. The house is 36 years old and sheathing is in great shape so it must not be too bad.

I covered the shed roof on the addition I just built with Ice & Water shield (100% coverage). They warn that the sheathing will rot unless there is proper ventilation with 100%. Interesting thought about using Tyvec on the roof.

[Wgoodrich]

I see no savings in removing the mansford framing all together. If you remove the mansford roof then you will need to install siding on the outside wall now holding hte insulation.

The mansford area is considered outside your insulation envelope. Not designed to insulated or packed shut. No vent piping required. ONly soffit vents install at the bottom of the mansford roof facing down.

Installing a ridge vent will not replace the soffitt vents and you will not increase your air flow. When air goes out it must come in. You want it to come in from the lowest area of the attic and venting out through the highest point of the attic.

Installing a ridge vent above your roof vents will only cause a flow of air from the roof bubble vent to the ridge not helping the rest of the attic at all.

Might want to rethink your plan

Wg

[AllanJ]

If not already positioned so, the passive mushroom vents could be repositioned near attic floor level and the power vent positioned near roof peak level. This is best done during a roofing job. A ridge vent will hinder rather than help a power vent although if a ridge vent is inadequate, you have no choice but to keep the power vent on in summer.

The mansard venting is a separate "vent circuit" according to your description. With no slits let alone big openings into the attic, the only escapes for any moisture that manages to migrates in winter from the living space outwards is via air swirling up and then back down to the soffit, and through the mansard sheathing.

[stepheva]

Guys, I'm new here and desperately looking for help. I am have been the president of my condo corp for the past year here in Canada, and need to find a solution. We have 40 units in our complex, 10 seperate buildings with 4 units in each one. These are row townhouses. They have mansard roofs. 3 of the buildings have low slope roofs on them. These units are about 30+ years old. Since I can recall, we have been having issues with ice jamming and water running into some units. They have replaced the mansard 2x in the past 15 years. The product they put on was a pvc 'shake' like material. Apparently not for this type of application. There is an aluminum cap at the top. Well, these roofs continue to leak on the outside walls. There is about a 2" gap leading into the attic. Our property managers have called in numerous experts, and they have tried everything from pumping insulation into the attics, to cutting venting that goes right around the unit. This venting is right where the mansard/attic opening of 2" would be. Well, no difference. On 2 problematic units, new water/ice shield was put down, but water still leaking in, and ice still forming terribly. All the outside walls in my condo on the top floor are freezing! I don't know that I have any venting into my attic. I popped the hatch open and couldn't feel a thing. Couldn't get in either due to the low slope and all the insulation. Anyone have any ideas???

[AllanJ]

Checklist:

Is there adequate venting that causes warmer air to rise and exit from the attic somewhere near the peak and colder air from outside to enter the attic from near attic floor level to make up for the warmer air that just exited?

Is there insulation to reduce the heat from the living spaces migrating up to the attic?

Is there enough insulation in the walls inside the mansard so make up air that enters soffits and must pass through there into the attic does not get heated by the living space?

Is there a waterproof layer such as Grace Water Shield (tm) on the roof deck under the shingles and going around the perimeter of the roof from the outline of the exterior walls inward/upward about three feet?

Refinements:

Install roof heating cables at the perimeter of the roof at the outline of the exterior walls (not at the bottom of the mansard). Include vertical cable sections here and there from the zig zag cable installation down to gutters or through downspouts. These will lay on top of the mansard shingles.

When the shingles are next replaced, install the waterproof layer on the roof deck going in six feet rather than just three.

Note: If tar paper is overlapped improperly on the roof deck, leaking due to ice dams will be greatly increased.

[Wgoodrich]

A few pics linked to this post would greatly help us help you. Many possibilities can exist that is causing your problem or problems. Pictures would be a great help.

Wg

[stepheva]

Thanks guys. Will post some pics asap. There is a question about heat loss in the walls (between the Mansard). I will answer some more tomorrow, as it is really late now.

Thanks