Hello pros.

Please see the attached pictures.

Any insights on how you would insulate the 2nd story walls of this house? The house is located in the suburbs of Washington D.C.



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As there are a few different types, impossible to answer without some pics & more specs - Patrick you need to either save the files locally (like on the desktop from your Google Mail program) or give everyone your email password - I would recommend the save locally & then upload : )

Pics are hopefully attached this time.

As for details\specs I have not as of yet. 

The homeowner reached out to me with concerns about other proposals he has received.

Other companies proposed filling the walls with blown-in fiberglass (from the inside). The foam insulators said such a wall (as part of a mansard roof) cannot be foamed.

My big question is whether there is wall sheathing and roof sheathing or just roof sheathing?




Well one would have to look into the wall/roof to see how it was done but yes both should technically be sheathed - 1 layer only would be typical (remember a roof is essentially just a wall on a different plane) though the little kickout / flare might be two layers depending on how it was done.

I also would not just use fiberglass for the "wall" portion & instead would treat anything with shingles on it like a roof. I am unsure why the foam applicators said it couldn't be foamed though that would require the drywall inside to come out to spray it properly. If that might be an issue, I would definitely look into dense packing cellulose in those cavities (not sure dense packing FG would eliminate convective current issues). With that the biggest issue is what does the local AHJ require / consider those areas & it is well worth checking with them on what specs you must meet

Thanks a lot Sean.

I spoke with the HO last night and he is now considering having the shingles and sheathing removed. I told him that a hole should first be made on the inside to see what we're dealing with. Ultimately, the HO does not appear likely to be interested in tearing out the drywall.


If the sheathing is being removed, then installing a layer of rigid foam under new sheathing is appropriate.  There should be a non condensing insulation layer adjacent to the "roof" sheathing.  In the DC climate zone this is R15. (2 inches polyiso) the remaining cavity could be filled with fiber insulation of your choice.

2009 IRC Table R806.4 and 2012 IRC Table R806.5 set minimum requirements for minimum rigid board insulation to be placed on top of the roof deck if air permeable insulation is installed in direct contact with the underside of the roof deck. The addition of the rigid board insulation will keep the monthly average condensing surface temperature at or above 45°F.
Note that the additional rigid board insulation is not required if air impermeable insulation (ie spray foam) is installed in direct contact with the underside of the roof sheathing.

Thanks for your insight David.

I have done the exterior rigid foam deal for re-siding projects. I have been seriously considering using rigid foam for above-roof applications as well.

One thing you said has bothered me for some time, and it has to do with spray foam directly to the roof deck. I don't do it, because I am very concerned that this will lead to issues with the roof sheathing and shingles (due to heat build-up). I think Joe Lstiburek speaks to this.

My approach (while horribly painstaking) is to use polyiso under the roof sheathing, leaving a 3/4" to 1" air space between the underside of the sheathing and the polyiso. 

I am thinking (based on my sense of the situation and the combined input received from y'all thus far) that polyiso under the sheathing (with an air space) and cellulose to fill the remainder of the void is the way to go. This still irks me because there will be no intake\exhaust ventilation. Will I just trap heat behind the shingles and bake them?



I wrote a piece on this some time back:



Greg Labbe

BlueGreen Group

Thank you Greg. This is very helpful.

I will be meeting with the HO next week to perform some testing and make an access point through the drywall to see what we're working with.



Nice explanation.  thanks

Greg, I hate to disagree when I don't have real world experience with this style of construction, but I have some concerns with the proposal at the link you posted.

Here are a few for all to consider.

1.  Isolating the two volumes, 5 and 8 will reduce the available stack effect pressure, especially on the attic 5 volume.  Instead of the upper vent seeing 1/2 of the total SE pressure it will see 1/2 of 1/4 of the total pressure (assuming the height above is 1/4th of the height below.  Vent areas also play a role)

2.  In the picture of the mushroom on top of the steep slopes, if there are soffit vents in that overhang, they will serve as an intake for any heat coming off of the shingles below.  The concern that the heat from zone 8 will affect zone 5 isn't much different from a normal attic.

3.  Area #1 is a concern as shown it provides very little room for insulation.  One possible improvement could be a 45° soffit on the inside to provide both better insulation and venting to the upper zone.

4.  As always with ice dams, air sealing and insulation are the first concerns.  Once that is done, then venting can be addressed.

5.  An option on a mansard style roof would be a cupola on top for venting.  The advantage would be two fold, better upper vent area and increased height, ie increased stack effect.


Two things

First, the house looks new enough (after 1980??) to already have fiberglass in the wall.

Second, I am not sure, but I think you are misunderstanding the construction.  Note that the angled surface with the shingles on it is NOT the wall that needs attention.  The wall is vertical, and the wall and that angled roof plane form a skinny triangle going to zero at the top where the steep roof plane meets the less steep upper roof plane and the wall, all at the same place.

If the soffit is removable (does not look like it is) you can work strips of foam board sheathing about 2/3 the way up the wall and nail them on.  This is less than perfect, but IR show a slight increase in interior wall temperature from the inside for the lower part of the wall when we can do this.  Do not put foam sheathing under the roof sheathing as there is air passing from the (unintentionally) vented soffit, up to the upper attic which would negate its effect.  It does not matter if the two attics are connected.

While in there, don't forget that one of the two biggest problems here is the open ends of the floor joist cavities - if you can only reach in far enough to seal these enormous holes, then you have done the bulk of the fix.  The vertical wall is essentially a kneewall, so treat it like any kneewall.

Since this is a townhouse with a common wall, the common wall is the other of the biggest problems here.  My guess from the shot is that it is a gypsum 2" thick party wall, and the house framing stands about an inch from that common wall.  You need to seal the common wall to the framing vertically all the way up the side, then get into the attic and seal the depth of the house at the same joint.  You would like to do the same to the lower part of the wall, but the brick veneer prevents it.

If there are complaints, I would suspect the combination of this party wall gap and the open floor joist cavities are the source of the problem.

If the roof is coming off, then you will work foam board in to fasten to the back of the vertical wall, making it airtight from top to bottom.  While you are in there, seal the floor joist cavities at the bottom of the wall, and seal the common wall for the height of the wall, top to bottom.  In my opinion, this is not worth doing unless there will be roof work anyway - by the way, this is a great technique and we have done perhaps 10 such jobs in inaccessible "attics".

I would not worry about ventilation in DC if you can make the vertical wall, common wall, and floor joists airtight and therefore keep moisture out of the triangular "attic" cavity.

Ed Minch

Thanks for your input Ed. I have a lot to consider before I propose a final work scope for this client.

- Patrick


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