insulation in crawlspace of flat roof duplex


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Old 08-04-06, 10:27 AM
M
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insulation in crawlspace of flat roof duplex

Hello all, I am new to this site and look forward to helping out where I can.

I have a 105 year old brick duplex in Minneapolis. I have taken the last 4 months to update the upstairs and it has been a real interesting project to say the least.

The upstairs had some 220v electric heat and a huge space heater. I installed a Bryant furnace in a closet which I elevated and extended over a stairway so as not to add anything in the house. I insulated all the duct.

The crawlspace above the 2nd floor is Very cramped and the slope of the roof makes it impossible to get to the rear of the house. There is currently NO insulating in this structure , none in the walls or the crawlspace.

The roof is a heavy lapped torchdown rubber , and I am thinking to blow in the insulation at the rear of the building perhaps as far as 1/3rd of the way to the front by cutting some access holes in the Roof and then patching them with Eternabond Tape.
I can get to the front 2/3rd fairly easy inside the crawlspace above the 2nd floor.

Will it hurt if the insulation comes in contact with the sheathing ? I will not be able to do a perfect job here and the back of the house has no real room for much insulation

I hope some smart person takes pity on me , and helps out LOL

I will be out of town for a couple days and look forward to replying then

Thanks

J
 
  #2  
Old 08-04-06, 08:27 PM
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Your question has been a thorn in my side for over a decade. Several years age (10+) an application like yours came up at an Energy Conservation convention. I do not attend these conventions for personal reasons. Several colleges of mine Emailed me with the solution to this problem and asked me for my opinion. Their solution by consensus was to dense pack cellulose insulation in the low clearance of the attic and blow in cellulose in the rest of the attic that had adequate clearance. As much ventilation for the attic should be provided as possible.

You have to remember that this was several years ago and the favorite reply for a dumb solution was "DUH!" Which was my reply to my colleges and for some reason got published. Mind you my reply was published, not my explanation.

These are several factors to consider here.

1. The sole purpose of attic ventilation is to bypass the low moisture vapor permeability of roofing materials.

2. Insulation impedes heat flow from conduction, in both directions. In other words, insulation prohibits heat flow through it during the winter from inside the house and outside the house during the summer.

3. The rule High to Low always applies, with an exception. what this means is and object of higher temperature or moisture will always give moisture or heat to objects of lower temperature or moisture level and not vice-versa. The exception is moisture during the winter. whenever you heat air within a confined space (house) the air expands and that creates an area of high pressure inside the house. The lower pressure outside causes the pressure inside the house to be attracted to the outside, which follows the rule high to low. This pressure will dominate moisture flow and cause it to give moisture to objects of lower moisture levels. FYI this is the reason for vapor barriers.

If we apply this to your situation what will most likely occur is that failure of the roofing will occur in the section of the roof that is in direct contact with the insulation way before it occurs in the section that is not in direct contact with the insulation. So any amount of savings incurred by the insulation is negated by the costs of roof repair/maintenance and in most cases you will probably end up losing money. If you want me to explain how and why, I can do so but I don't want to bore you with it.

My advice to you is to insulate the attic as much as possible without the insulation touching the roof decking/sheathing. The R-19 stands for 1/19th of a BTU traverses a square foot of this material per hour. So if the section of the attic you can only install R-19 without touching the roof decking/sheathing verses dense packing this area and accomplishing let's say R-30, the savings is not that significant. I can do the math for you if you like. This is referred to as the "Law of Diminishing Returns". However, if I consider the costs of roof repair/maintenance, the installation of R-19 wins hands down.
 
  #3  
Old 08-04-06, 09:47 PM
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thanks for the reply, I do not think I can get so deep into the crawlspace near the rear of the house to install even R19 batts. It is so tight near the back and there is a bedroom back there and the heat always gets turned up and or supplemented in that bedroom (I have the new furnace this year so we shall see if that changes) and so the heat rising Always causes a huge ice dam on the gutter starting there. Butting up to that bedroom is the stairwell which being Unheated never has too many issues. Frankly I am leaning to leave it as is, there is not a spcek of insulatin in this building , the brick is Very tight and I have updated all the double hungs with the new trackliners and all the windows are now very well insulated along with the storm windows.

As for damaging the sheathing and roof materials , well there is some type of 1 inch hardboard insulation on the old original sheathing then tar paper and then a 3/8 inch torch down lapped roll rubber roof and currently there are no vents on this roof expressly for venting the space , tho I am sure there is Some airflow due to me installing a dryer vent, bath fan vent, flue vent and 1.5inch plumbing vent thru the roof. I guess I might just let it slide this year and see how the heating bills go for the tenant, my biggest issue is that I want to add a condensing unit and be ready for the cooling season next summer and it would greatly benefit I think if the roof were insulated.

Eeks I do not know how I should proceed !

thanks for you kind assistance and let me know what you think or address any issue as you see fit I am very handy at everything else LOL

J
 
 

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