12 Comments

1. 
   Hatherly | Jun 20, 2011 08:07am | #1

   Thermal bypass
   But if you insulate on top of the existing roof, how are you going to deal with the thermal bypass that will occur around the walls in the attic cavity where the existing batt insulation is poorly installed?

2. 
   user-998246 | Jun 20, 2011 10:06am | #2

   Baltimore City Codes
   FYI, Baltimore City often lags in code adoption so they may be using an older code than what the state of MD has adopted.

3. 
   gusfhb | Jun 20, 2011 12:15pm | #3

   re: Amanda
   I would think insulating the walls/edges is much less complex than the whole ceiling

4. 
   Hatherly | Jun 20, 2011 01:34pm | #4

   I think either way is
   I think either way is difificult - walls or ceiling - when you can't access easily from the inside and you don't know what it in the cavity.

5. 
   user-998246 | Jun 20, 2011 02:45pm | #5

   Also be careful insulating in existing cavities
   Especially in older, existing building, insulating in wall and ceiling cavities can be a bit dangerous considering the state of some older wiring and connections. There is some talk brewing about on whether fires have occurred due to insulation around wires and insulation. If a wire is over insulated or the insulator is degraded, an arc can occur along the surface of the wire insulation, causing a fire. Note the difference between wall insulation and wire insulation in this context. Basically, if you spray or stuff insulation into a wall cavity in an existing building, pay close attention to the type and condition of the wiring that may be in the cavity. The insulation itself can burn or melt in some cases or can over insulate a wire or connection, causing a fire, which causes the structure itself to burn. Just an FYI.

1. 
   severaltypesofnerd | Aug 17, 2021 02:07pm | #11

   And as well, the number of amps the old wire is carrying. Since lighting loads have dropped so dramatically, and high use areas get modern circuits, changes are those wires are carrying very little load, and can be protected with a smaller breaker.

6. 
   Hatherly | Jun 20, 2011 04:35pm | #6

   Agreed - that's why it is so
   Agreed - that's why it is so difficult to do and why the thermal bypass ends up being an issue if you just insulate above the roof.

7. 
   user-943732 | Jun 20, 2011 05:22pm | #7

   Amanda is right
   If you want to add insulation on top of the roof then you must also deal with the entire perimeter of the attic/roof cavity -- which is quite challenging unless you rip open / trench the roof from above or else cut an access and send in people to crawl and seal. if you are going in to crawl and seal in the roof cavity to get the perimeter, it would actually be easier to just seal the bypasses and do some dense pack in the small section in the rear that is too small to crawl. You can then blow cellulose in the rest of the cavity. This approach can be done for about $1.50/sqft -- far less than any of the foam approaches. it also leaves you with a solid roof for adding a deck.

   You should perform voltage drop tests on all of the wiring that runs through the attic before and after the work and fix any problems found or created.

8. 
   kim_shanahan | Jun 23, 2011 06:33pm | #8

   Amanda ususually is
   If the joist space has been ventilated, as would have been proper and expected, then adding insulation above a ventilated space will provide no insulation value. Unscrupulous and/or ignorant spray foam roofing contractors in our market, of which there are many, will often tear off old built-up tar and gravel roofs and replace them with sprayed on polyiso systems. They make good roofs and provide good insulation, but not when the space below the polyiso is ventilated. These contractors make false claims to the homeowners on improved efficiency and lower energy bills. Incidentally, it is common for gravel to be placed on the foam roofs to protect them from UV degredation rather than elastomeric paint, which is only used around pipes, penetrations, flashings, etc, that the gravel does not cover. The skin of the polyiso is considered durable enough in and of itself. Most reputable roofing contractors offer 10 year warranties.

9. 
   toddoskin | Feb 06, 2013 05:35pm | #9

   Gaco silicone liquid applied roof?
   I know this is a very old thread, but in case someone else checks it out...

   How about this : http://www.gacoretail.com/gacoroof.html

   It's a silicone based liquid applied roofing 'coating.'

   You can DIY pretty easily..

   I've done it on a couple very small 'low-slope roofs'.. and so far so good (I have no long term experience with these roofs however).

   In terms of insulation, I've also done a lot of blown-in cellulose installations in pitches roofs, and low-slope roofs... in really old buildings (usually are 80-100 years old).

   I blew in about 10" of cellulose in my own cathedral ceiling...and has been doing well for at least the past 2 years.... I'm thinking about using Gaco coating for roof too... as I will need a new roof soon...

   Anyone else have experience with Gaco coating?

   -Todd

10. 
    metamerman | Feb 07, 2013 12:16pm | #10

    Don't forget the coverboard!
    I note that "coverboard" (such as DensDeck) is not mentioned in this post. Putting coverboard between the foam and the EPDM (or TPO) membrane is recommended in the manufacturer's installation instructions, may be required by fire code, and is the only smart thing to do if there is any chance that the roof will be walked on (i.e. if there ever will be a roof deck or drains that need to be cleaned) because it will keep the foam from breaking down at the edges of the panels. It's just a bad idea to put a membrane that should last 40 years down on a substrate that will start failing decades earlier.

11. 
    severaltypesofnerd | Aug 17, 2021 02:08pm | #12

    Consider also in some areas a Class A fire rating is required for such roofs, which ends up as a layer of glorified drywall just below the roofing membrane.