Discussion Forum

Bob Monnet wrote:

I would like to increase the insulation and add new siding to the envelope of my home. The existing envelope is 2x4 stud wall, fiberglass bat insulation, sheathing, 'fiber' siding. I'm thinking of leaving the old siding in place for several reasons which are; save the labor of removing, save from placing it in a landfill and, utilizing a layer of the wall for what value it may have in vapor or insulating qualities. For the exterior siding, I'm thinking 1 or 2" EPS insulation with seams taped, cedar board and batten, and Robinson Thinrock for a waynes coating.

Would it be better to remove the old siding or try to leave it in place?

One issue is the lapping of the siding doesn't leave for a smooth flat surface to face with EPS insualtion. I figure the insulation is rigid enough not to show horzontal 'waves' and I would also only screw or nail where the old siding touched the EPS. I don't know if the air gaps created between the siding and EPS are a good thing or a bad thing.

Thanks

I just finished a simular project on my own home that included removing two layers of old siding (one fiberboard and one steel), adding 1.5" of taped exterior rigid foam board, a Tyvek moisture barrier, a "rain screen" detail and new siding on top of this assembly.  The project included replacing almost all the windows and spraying closed cell foam in many of the wall and roof framing cavities.

Having gone through the process I would recommend the following:

1.  I suggest using polyisocyanurate rigid foam board instead of extruded polystyrene.  Even though it is a bit more expensive it has a higher R value per inch and it does not have the bio-accumulating brominated flame retardants that all rigid polystyrene (both XPS and EPS) contains (see Environmental Building News Aug.'09).

2. I chose to remove my old siding to get a smooth substrait to attach the foam to.  This allows easy  upgrading of the wall insulation from the exterior of the house (dense packed cellulose works well here if there is sheathing under the siding) and to more positively locate the framing for better nailing.  I attached the foam boards to the existing studs and then taped the seams.  In theory that should create a code approved water resistive barrier but since I don't trust the tape to last for the long haul I added a layer of Tyvek and nailed 1 X 2's through it and the foam and into the studs.  The 1 X 2's created a "ventilated rain screen" to let the cedar siding dry evenly and direct any moisture out of the assembly.

3. In theory small contained air voids should not degrade the insulation (as long as they are not large enough to start convection loops) but given the vageries of site construction I am concerned that air could get between the old siding and "short circuit" the foam insulation.

4. One concern worth investigating is if your house has a polyethylene vapor barrier between the studs and the drywall.  Since exterior walls need to dry either to the inside, the outside or both one doesn't want to create a situation where water vapor is trapped within the wall assembly.  Since ridgid XPS and foil faced polyiso foam both act as a vapor barriers be sure your wall can dry to the inside of your home. 

Best wishes