Where to insulate with 3.5" Foam Board
 

We have a stack of 4x8, 3.5 inch poly sheathed ridged foam board. It has the reflective material on both sides. Some ideas floating around are:

1. Put it on top of the 2x4 roof trusses in the attic?

2. Put it between the 2x4 roof trusses?

3. Install it under the house in the crawl space and fasten it to the bottom of the floor joists. We already have about 6+ inches of fiberglass insulation in there already?

4. Pull the siding off the house, (we have mostly 2x4 stud walls) and replace the siding on top of the 3.5 Foam board?

If we did number 1, would you leave some of the blown-in existing insulation in the space between material and the ceiling?

If we did number 2, would you blow the insulation back on top of the foam board?

If we did number 4, would it be better to pull the siding, pull the existing fiberglass insulation out replacing it with the foam board?

I would do the floor/crawl space roof by securing it over the pink insulation.
You would have a super insulated floor which would add to the comfort / warmth of your floor and rooms.

how much insulation is currently in the attic, the crawl space, the walls?

is their vapour barrier on any of them?

If you do the roof make sure that you are venting the underside of the roof decking or you will end up with a hot roof.
I would insulate the walls with it because you can stop thermal bridging in the walls this way and insulate the attic with blown in cellulose insulation, you can also cellulose insulate the wall cavities if you want to pull the fiberglass, but a foam shell around your house is going to help you a lot, make sure that it goes down to the ground as well, but for below grade you are going to need a foam that is made for ground contact, the foam you have is most likely not if it's foil faced.

The house has the insulation requirements of the late 1990's. The attic has 12-14+ inches of blown-in, crawl space is 6" fiberglass batts unfaced, walls are mostly 2x4 with fiberglass batts. The exterior of the house has the vapor barrier wrapped on it also.

Here are two of the attic.

The venting in the roof was poor in my opinion. The original HVAC crew installed 5 vents in the roof. Two of them were for the fans from the bathrooms. One was ran 15 feet at an angle to the roof vent which I don't think is good. Recently a new vent was installed a couple feet from that fan. After removing the bath vents and installing some new vents, the attic is cooler and so is the living area.

The 3.5 inch insulation that I bought was meant for the walls according to the manufacturer. Roof Insulation Wall Insulation Polyiso Specialty Products | Rmax

OMG Geo NR Gee

IMHO if the removal of the siding is an option to clothe your home in a 3.5" of foam I'd be all over that.!!! Remove the siding leave the R12 fiberglass and put a insulated barrior on your home. (Long Screws with plastic washers) Put the foam boards on the 2 x 4s tape the joints then strap the walls and new siding. Or better, look into stucco. (Dryvitt or Synergie) this is an additional layer of foam board (beaded foam) and then covered with a modified mortor and a latex type colour of your choice stucco. You will have a tight home with super insulations and insulating areas that are otherwise almost impossible to do like the rim joist. NO more thermal bridging thru the 2x4s check out some of the thermal pictures of homes with 2 x 4s you can see almost all in the thermal images. Good luck,. It will be a little work but probably a lot less than the work required to pay for heating fuels the rest of your life.

Randen

I agree wrap the outside of the house depending on what you mean by "The exterior of the house has the vapor barrier wrapped on it also."

if the vapour barrier is on the inside behind the drywall don't wrap the house with rigid as you'll trap moisture between the two. If the barrier is on the outside behind the siding you can remove it easily and not worry about trapping moisture.

What moisture will you trap?? If the wood hasn't dried out yet its not going to.This sort of construction is going on all the time. And really it's not going to be hermetically sealed there will be some, albeit little air movement. If Geo NR Gee does install this system and with good windows he will have a minimal air exchange envelope that of which new homebuilders strive to obtain. He will in all probability need a air exchange unit to insure his family has fresh air to breath.

The goal is to heat it with a match. He is very fortunate to have siding on the outside that he has a chance to do this. If it was anything other veneer like brick not a chance.

We have first hand experience with this. Our home is 2x6 construction with vapour barrier on the inside. The outside is Dryvit 1 1/2 foam very nicely sealed. The air exchange performance is 1.9 ACH @ 50kp.
During some home renos we had seen the wood structure looks as good as the day it was installed. No dry rot no mold and that was wood that was exposed to the elements before we sealed it all up. If anything it was a lot dryer.

Randen