Roof insul. from outside

[Curt S. - Minn.]

I am in the process of upgrading the roof insulation for a customer here in Minnesota. We are working on a 1 1/2 story home built in the 1940's. It has the typical second floor with pitched ceilings in the bedrooms and kneewalls along both sides. We are replacing the roof and I suggested to the homeowner that we upgrade the roof insulation from the outside since they have some ice damning issues and the rafters are only 2 x 4's with who knows what for actual insulation.

My idea was to remove the shingles and tarpaper and lay 1 1/2" or 2" of rigid foam across the entire roof. On top of the foam we would lay 1 x 4's purlans up and down the roof directly over the existing trusses, screwing thru foam in to trusses. This would give us a nice ventilation path from eave to ridge. We would then install new roof sheathing over purlans leaving a space at the peak for ridge venting. The final step would be installation of a typical asphalt roof.

A fellow contractor mentioned his concern for the possibility of condensation build-up on the under side of the rigid foam (on the top side of the existing roof sheating). I am not certain of the existing sheathing or what insulation is in the current rafter spaces. I am guessing the sheathing is most likely the typical 3/4" lap boards from that era. Inusaltion - I am gussing fiberglass.

My goal is to add value for my customer by suggesting he upgrade his insulation but I don't need him calling me in a year or two with moisture dripping from the drywall in his bedroom. Does anyone see any concern with moisture buildup?

Sincerely,

Curt S. in Minnesota

[Mark Parlee]

Curt
1st post; welcome to the forums.

send me your email and I will send you an article I wrote on that deal with this. While you have the roof stripped why not go ahead and add some more insulation in the rafter bays. You would need to consider what type of insulation to add. If you don't do this right you very well could get a call.
Go to my profile for my email.

[vegoilburner]

I did almost the exact same thing to my own home 2 years ago. I have a 1.5 story bungalow built in 1910 northwest of Chicago. Most of the roof had no insulation between the 2x4 rafters and the rest had 16" R-11 fiberglass batt between the 24" OC rafters!

From the outside I tore-off down to the rafters; seathing too.

Between the rafters I filled them with 3.5 inches of polyisocyanurate and spray foamed the sides.

I then applied a bead of 100% silicone to the rafters and laid 1.5" foil-faced poly iso across the rafters.

I laid 2x4's on their side and nailed them to the rafters through the foil-faced foam using galvanized 30# box nails.

On top of the 2x4 sleepers I installed the sheathing, tar paper, and shingles as usual.

This provided a 1.5" air gap between rafters from soffit to ridge.

The foil-faced polyiso provides a radiant barrier since the required air gap exists.

So I now have a ~R30 roof. It is a bit below code but is WAY better than before. The past 2 winters, the difference has been noticable. Summer to.

This is about the same thing you were thinking of just with the sheating changed. Give's you a chance to upgrade the between rafter insulation too.

Just wanted to share, hope this helps!

[davenorthup]

I'd consult with an engineer. More insulation = more snow load since your not melting as much. More snow load ='s ?????

Last time I checked a 2x4 holds very little. With out knowing much about the system(snow load, spans, pitch); I'd suggest re-raftering the roof with something that can hold a lot of insulation and snow loads.

I come from the world of 100# + snow loads.