Published on March 24th, 2016 | by Henri


Improving insulation in old farmhouse

Q.  You have pointed out in several articles the damage that can be caused by ice dams. I have a friend who has a 100 plus year old two-story farmhouse with a one-story addition. It has an open ceiling with 4×4-inch rafters and no ceiling joists. It has 1×6 sheathing  over the rafters, with 2×4-inch strapping on top of the sheathing. Blue-board foam, 1-1/2-inches thick, is placed between  the strapping, with a metal roof on top of that.

There is a lot of ice build-up on this roof and my friend thinks he wants to take the metal off and add more strapping and insulation on top, and then replace the metal. I think he should add a second layer of strapping and a new roof, to create a cold roof. What would you do? If he was to go the cold-roof way should he add insulation and use 2x6s or 2x8s to help stop some of the heat loss?

A.  I assume that the metal roof is of the screw-on type and not standing seam. Your friend’s addition suffers a lot of heat loss through the joints between the 2×4-inch strapping and the 1×6-inch ceiling boards.

The blue-board insulation between the 2x4s has probably also shrunk a bit, allowing warm air to reach the cold metal roof through the spaces between the strapping, the ceiling boards and the insulation. The 1-1/2 inch layer of blue board does not provide much insulation anyway.

His best bet would be to take off the metal roof and add 2-inch thick, 4 feet wide XPS rigid insulation cris-cross to the existing layer, which I assume is running from side to side between the 2×4 strapping. So the new insulation layer would be running up and down the roof.

Since it will be impractical to cover the insulation with a non-asphaltic roofing membrane, all joints between the insulation panels will need to be taped with a compatible tape (not duct tape) to prevent condensation that always forms under metal roofs from getting through the insulation and onto the ceiling boards below.

Then he should run new strapping from eaves to ridge screwed to the existing 2x4s. This strapping need only be 1-inch by 3-inch and should be screwed, using screws long enough to go through the new insulation and one and a half inches into the existing 2×4 strapping.

This will provide a drainage channel for any condensation under the metal roof.

Next, he should screw strapping from rake to rake (side to side) to the 1×3 strapping, and re-install the metal roofing to it. This will give him much better insulation without thermal shorts, as well as a drainage plane for condensation forming under the metal roofing.

An Alternative

An alternative would be to install CDX plywood sheathing over 2x3s running from the eaves to the ridge and screwed through the insulation into the existing 2x4s. The 2x3s should extend past the eaves enough to install an off-the-shelf continuous soffit vent, and be capped with a new fascia board.

If this method is used, a ridge vent will also be needed – a somewhat tricky thing to do with a metal roof.

Cover the sheathing with one of the modern roofing felts and install the metal roof over the sheathing. This will give the added protection a cold roof provides.

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