Today I surprise my readers with an actual “how to”!
Proper vapor barriers and ventilation system design and installation are important to prevent condensation and resulting problems from moisture damage.
Condensation occurs when moisture laden air comes in contact with a surface temperature equal to or below air dew point. This phenomenon creates problems which are not unique with steel covered buildings. These problems are common to all construction types.
Any metal roof underside is to be protected from condensation by insulation, which reduces condensation forming potential on panel undersides.
Most Common Mistakes:
- Overlapping reflective radiant barrier rolls (other than the appropriated side tab).
- Cutting off at eave girts to create “waste”.
- Placing reflective radiant barrier in overhangs (beyond column building lines).
- Not placing roof reflective radiant barrier under all steel roof surfaces within building lines (including roof only shed or carport areas).
- Failure to square roof plane before installation.
- Not straightening eave purlin to a string line before installation.
- Failure to adequately seal joints, rips or tears in reflective radiant barrier.
- Not using roof reflective radiant barrier.
When installing, reflective radiant barrier either A1V or A2V (white vinyl on one side), white vinyl side will face the building interior (with few exceptions).
Side bar – what is all this A1V type terminology anyhow? The “A” or “V” are the facings – aluminum or vinyl. The number 1 or 2 corresponds to the layers of air cells which are sandwiched between the facings.
A package insert may give directions for installing reflective radiant barrier. The directions refer to “airspace”. This is not the airspace on building inside. The “aluminum” reflective side is installed facing the steel (sunny side up).
Reflective radiant barrier installation does not change construction methods. Reflective radiant barrier facings are not affected by cold temperature. Except for underneath a concrete slab, reflective radiant barrier is not to be overlapped. Depending upon product received, install by one of the following methods.
Prior to of any roof reflective barrier installation, make certain roof planes have been squared.
Reflective radiant barrier is not placed in overhangs. If reflective radiant barrier is installed in overhangs, there will be a SHORTAGE !
Start roof reflective radiant barrier at same end of building roof steel installation will start. The reflective radiant barrier roll end begins flush with eave purlin outside edge (also known as eave girt). The reflective radiant barrier leading “long” edge begins flush with building end truss outside.
Reflective radiant barrier is installed to run eave to eave over ridge. Splices, if needed, are best made directly on purlin tops.
Other than to make a roll end square, do not trim the starting edge. Start flush at eave purlin outside edge. Opposite end is cut flush with opposite eave purlin outside edge (or ridge purlin upper edge for translucent or Vented Ridge applications).
Verify adequate material exists before trimming off large amounts.
Using a minimum 5/16” galvanized staple, staple through reflective radiant barrier to eave purlin top. As an alternative to staples, 1” galvanized roofing nails (with the big plastic washers) also work well. Roll out reflective radiant barrier across purlins (up and over ridge) with aluminum side up and white side down (towards building inside).
Pull reflective radiant barrier past opposite eave purlin edge and staple to top. Trim roll off flush with opposite eave purlin outside edge.
As an alternative, double-faced tape may be used to attach reflective radiant barrier to eave girts.
Install next roll in same manner, stretching roll tightly, align properly and close butt sides.
For reflective radiant barrier with an “adhesive tab” (typically A1V): These have a 1” tab (without air cells) extending along one reflective radiant barrier roll long side. At the seam, where two reflective radiant barrier rolls are joined, pull tab across adjacent roll by 1”, remove “pull strip” from the adhesive, and firmly press the two rolls together. Properly installed, each roll will have a 48” or 72″ net coverage, depending upon product purchased.
For square edge rolls, use a butt joint and seal seams properly with tape. (Good to Know! A butt joint is where two pieces are placed end to end or side to side without overlapping.)
Foil tape (for A1A or A2A – faced both sides with aluminum), 2” white vinyl tape or a silicone bead can be used to make permanent seams between ends and reflective radiant barrier roll sides.
For maximum air and vapor tightness, keep perforations in reflective radiant barrier to a minimum. Seal all perforations with reflective radiant barrier tape.
If this process sounds like work, it can be! A far less labor intensive solution for condensation can be read about here: https://www.hansenpolebuildings.com/2014/07/condenstop/
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