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Below Deck Insulation and Cool Roofs — What’s To Go Wrong?

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For most of us in the low slope roofing industry, it’s considered to be absolutely standard to have insulation placed above the roof deck. For standard steel deck construction, this approach has many advantages, such as:

  • The dew point is typically within the insulation layer, lowering condensation risks.
  • Construction of the roof system can be done from above, on a stable platform.
  • The membrane is installed on top of what is normally a stable surface such as polyiso or a cover board.
  • A well installed system of two layers of polyiso with staggered joints can be a fairly effective vapor retarder.


However, there are other approaches, some of which are regional and some dependent on the building type. For example, in the coastal regions of California, Oregon, and Washington, i.e. the West Coast, low-slope roofing is frequently installed over plywood or OSB decks above wooden rafters. Fiberglass insulation is installed between the rafters as shown schematically below.

A wood deck roof system. For some building uses, the ceiling might be absent.

This type of system can also be found in low slope residential buildings in inner city areas. Also, light commercial construction that has some slope can also be built this way.

In practice, it’s very difficult to install fiberglass insulation in a way that air doesn’t readily pass up through the system and into the space below the deck. Let’s look closely at the features of this system that can contribute to condensation issues:

  • The roof deck is “cold” –Wood decks without insulation above them will readily equilibrate to the exterior temperature. During cooler night-times and in winter periods the deck temperature can then be below the dew point of the interior air. Hence the term “cold decks.”
  • Construction materials – Wood-based products absorb moisture. Once the water content gets above 30% by weight, rot is inevitable. Even if the air space is claimed to be vented, air flow is often insufficient to dry out the wood after condensation has been absorbed.
  • Insulation Type – Fiberglass insulation is very porous, and if there are any leaks around the edges of the facing, then air readily passes up through the system.

The schematic below shows these points in detail:

Wood Roof Cold

The wood deck system showing moisture ingress. A – air flow from walls and elsewhere in the structure; B – through penetrations; C – diffusion through vapor-permeable ceiling and paint.

So, Cold Roof Decks Have Always Been a Problem? No!

Historically, membranes were not “cool” but usually black, and they got very hot during sunny days, typically throughout the summer. These were self-drying roofs, with damp plywood drying out during summer days. Cool membranes changed that.

Wood Roof Warm

The wood deck system showing drying out. A – air flow down into building; B – through penetrations; C – diffusion through vapor permeable ceiling and paint.

As pointed out by Phil Dregger, of Technical Roof Services, CA, the balance between wetting and drying-out can be very delicate. Some roof systems might be “on the edge” of having problems, and a reflective membrane simply forces the issue.

So, Can I Use a Cool Roof with Below Deck Insulation? Yes!

West coast type roof systems can be retrofitted with cool roof membranes so long as some basic changes are made. It’s important to review any final plans with a design professional. What follows are some of the basic considerations that, if implemented correctly, can lead to success.

1. Add Insulation Above the Roof Deck 

This approach fits current “building science” thinking in terms of putting the thermal control layer on the outside of the building structure. Typically this would be achieved by adding polyiso insulation above the plywood deck. The right thickness would need to be calculated to move the dew point above the plywood deck, where air infiltration is minimized.

2. Use Fully Adhered Membrane Attachment 

By fully adhering the membrane, one of the major differences between traditional built-up (BUR) and mod-bit roofs versus many single-ply installations is eliminated. The importance of minimizing air flow up into the system can’t be emphasized enough. Mechanically attached membranes can billow in wind events and pull up moist air into the roof system. It’s best to reduce or eliminate this effect by fully adhering the membrane.

3. Pay Attention to Penetrations 

  • In reroofing situations, it’s not uncommon to see penetrations that have deteriorated over the years. Worse, some may have been repaired by a local maintenance person who didn’t really understand the design and function.
  • Examine every stack and exhaust duct to make sure it’s functioning properly. Is the exhaust truly exiting the system out into the exterior or is some leakage possible back into the roof system?
  • Don’t just flash the penetration in; also seal any space between it and the roof deck.

What About Vapor Retarders?

  • It can be tempting to think that a vapor barrier will automatically prevent moisture from entering the system. However, the result can be to enclose the roof system with two vapor barriers: the membrane on top, and the vapor barrier on the underside. Any moisture that does get in will not be able to leave.
  • In those instances where the building designer deems that a vapor retarder is essential, that barrier should have some vapor permeability. Never add a vapor retarder without first consulting an experienced roof designer. It’s important that any retarder be placed correctly, have the right degree of permeance, and is correctly interconnected into the system.

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  1. William Hendricks

    Thomas- for a concrete slab roof deck that has a torch down roof with no over deck insulation, would placing mineral wool insulation under the deck be appropriate? Does it need ventilation space btw the deck and insulation and/ or a vapor barrier facing oriented to the inside? In Climate zone 4 non-marine.

    • Thomas J Taylor, PhD

      Ordinarily, I don’t recommend using mineral wool in a roof system due to its high permeability. That can lead to moisture transport towards a cold surface, followed by condensation. In the situation you describe, installing polyiso foam under a rood deck would be very difficult, and so it could be that mineral wool is the better option. As you note, I would then advise installing a vapor retarder on the inner side of the resulting assembly.
      I’d be curious to hear from others in climate zone 4 either designing roofs or roof consultants as to their experience and advice in this type of situation.

  2. Ruth

    Dear Thomas, I live in Jerusalem, Israel. My roof – flat with a little inclination for the rain-is made of stone…very badly insulated. Could I build a deck above the roof with some kind of insulation adhered to the deck to insulate the house from cold and heat.?
    Thank you , Ruth

    • Thomas J Taylor, PhD

      Ruth – I’m not sure of the exact construction used, so giving guidance is not easy and you’d be best served talking with a local builder. Is the roof really stone or is it a concrete deck? In theory, adding insulation above the roof deck, and then a membrane above that would help – but the devil is in the details. The edges would need a lot of care – so, the membrane would have to be carried down to the wall in some way. Best to consult with a builder.

  3. Thomas J Taylor, PhD

    John – the project you are describing sounds like residential steep slope. As you probably realize, that would typically require a 1:300 ratio of soffit to ridge vent areas to get sufficient air flow. Some people would recommend having a higher soffit vent area, closer to 1:150. For an older building you would really need to consult with a local engineer, architect, or contractor who can give specialized advice for a unique project such as this. By the way, polyiso board provides around 5.7 R per inch of thickness, which is the highest value of any conventional insulation materials.

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