Thread: Insulation Roof Deck Clearance

Hey guys,

I've been reading posts here for almost a year now and get a kick out of some of the stuff you come across. I'm a DIY-er and have decided to take your advice and hire a licensed and bonded contractor to insulate my attic. I've had 2 different contractors out and I'm getting conflicting information from each of them.

I have a low slope roof, 2/12 or 3/12, and very little room in the attic. At the peak it is around 24" high, fairly tight. The first contractor said that it is not good to have cellulose insulation compacted tightly against the roof deck. Another contractor is telling me since we are in such a moderate climate, Phoenix Arizona, that cellulose can be densely packed into the low clearance areas and touch the roof deck.

Basically my question is should blown in cellulose insulation come in contact with the roof deck?

Any advise is much appreciated.

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Originally Posted by Ryan Schemm

Basically my question is should blown in cellulose insulation come in contact with the roof deck?

Their should be a 1" air space between the insulation and the roof sheathing for ventilation.....I'm assuming your attic has ventilation.

Proper applications can vary based on geographic locations. But like Chris, I would recommend maintaining a 1" air space between the roof decking and insulation.

Thanks for the information. Yes my attic is ventilated, gable vents and 2 Whirlybird's. The house was finished sometime around 1960 so no ventilation in the bird blocking.

Originally Posted by Ryan Schemm

Basically my question is should blown in cellulose insulation come in contact with the roof deck?

Generally, no.... but not that the contact really causes much of a problem. My main objection would be that it obstructs airflow and ventilation throughout the attic.

And, compacting it is not good... aside from wasting materials, I've heard it loses some of its effectivness when it's compressed. Basically, it relies on the air being trapped in the "fluffy" space to work right.

If code requires "1" air space" between Densed packed cell and any building envelope strcuture, code is not understanding DP cell insulation properties. DP cellulose is supposted to be packed tight and cannot be dense packed if not tight (i.e. needs a boundary to enable pressurizing the cell to gain and hold the density).

DP cell is hydroscopic (absorbs humidity and vapor) - absorbs humidity when outside or inside humidity levels increase and then dries off through the envelope when relative humidity levels reduce. That and its no-infiltration feature are why it works as insulation. The issue is NOT, can it be tight to the deck; it must be tight to the deck to work. And vapor barriers must allow vapor exchange ... has to be able to breathe into or out though the building envelope.

From the The Cellulose Insulation Manufacturers Association website; CIMA Technical Bulletin #2: Standard Practice for Installing Cellulose Building Insulation

6.2 Existing Structures
In joist areas, where soffit vents are installed, the opening
from the attic into the soffit area may be blocked by batt type
insulation between and at the ends of the joists (See Figure
2). Insulation should not totally fill the space between ceiling
and roof. There should be a 1 inch (2.54 cm) opening next
to the roof for ventilation from the soffit area. Or a chute
may be installed per Section 6.0 and Figure 1B.

Thank you for the link and the summary. Although I never doubted the other members, documentation always helps.

I never doubted the 1� air space would be code …. but that code standard is not up-to-date with the features and performance of Dense Packed Cellulose. The poster should NOT use DP cell if a 1� air space (using foam channels) is to be used for two reasons: (1) you cannot dense pack cellulose without being tight to the cavity walls (i.e. the underside of the deck) as there cannot be any pressurization and density achieved under this condition of a 1� air boundary and (2) if one did try to dense pack this arrangement (baffles and channels installed to retain the 1� air space), dense packed cell will collapse the channels and eliminate the air space. This situation calls for Blown-in Cellulose as it lofts without pressure.

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Dense Packing Cellulose Insulation

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