Flash & Fill

[builder49]

Existing building has a relatively low pitched roof (2/12) with membrane roofing in adequate condition. Rafters are actual 2x8" and 18 1/4" o.c. Proposed insulation is to spray 2" closed cell foam to underside of roof deck and fill remainder with dense pack cellulose. It would then be strapped 16" o.c. and 5/8" drywall. The building is being remodeled into a restaurant with 1/4 space for kitchen area and remainder for patrons. I would appreciate suggestions or comments on this assembly. Thanks, builder49

[Bluewoodrock]

Climate Zone/Location?

[builder49]

Hadley, MA. 5A Cool/humid

[calvert]

How is the insulation contractor going to provide the lower surface to create the cavity in which to blow the dense-pack? Is he willing to tell you how many bags of material it will take to accomplish the dense-pack and are you going to verify his calculations?

In view of the fact this is a restaurant, the insulation is almost a secondary concern, they are usually looking to get rid of heat rather than conserve it. The moisture could become a concern if the ceiling has an inadequate air barrier so I would pay close attention to that.

I think predicting potential moisture and insulation problems for a building such as this becomes very problematic in the sense that no matter what you design for....the dynamics of heating, cooling and humidification will be tremendously impacted by the day to day variables of doors opening and closing, utilization of the full capacity of cooking equipment and related venting equipment,etc..

What kind of support do you have under those 2x8's. A tight insulation job is certainly going to contribute to an increased snow load. How did you fare in that last blizzard? Might be worth taking a look at with that low slope.

[Ted S.]

I would strap first, then denspack so that the drywall will be in contact with the insulation.

[Bluewoodrock]

I agree with Ted-strap first so cellulose contacts drywall. I might also add a bit more CC-maybe bump up to 3"-I was talking with some Cellulose manufacturers last week and they are much more comfortable with this system when the R values from the different materials is about equal.

[ThingOfBeauty]

There's an IRC code section dealing with this situation, R-806.5, accessible here.

In Zone 5, a minimum of R-20 of air impermeable insulation must be applied to the roof sheathing above the air-permeable insulation.

BSC has mentioned two different 'rules' that I remember. One is that the foam should be thick enough that the inside face is above the dew point of 70 degree--40RH air when the outside temp is the coldest monthly average temperature in your area.
The other is that "this usually comes out to" 1/3 R-value spray foam, 2/3 fibrous insulation.
Better look up those numbers before you rely on them! I probably mixed something up.

[worthy]

In Zone 5, a minimum of R-20 of air impermeable insulation must be applied to the roof sheathing above the air-permeable insulation.

However, most closed-cell foams are .1 perm at 2", making them a Class II vapour retarder that is semi-permeable; and even a thicker application of oc spf would be considered air-impermeable.

the inside face is above the dew point of 70 degree--40RH air

The 70 degrees refers to the "interior conditioned space at an interior relative humidity of 45 per cent".* The key is that the interior of the roof deck is warm enough to prevent condensation.

See pp. 166-67 in "Builder's Guide to Cold Climates" Lstiburek

A tight insulation job is certainly going to contribute to an increased snow load.

A good point also made by property insurance companies.

[builder49]

How is the insulation contractor going to provide the lower surface to create the cavity in which to blow the dense-pack? Is he willing to tell you how many bags of material it will take to accomplish the dense-pack and are you going to verify his calculations?

In view of the fact this is a restaurant, the insulation is almost a secondary concern, they are usually looking to get rid of heat rather than conserve it. The moisture could become a concern if the ceiling has an inadequate air barrier so I would pay close attention to that.

I think predicting potential moisture and insulation problems for a building such as this becomes very problematic in the sense that no matter what you design for....the dynamics of heating, cooling and humidification will be tremendously impacted by the day to day variables of doors opening and closing, utilization of the full capacity of cooking equipment and related venting equipment,etc..

What kind of support do you have under those 2x8's. A tight insulation job is certainly going to contribute to an increased snow load. How did you fare in that last blizzard? Might be worth taking a look at with that low slope.

Calvert,
Thanks for your reply. Fabric netting would be stapled tightly to inside of rafter faces and dense pack blown in at 3.5-4.0 lbs/cu.ft. He's a reputable installer in the area and has done a number of successful jobs for me. Bays bulge a bit, but are rolled back. tight as a drum. My plan is to strap 16" o.c. over rafters after netting but before blown in to ensure good drywall install.
HVAC contractor will be installing units with the above insulation in mind. Yes, this all can be problematic and with today's new materials and assemblies-thus my questions to learn.
The roof rafters (actual 2x8 solid hemlock)in very good condition are 18.25" o.c. and span from outside wall to a solid 8x14"middle beam supported every 12' by 8x8 solid posts with direct bearing This was an old coal and grain depot. The floors are on 3x10" (actual) joists 18.25" o.c.
We get out share of snow and it is possible to get 24" with follow up rain adding to the problem. We have gotten this, but it's not every day. Today, anything is possible!
Thanks for your input, builder49

[Mark Parlee]

B49
what would the additional cost be to just fill this area with foam.

closed cell a r6/"= 42

2" of CC = r12 + 4" of Dense pack at 3.5/" adds up to r26

how much insulation value do you need here. I realize in a retail establishment the client is not as concerned with what it costs to heat and cool just as long as the cashflow is there.

[builder49]

Mark,
One quote to get r-38 closed cell is $8,000.00 The board foot price would come down to do entire building but that still would come out to over $16,000.00 2" of closed cell would be $3,700.00 Full dense pack comes out to from $11,000.00 to $12,000.00 depending on installer.
Thanks to worthy,thingofbeauty, bluewoodrock and ted as well. builder49

[NW Architect]

[QUOTE=worthy;684311]In Zone 5, a minimum of R-20 of air impermeable insulation must be applied to the roof sheathing above the air-permeable insulation.

However, most closed-cell foams are .1 perm at 2", making them a Class II vapour retarder that is semi-permeable; and even a thicker application of oc spf would be considered air-impermeable.

0.1 perms = Class I, aka impermeable
1.0 perms = Class II

[worthy]

Dr. Lstiburek categorizes vapour barriers and retarders as below in BSD-106 "Understanding Vapor Barriers"

Class I Vapor Retarder: 0.1 perm or less

Class II Vapor Retarder: 1.0 perm or less and greater than 0.1 perm

Class III Vapor Retarder: 10 perm or less and greater than 1.0 perm

In any case, the point is that the above roof insulation would not seem to be needed.