Insulation- spray foam or cellulose in attic?

George_14

I have a 1930's house with a few inches of fiberglass insulation in the attic floor covered by floorboards. The attic is about 1000 sq ft and has a high ceiling (10 feet). The roof is slate. I'm planning on adding some more insulation and had 1contractor come in and suggest spray foaming the rafters. They basically made the spray foam out to be a miracle product. Then I had someone from energy company out to do an assessment in order to be eligible for a rebate on the insulation. he said he would do blown in cellulose in the floor boards and thought it would be better to reduce the size of the conditioned space. He said the spray foam might be a better product but since you are increasing the size of the conditioned space that would negate the benefit. I forgot to mention I have ac ductwork and a air handler in the attic. I may decide to add a room up there in the future.

Does anyone have any comments on which way is should go? After the rebate neither option would be very expensive. Thanks for any help.

George_14

Forgot to mention

Forgot to mention I live in new England.

The Steam Whisperer (Formerly Boilerpro)

Decisions.......

When comparing the two keep a few considerations in mind.  One is that you may be finshing the attic.  That means you will need to insulate the rafters eventually.  The next is that the volume of the heated space is generally not as important as the size of the exterior exposure (unless we're talking really leaky spaces)  The roof will typically have at most 40% more area than the ceiling (a 45degreee pitch or 12/12).  Typically light density blown in cellulose only insulates, it does not stop air leakage.  Spray foam, if it covers the whole roof and seals right down to the top plates of the walls, will provide an airtight cap on your home.  This is an enormous benefit, as the 2nd floor ceiling is the largest place of steady air leakage in the winter.  If the air can't get out the top of the home, the intake of cold air lower in the home (chimney effect of hot air rising) will be greatly reduced.  the only way around this is to go through and seal up every potential air leakge point in your ceiling before adding the cellulose.

All things considered I'd go with the spray foam.

Mark Eatherton

R U Kidding me???

Foam has an inherently higher R value per inch, and comes with an inherently higher price.



What a person has to be careful about is moisture retention. Some foams (closed cell) have a low permeability rate. THis is good for keeping moisture out, but it has a tendency to trap moisture IN (internally generated) which can create a whole other host of problems.



Each application varies. I think it would be a good idea to get a building scientist involved (i.e. Building Performance Institute energy auditor) and have them advise you as to the proper application, and ventilation requirements.



ME

Robert O'Brien

Another benefit

of foam is the airhandler/ductwork is now in the envelope of the home.

Patchogue Phil_2

Whole House Is A System

Mark is right.  You have to treat the whole house as a "system" where each component and design needs to play nice with each other.



Get a Building Scientist involved.  Also consult with a real slate-roof specialist. Not someone who dabbles a little in slate roofing.  You don't want any moisture getting past the slate and then has no where to "breathe" away -  the spray foam will trap the moisture and your wood roof components will rot.



The following may or may not be best for you...  consider creating an air breathing channel underneath (attic side) the roof deck by using PIC board with standoffs (to create the air breathing channel).  Foamed in place (DIY or by a pro) to the rafters.  Spray foam or spray cells the rest of the void.  Add strapping (perpendicular 1x3 nailers) for your drywall.  You have the height.  If that is a living area,  having a thermal break with MAX insulation will be more comfortable in both Summer and Winter.



No gable vents.  Soffit intake vents and a ridge vent.  The roof breathes underneath the deck and shingles.  The ceiling is then well insulated and comfortable.



But do get a Building Scientist involved.

Jamie Hall

As Mark notes...

every application is different.  When I researched insulation for the building which I supervise, I came up with using both!  A portion of the attic, which is unheated and ventilated, has 12 inches of fiberglass batts laid out on the floor.  The remaining part of the attic, however, has 8 inches of sprayed on (open cell -- Icynene) foam.  The fiberglass batts have a higher R value -- because they are thicker -- but this is almost completely negated by the fact that they don't seal all the ways in which air can get from the heated space below into the attic space.  At some point, the roof will get sprayed and the ventilation to that attic space controlled.  The foam does a superb job.  It was not cheap.



The one major problem with the foam -- which I don't think you have -- is that it can't be used (or at least the application outfit won't give any guarantees) in spaces which have a finish on both sides -- that is, for instance, a sloping ceiling in the attic rooms which is plastered.  First, they won't guarantee to fill all the space.  Second, they won't guarantee that as the foam expands in place it won't pop the plaster off.



On the whole, in your application, I think I would go with foaming the roof -- particularly since you might finish it and also you have ductwork and air handling equipment up there.

Gordy

Foam IT

 For reasons stated previously.

HVAC ends up in conditioned space.

Creating an insulated space for future expansion plans.



I do suggest doing what Phil mentioned about creating a breathable channel in each rafter bay, and then foam over that.  You need to be able to let moisture disipate from the roof plane, or you will rot the wood, and or rafters.Biggest concern besides interior generated moisture is if the roof developes a leak you may never know until some serious damage is done. With a channel built in the roof plane will ventilate, and any potential leakage will make its way to the soffit where it may be noticed if bad enough.



 Slate roofs last decades, but if you ever have to RR roof you may go with asphalt composite shingles. I have read where the spray foam against the roof plane does not effect composite shingle life substantially I tend to disagree I still say you need a little ventilation to help cool the roof plane with that type of shingle.



Gordy

NH03865

Rember the codes

Check with your local code officials, you may not be allowed to have an air space below the roof sheathing because of condensation. The IRC requires the foam to sprayed directly on the underside of the roof sheathing.

2009 INTERNATIONAL RESIDENTIAL CODE

Effective July 1, 2010



R806.4 Unvented attic assemblies. Unvented attic

assemblies (spaces between the ceiling joists of the top

story and the roof rafters) shall be permitted if all of the

following conditions are met:

1. The unvented attic space is completely contained

within the building thermal envelope.

2. No interior vapor retarders are installed on the ceiling

side (attic floor) of the unvented attic assembly.

3. Where wood shingles or shakes are used, a minimum

1/4-inch (6 mm) vented air space separates the

shingles or shakes and the roofing underlayment

above the structural sheathing.

4. Any air-impermeable insulation shall be a vapor

retarder, or shall have a vapor retarder coating or

covering in direct contact with the underside of the

insulation.

5. Either items a, b or c below shall be met, depending

on the air permeability of the insulation directly under

the structural roof sheathing.

a. Air-impermeable insulation only. Insulation shall

be applied in direct contact to the underside of the

structural roof sheathing.

b. Air-permeable insulation only. In addition to the

air-permeable insulation installed directly below the

structural sheathing, rigid board or sheet insulation

shall be installed directly above the structural roof

sheathing.

c. Air-impermeable and air-permeable insulation. The

air-impermeable insulation shall be applied in direct

contact to the underside of the structural roof

sheathing for condensation control. The air-permeable

insulation shall be installed directly under the airimpermeable

insulation.Unvented attic

assemblies (spaces between the ceiling joists of the top

story and the roof rafters) shall be permitted if all of the

following conditions are met:

1. The unvented attic space is completely contained

within the building thermal envelope.

2. No interior vapor retarders are installed on the ceiling

side (attic floor) of the unvented attic assembly.

3. Where wood shingles or shakes are used, a minimum

1/4-inch (6 mm) vented air space separates the

shingles or shakes and the roofing underlayment

above the structural sheathing.

4. Any air-impermeable insulation shall be a vapor

retarder, or shall have a vapor retarder coating or

covering in direct contact with the underside of the

insulation.

5. Either items a, b or c below shall be met, depending

on the air permeability of the insulation directly under

the structural roof sheathing.

a. Air-impermeable insulation only. Insulation shall

be applied in direct contact to the underside of the

structural roof sheathing.

b. Air-permeable insulation only. In addition to the

air-permeable insulation installed directly below the

structural sheathing, rigid board or sheet insulation

shall be installed directly above the structural roof

sheathing.

c. Air-impermeable and air-permeable insulation. The

air-impermeable insulation shall be applied in direct

contact to the underside of the structural roof

sheathing for condensation control. The air-permeable

insulation shall be installed directly under the airimpermeable

insulation.

George_14

thanks

Thanks for all the great feedback.  Sounds like I'm hearing recommendations for the spray foam (with some type of vent for moisture) and to get an energy consultant involved to confirm.  I was ready to go the spray foam route when the energy consultant from the electric company came out and suggested doing the fiberglass on the floor.  They are sending another guy out that will quote that work.  He said they don't do spray foam.  Am I hearing I should get an independent auditor familiar with spray foam?

Thanks

Jamie Hall

Yup...

one's best bet -- if one can find one -- is to find an energy auditor who either sells both kinds of insulation -- or neither one!  After all, if all one sells are Fords, then Fords are the best cars on the road, is it not so?

Gordy

R806.4

 Yes they are worried about condensation in UNVENTED attic assemblies, as this code is pertaining to unvented attics. This is why I'm suggesting to vent the roof plane.



 The demands listed to be met are so you can have an unvented attic. This does not pertain to requirements for a vented attic.

D107

Different Options from BuildingScience.com

RR-1001: Moisture-Safe Unvented Wood Roof Systems

This recent article discusses humidity in unvented attics. I think it behooves any homeowner to read a few of the more than dozen articles on this subject alone from Buildling Science so that when they hire their insulation specialist they can ask the right questions.



http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems/view?searchterm=unvented%20roof%20design

D107

Different Options from BuildingScience.com

RR-1001: Moisture-Safe Unvented Wood Roof Systems

This recent article discusses humidity in unvented attics. I think it behooves any homeowner to read a few of the more than dozen articles on this subject alone from Buildling Science so that when they hire their insulation specialist they can ask the right questions.



http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems/view?searchterm=unvented%20roof%20design

rlaggren

It's all about moisture

We put moisture (water vapor, which is a gas) in the air by living in the house and that moisture happily migrates where ever it can reach. It doesn't need an actual air leak to do this because it can go right _through_  many surfaces, but an air leak makes it happen lots faster. If it contacts a cold surface on it's travels it can condense and turn to water on that surface Often that is bad news for the building because that surface may be sandwiched between materials and out of site in your wall or ceiling and once the moisture becomes water it stays that way and accumulates and wets the material around it. If there is lots (relatively) of air movement to dry up the water, damage is minimal; if there is no air movement (as in a wall sandwich) the materials stay wet and mold and rot begins and grows. Old days, the houses all leaked like swiss cheese so water problems that didn't drip on the owners dried up quickly and nobody much the worse. New houses and houses retrofitted to reduce leaks and improve insulation don't have all that air movement inside the walls and water condensing in those walls or ceilings is a real problem.



So take note of where your living moisture is going to end up. The big challenge for the building envelope is to keep the moisture away from any cold surfaces while keeping the inside either hotter or colder than the outside. Methods vary by climate because houses that use AC more than heating condense on the outside of any cold surface, ie. your air conditioned  _interior_ wall in the summer, while in so-called heating climates moisture is more likely to condense on the inside of your _exterior_ wall in the winter. Current design incorporates moisture retarding layers in all wall and roof construction to keep the water vapor away from cold surfaces; exactly where that layer gets placed depends on your climate. This incidentally is what those gaskets for your electrical boxes are all about.



The other problem is when things fail and you get a roof leak or wind blows rain into your walls through the edges of windows, doors, or even loose siding. And things are definitely going to fail, life is like that. So the building envelope ideally has several layers of protection which will allow water leaks to drain down and out or at least to deposit water in a ventilated space where it will dry over a short time. There are various methods that more or less accomplish this again they vary by climate. The newer codes are starting to reflect these issues as referenced above.



Before spending a huge amount of money, it would be good to read a little and then talk to several people and try to find one who to give you some well informed and current options for insulating your house based on your type of building, climate and on the products available in your area. A few hundred spent ahead can save a LOT of grief down the road.



Rufus