Is it okay to insulate against the underside of roof?

[weejub]

To me that means vapor is an issue with closed cell foam. In the climate hear in Wyoming and N. Colorado, water vapor is not an issue if you have some sort of roof vent.

I have become sold on closed cell foam insulation. I used it in my cabin, which is at about 10,000 feet and has about 12 feet of snow around it now. I have 2 vents on the eaves only and I have never had a problem with vapor. It stays nice and toasty and dry in there for the 5 years it's been up.

My brother-in-law's home in Pooler, GA has blown closed cell foam right up under the plywood - like 1 1/2 feet of it. All the houses there have that stuck up above the attic under the roof. A developer did this, so it has to be correct lol

But seriously, it got me thinking about doing it just to keep my attic cooler and warmer to my already insulated attic in my new home.

 

[PabloCruise]

But I think your attic space would/should be ventilated as was the question earlier.

You wouldn't want to mess with the airflow from soffit to ridge cap...

 

[PabloCruise]

Well, if you use a propane heater you will get moisture as a by product of combustion. If not then no worries...

Ah, good point.
It would be nice to have heat in the garage...

 

[45Kevin]

So my understanding is:

You don't need ventilation in the space between the rafters, if it is filled with insulation.

You need a vapour barrier on the warm side of the insulation. This because where the warm air meets the cold air, the moisture in the warm air will condense into water, ergo wetness and likely mold.

The vapour barrier stops the moist warm air from getting to the cold air.

I don't know how it works in very warm climates. Most of the time the warm side of the insulation is on the outside. Does the vapour barrier go on the outside? Or is it because A/C air is so dry that there is very little difference in the specific humidity of the inside and outside air.

Anyone know?

 

[se2000]

But I think your attic space would/should be ventilated as was the question earlier.

You wouldn't want to mess with the airflow from soffit to ridge cap...

If you use a permeable insulation that is true. With 2" of closed cell, there is no permeance so you want no air flow.

I forgot to mention before that there is "paint" that can be applied to the underside of the foam that acts as a fire barrier.

 

[Delancy]

Why insulate a non-climate controlled environment?

 

[PAToyota]

Why insulate a non-climate controlled environment?

To moderate temperatures. Even if you aren't heating the space, it will stay warmer with insulation. And even if you are far enough south to not require heat, the insulation (particularly in the roof) will keep the space cooler when the sun is beating down on it.

 

[Delancy]

To moderate temperatures.

Won't the heat retention afforded by insulating an unconditioned space increase the relative humidity dramatically?

 

[PAToyota]

It is always a balance of factors, but an uninsulated structure will heat up above the ambient temperatures like a car in a parking lot with the windows rolled up on a sunny day. And opening a couple windows or doors will solve the relative humidity issues.

We've taken temperatures over 180℉ from the underside of a roof deck when the outside temperature is in the 90s and you have a "pleasant" breeze. Just illustrates how much hotter it can be inside a building than ambiant external temperatures.

 

[90FJ]

Can't say that I agree, but I've been wrong before.

1. A roof is designed to be water tight and properly constructed and maintained it will be.

2. A roof does get hotter than the walls in the summer, but doesn't matter, at least in colder climates.

3. If the cavity between the rafters/trusses is filled with insulation and is air tight, like the walls, you will need a vapour barrier.

4. Flat roofs are built this way, as well as some sloped roofs with vaulted ceilings.

5. Soffit and ridge vents are needed in unheated attics.

Since the OP is in Colorado I would think that the buiding practices are similar to where I live.

I've been drinking but I'll give it a shot. Roofs are never completely water tight especially when comp shingles and felt are used, this is not an issue when adequate ventilation is provided as the evaporative effects compensate for any any leaks, interior sources or solar driven moisture occur. Hot roofs or conditioned attics are less likely to fail when vapor impermeable underlayments and waterproofing practices are used but the practice still relies on complete faith on the roofing contractor.

Moisture moves from hot to cold.... always. Insulation on the warm side of the sheathing keeps it within the dew point for greater time periods of the year without opportunities to dry to the inside. You will need to pay special attention to complete air sealing which is difficult as air is a huge transportation method for moisture.

I have seen tons of flat roofs fail as well as vaulted ceilings with vapor barriers, as long as temperature levels from inside to outside have the capacity to be warmer and cooler than their counterparts throughout the year moisture will will transfer from both sides. The goal is to prevent impermeable surfaces within the assembly for vapor to condense, closed cell spray foam is not preferable in this application.

The goal is a forgiving assembly to provide for any scenario.

So my understanding is:

You don't need ventilation in the space between the rafters, if it is filled with insulation.

You need a vapour barrier on the warm side of the insulation. This because where the warm air meets the cold air, the moisture in the warm air will condense into water, ergo wetness and likely mold.

The vapour barrier stops the moist warm air from getting to the cold air.

I don't know how it works in very warm climates. Most of the time the warm side of the insulation is on the outside. Does the vapour barrier go on the outside? Or is it because A/C air is so dry that there is very little difference in the specific humidity of the inside and outside air.

Anyone know?

Hot climates experience most vapor flow from the outside in as AC dehumidifies and cools the interior, warm moist air is a threat from the outside finding it's way in. Vapor barriers again are discouraged for all climates within wall assemblies, the focus being air tightness.

It sounds like conjecture but the laws of physics or constant as is the probability for human error when homes are assembled. I too still have things to learn, I applaud everyone in this this thread for their knowledge and the opportunity to debate.

..........

Dude, you're smart.

 

[snobdds]

90FJ, I think you need to drink more often...

 

[45Kevin]

Ok, I have been doing some reading here. http://www.greenbuildingadvisor.com/blogs/dept/musings/creating-conditioned-attic and here http://www.greenbuildingadvisor.com/green-basics/insulating-roofs-walls-and-floors .

This one is a little more techinical and has some good diagrams and charts. http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems


.

 

[PabloCruise]

Ok, I have been doing some reading here. http://www.greenbuildingadvisor.com/blogs/dept/musings/creating-conditioned-attic and here http://www.greenbuildingadvisor.com/green-basics/insulating-roofs-walls-and-floors .

This one is a little more techinical and has some good diagrams and charts. http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems


.

Thanks - Looks like I need to read up on these resources provided - thanks all...

 

[se2000]

Ok, I have been doing some reading here. http://www.greenbuildingadvisor.com/blogs/dept/musings/creating-conditioned-attic

.

I read this article and while it is very good, it is not fully accurate.

Before I conditioned my attic, I talked several times with the iso spray group because I was going to use foam boards under the sheathing along with spray to complete the seal.
I believe the people when they said you could never get a complete enough seal.
Unless you are facing an inspection for R value, the effective insulation is higher than the thickness of the foam.
I went down in calculated insulation but my heat bill was up to a $100 less a month than last year when it was a lot milder winter.

 

[90FJ]

The conclusion is that yes, closed cell spray foam can be used on the backside of roof sheathing in cold climates as long as you have dark colored roofing material, a completely watertight roofing membrane and you complete air sealing from the interior to attic space.

Too many variables and potential failures IMO, especially in a retrofit scenario.

 

[se2000]

who's conclusion?

It was very easy to seal. The foam does the sealing.

I used eps to stuff the eaves. the ridge had screening. As long as you spray so no wood shows from the roof, you are sealed. Far more sealed than the batting covering the ceiling and truss lower member

 

[90FJ]

who's conclusion?

It was very easy to seal. The foam does the sealing.

I used eps to stuff the eaves. the ridge had screening. As long as you spray so no wood shows from the roof, you are sealed. Far more sealed than the batting covering the ceiling and truss lower member

John Straube, Jonathan Smegal and John Smith's conclusion via the research report that 45Kevin posted.

Who are you arguing with? I said that it's probably ok to use closed cell foam in cold climates as long as you control every single variable, sounds like you're quite sure you did. I only added that I personally did not feel comfortable with it but that's just me and my 15 years of industry experience plus SBA, LEED and RESNET energy modeling accreditation. There's better ways to air seal and insulate a home's roof, I certainly wouldn't advocate for batts.

Foam is great when used in the right spots. You can't just trust the foam guy btw, he sells the foam.

 

[PAToyota]

There's better ways to air seal and insulate a home's roof, I certainly wouldn't advocate for batts.

Ok, then what do you recommend?

 

[90FJ]

New construction:

Ducts inside conditioned space
Raised heel energy trusses
Eliminate can lights and lid penetrations
Seal top plate plumbing and electrical penetrations
Blown in cellulose R48
gasketing material at top plate and drywall intersections
sealant around roof and perimeter wall drywall connections

you could also use Dupont's Atticwrap to tie into exterior air barrier: http://www2.dupont.com/Tyvek_Weatherization/en_US/products/residential/resi_atticwrap.html
SIGA makes one as well that you can install under the sheetrock lid although we used it recently in a Net Zero home that did not meet expectations with 1.4 ACH50: http://www.siga.ch/en/product-overview/majpell-detail.html

These are all best practices regardless if you go to a hot roof design. As I said before there are experts far smarter than I that pioneer designs around the detail but they are certainly experimenting and I am more willing to let them fail in the name of research than I am to endorse the practice in layman construction.

I enjoy tinkering with this stuff and am possitive that some of my current assumptions on high efficiency and durability will prove to be deficient in the long run. So again, I welcome debate and am always learning thank you for all that have shared.

 

[PabloCruise]

What is hot roof design?