Sistering roof rafters to increase space for insulation

[phinsperger]

Originally Posted by Brian A. MacNeish

Paul:

One of the worst items handled in an attic renovation/conversion is the air leakage at the attic kneewall down through to the ceiling below. This is usually never properly airsealed and the outer attic is usually warm. This system can be made worse by adding gable end vents for the outer attic as they allow more air movement from the attic floor system and house below.

Yes I know that Brian. I didn't say the low insulation level in the cathedral was the only reason, but one of the reasons. Both convection and conduction heat loss contribute to the problem. You must address both to have a well performing assembly.

[Nathan W. Swilling]

An update on the situation for those who are interested. After doing some research, my plan was to use baffles to allow for ventilation from the soffit vents up to the top of the roof and put in some gable vents. But, after going over the roof with some contractors over the weekend, there were some new learnings.

1) The house has no soffit vents. Actually, none of the houses on my street have soffit vents, either (built in the early 1900's when houses were drafty)

2) There seemed to be a consensus opinion that gable vents are not worth anything at all. They don't ventilate, and are more likely to allow snow or driving rain into the house than air to escape (well, I exaggerate a bit).

3) The only good approach for ventilation would be to put in a ridge vent, unfortunately, we have a slate roof, which would make the ridge vent tricky and expensive.

So, the question then became, why worry about ventilation at all? The contractors suggested using foam insulation, directly to the underside of the roof. They said they've been doing it in all of their similar projects. It creates a real thermal and air barrier, and you don't have to worry about ventilation.

It's a dramatic step, and I've read a lot of research on the internet about it. However, has anyone seen any practical experience where this approach has led to problems? One question that no one seems to know the answer to is what happens if the roof leaks? Where does the water go? Do you even know it is leaking?

[Marcel R. Cyr]

Originally Posted by mcyr

Nathan, Larry is giving you good advice and there are different alternatives and solutions, and this would be an alternative;

Because warm air naturally rises, the attic or roof area of your home is your first priority for insulating. Insulation reduces the upward flow of heat, keeping it inside your home longer. That means you’ll stay warmer, and your heating system will not come on as often—reducing your utility costs! In Main's cold climate, insulating existing attics to an R-value of at least .38 is recommended. R-value is the measure of an insulation material’s ability to resist heat flow. It’s measured per inch of material. For example, glass fiber batt or blanket insulation has an R-value of around 3.2 per inch, and the R-value of loose-fill cellulose is about 3.7 per inch. Both of these insulation types are commonly used to insulate attics. Twelve inches of the glass fiber batt insulation achieves R-38, and about 10 1/2 inches of cellulose will do the trick. How much insulation is in your attic? In this case, how much do you need?

As Larry has suggested would give you an R-factor of 19+ 2" of thermax insulation which is 14.4 would equal 33.4. The thermax insulation would have to be foiled faced.

Another alternative would be to just spray the whole underside of the roof with SPF. This polyurethan spray insulation closed cell would provide you with a 8 R-factor per inch. So again r-19 + 21 @3" = 40 Just add another inch and your at R-48 and what is nice is you do not need to add a vapor barrier. Using the open cell Icynene spray, you would get a lot less R-factor and would need the vapor barrier as well.

It is kind of you get what you pay for.

Hope this helps.

Marcel

Since this product is closed cell, it will not allow water through and any leaks in the roof would take a long time to show up, so therefore, needs to be checked on a regular basis.

Unventilated space for attics is growing more popular all the time.

Just my opinion.

Marcel

[Peter C. Russell]

Nathan, I agree with Marcel.

I have renovated many vintage home and I suspect the reason for no soffit vent is due to timber framing which will not allow for any venting. If this is the case then more than likely the ridge pole would not allow it either.

With that said your alternative are, gable vents or spray foam, the latter is probably the best choice.

If this is the same project you asked advise for before you really should seek advise from a design professional. This way everything is documented and done right will add value to your home, not take it away.

[Per-Ola I. Selander]

Originally Posted by Nathan W. Swilling

An update on the situation for those who are interested. After doing some research, my plan was to use baffles to allow for ventilation from the soffit vents up to the top of the roof and put in some gable vents. But, after going over the roof with some contractors over the weekend, there were some new learnings.

1) The house has no soffit vents. Actually, none of the houses on my street have soffit vents, either (built in the early 1900's when houses were drafty)

2) There seemed to be a consensus opinion that gable vents are not worth anything at all. They don't ventilate, and are more likely to allow snow or driving rain into the house than air to escape (well, I exaggerate a bit).

3) The only good approach for ventilation would be to put in a ridge vent, unfortunately, we have a slate roof, which would make the ridge vent tricky and expensive.

So, the question then became, why worry about ventilation at all? The contractors suggested using foam insulation, directly to the underside of the roof. They said they've been doing it in all of their similar projects. It creates a real thermal and air barrier, and you don't have to worry about ventilation.

It's a dramatic step, and I've read a lot of research on the internet about it. However, has anyone seen any practical experience where this approach has led to problems? One question that no one seems to know the answer to is what happens if the roof leaks? Where does the water go? Do you even know it is leaking?

Nathan, the contractors do not really care about your house after a few years. They want to sell you a product/service and be on their way. Sad but true. Ventilation IS necessary, albeit maybe less so if you have a slate roof that "breathes" towards the outside (compared to torch down and other "sealed types").
If I were in your shoes, I would create soffit vents somehow, make sure you get 1"-1.15" of space between underside of deck and insulation (regardless of what kind). In order to increase the "depths", furr out existing 2x6" with a 2x2" or a 2x4" screwed and glued in place. It both allows for added space as well as stronger frame members. But, before doing this, allow the lumber to dry inside your house so it reaches a humidity level that is comparable to what is in the rest of the frame lumber.

If you plan on using fiberglass batts, make sure the ventilation baffles does allow the insulation to "breathe" into the moving air. Stay away from "Durovent" or other type of manufactured baffles out of styrfoam. Water vapor will only condensate on these baffles and eventually soaking the insulation. Custom made baffles out of "peg board" or some type of "mesh"/netting would be better. Only purpose is to hold insulation away from roof and maintain the needed air space.

Needless to sya, do not skip on the vapor barrier. Do NOT trust the paper on the insulation. Put on a proper PVC or similar type barrier with good overlaps at the seams.

The more I learn about American roofing systems, the more I am in favor for clay or concrete tiles that allow the roof to properly breathe. In (native) Sweden, there are very few problems with these roofs, in rain, or in snow, and they last 50+ years. Where I can see them less suitable are in eartquake prone areas since these roofs are very heavy (but still pretty common in CA).

[phinsperger]

Originally Posted by Nathan W. Swilling

One question that no one seems to know the answer to is what happens if the roof leaks? Where does the water go? Do you even know it is leaking?

Exactly. A leak will rott the wood. If there is a closed cell foam underneath it may prevent the water from entering the house but consider that the wood is there to provide support to a very heavy roof covering. In all likely hood your fisrt indication that there is a roof leak will be when a slate tile slides off the roof because the wood around its attachement is rotted.

[Peter C. Russell]

Originally Posted by Per-Ola I. Selander

Nathan, the contractors do not really care about your house after a few years. They want to sell you a product/service and be on their way. Sad but true. Ventilation IS necessary, albeit maybe less so if you have a slate roof that "breathes" towards the outside (compared to torch down and other "sealed types").
If I were in your shoes, I would create soffit vents somehow, make sure you get 1"-1.15" of space between underside of deck and insulation (regardless of what kind). In order to increase the "depths", furr out existing 2x6" with a 2x2" or a 2x4" screwed and glued in place. It both allows for added space as well as stronger frame members. But, before doing this, allow the lumber to dry inside your house so it reaches a humidity level that is comparable to what is in the rest of the frame lumber.

If you plan on using fiberglass batts, make sure the ventilation baffles does allow the insulation to "breathe" into the moving air. Stay away from "Durovent" or other type of manufactured baffles out of styrfoam. Water vapor will only condensate on these baffles and eventually soaking the insulation. Custom made baffles out of "peg board" or some type of "mesh"/netting would be better. Only purpose is to hold insulation away from roof and maintain the needed air space.

Needless to sya, do not skip on the vapor barrier. Do NOT trust the paper on the insulation. Put on a proper PVC or similar type barrier with good overlaps at the seams.

The more I learn about American roofing systems, the more I am in favor for clay or concrete tiles that allow the roof to properly breathe. In (native) Sweden, there are very few problems with these roofs, in rain, or in snow, and they last 50+ years. Where I can see them less suitable are in eartquake prone areas since these roofs are very heavy (but still pretty common in CA).

Per, excuse me but I for one take exception to your comment that contractors do not care about their clients, you sir are very wrong!!!!

I will admit there does exist unscrupulous contractors but I doubt you will find any on this message board.

As far as American building practices goes you should do more research before you post, or perhaps read the entire thread before you speak. This conversation has been going on for a while and we established that this house was built in the early 1900s, so being familiar with this type of construction more than likely you cannot install proper /ridge vents.

Spray foam is a proven product and as I have said before this project needs a professional to sort out all the details. Many here are only speculation on something we can't even see.

[Brian A. MacNeish]

Originally Posted by Per-Ola I. Selander

Ventilation IS necessary, albeit maybe less so if you have a slate roof that "breathes" towards the outside (compared to torch down and other "sealed types").
If I were in your shoes, I would create soffit vents somehow, make sure you get 1"-1.15" of space between underside of deck and insulation (regardless of what kind). In order to increase the "depths", furr out existing 2x6" with a 2x2" or a 2x4" screwed and glued in place. It both allows for added space as well as stronger frame members. But, before doing this, allow the lumber to dry inside your house so it reaches a humidity level that is comparable to what is in the rest of the frame lumber.
I have been working somewhere in the residential energy services/consulting/inspecting fields since 1977 and have seen thousands of 1.5 & 2.5 storey cathedral style steep roof cavities that were blown full with rockwool, fiberglass or (my favourite) cellulose. Most of these homes were blown in the 1950's-1980's. These roof slopes did not show signs of deterioration by moisture. Why? They did not have many or any holes in the sloping ceilings to let warm house air containing moisture into the cavities and the houses and due to general air leakage had fairly dry air. We can reproduce similar conditions in new homes/renovations as we now have the knowledge and techniques to prevent air leakage into wall/ceiling cavities by using air barrier systems and we can control interior moisture levels by cleaning up moisture problems and installing proper interior house ventilation.

There has been a movement afoot for 10-15 years to allow unvented cathedral cavities and roofs that has been recognized by some major building organizations incuding code producing orgs and even some roof shingle manufacturers.

https://www03.cmhc-schl.gc.ca/b2c/b2...00260000000011

http://www.buildingscience.com/docum...r-all-climates


If you plan on using fiberglass batts, make sure the ventilation baffles does allow the insulation to "breathe" into the moving air. Stay away from "Durovent" or other type of manufactured baffles out of styrfoam. Water vapor will only condensate on these baffles and eventually soaking the insulation. Custom made baffles out of "peg board" or some type of "mesh"/netting would be better. Only purpose is to hold insulation away from roof and maintain the needed air space.

This is a scare tactic!! This may happen if the house is run at very high humidities and the vapour retarder/barrier mentioned below is simply overlapped and not sealed to be the air barrier now required by Canadian codes (since 1985-1990)

Needless to sya, do not skip on the vapor barrier. Do NOT trust the paper on the insulation. Put on a proper PVC or similar type barrier with good overlaps at the seams.


From the former American Plywood Association (APA), now The Engineered Engineered Wood Association: http://www.contractorsinstitute.com/...20Flashing.pdf


See pages 9-10: "While polyethylene sheeting makes a very good vapour retarder, it is relatively difficult to install. In most cases, the use of polyethylene is not necessary, even in very cold regions. Ordinary interior latex paint applied over drywall can provide sufficient vapour retardent properties."


This last statement has also been said by researchers at the Canadian National Research Council, The US Corps of Army Engineers, Building Science Corporation (BSC), Advanced Energy Corporation.


From Can. NRC Building Digest #175 (March 1976):
http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd175_e.html


"Air leakage is now considered to be the prime cause of most condensation problems in walls and roof spaces. If, therefore, a building can be made tight against air leakage it may not need a vapour barrier, as defined. On the other hand, if there are openings that permit air to leak from the warm side to the cold side of the insulation, adding a vapour barrier (even of zero permeance) that does not seal off the openings will be useless."


From a BSC document: http://www.buildingscience.com/docum...por%20retarder


"Keep in mind however, sheet polyethylene on the inside of building assemblies in cold, mixed-humid, marine, hot-dry and hot-humid climates is not generally a good idea; drying of building assemblies in these climates occurs to the inside as well as to the outside.
Interior drying is typically necessary in air-conditioned enclosures. In other words, interior vapor barriers such as polyethylene and vinyl wall coverings should never be installed in air-conditioned buildings – even ones located in cold climates."


The more I learn about American roofing systems, the more I am in favor for clay or concrete tiles that allow the roof to properly breathe. In (native) Sweden, there are very few problems with these roofs, in rain, or in snow, and they last 50+ years. Where I can see them less suitable are in eartquake prone areas since these roofs are very heavy (but still pretty common in CA).

Food for thought!!