Insulate open rafter roof

We have a second lake home with an exposed 2X8 dougfir rafter ceiling. Shealthing is 1X10 pine boards with shingles directly on top of that. The building was built in 1956 and intended as a summer cottage but we want to use it year-around. I want to insulate the roof. I'm thinking of removing the shingles and applying 2" foam board, re-shealth with 1/2 OSB and shingle over that. I've also thought about installing an additional 1" on the inside between the rafters and covering that with beadboard and paint to brighten the ceiling while allowing the rafters to show through. Anyone else have better idea?

It seems like it would be a lot easier and more effective to add 6 " fiberglass insulation to the underside of the roof joists and cover with drywall or some other covering to fit your needs. You'd want to add vents to the soffits and ridge.

You probably need R38 or R40 for your ceiling/roof. Depending on what kind of foam insulation you install, you get anywhere from R2 to R6 per inch. Fiberglas bats are usually around R3-R4. So your first challenge is to get enough insulation into your roof system to avoid heating the outside.

Your second challenge is to ensure you have an appropriate vapour barrier preventing leakage of warm interior air into the cold spaces, where water vapour will condense and create moisture.

Unless you can construct your roof assembly as a sealed unit (ie, completely impremeable to water vapour), you also need to have adequate ventilation to dry out whatever moisture gets trapped in the roof assembly.

Bottom line is that converting a structure that is not intended to be heated or insulated into one that is, is a much bigger job than you are anticipating. I would suggest consulting with a qualified building engineer to understand what changes have to happen. Don't forget that you also have to deal with converting the windows, walls, and maybe even the flooring if you're changing the use from a 3-season cottage to a 4-season home. It can be done, but it is not a trivial undertaking. Better you get the uh-oh reaction now rather than after you've spent the effort and money and found out that there are condensation issues all over the place.

Welcome BusyFlyin:
I'm surprised PGriz didn't mention this method.
Cover the roof with 2" (or more) foam board and install steel roofing directly over the foam (forget the extra sheathing). You may want to remove the present shingles to reduce the weight on the roof but the most important part is to get the proper size screws. You want them to go into a solid wood base but not penetrate the inside of your ceiling.
Glenn

glennjanie, the reason I didn't mention the method is because it wouldn't actually deal with the issues that I brought up in my post.

Yes it can be done, but it won't provide adequate insulation, nor will that method introduce a good vapour barrier to eliminate condensation, or will it provide for ventilation. Busyflyin did not indicate whether the use of the cottage year-round was to be occasional, or frequent. That would determine how stringent are the requirements for vapour barrier, insulation, and ventilation. At this point, not enough information.

In order to do it correctly, you need alot of info concerning many things.
Is it a crawl space underneath.
Are there single pain windows.
Is the heat propane or gas. This is a big one.
How much do you use the cottage.
Do you need it just warm or are you going for the 4 season , live there someday solution.
These will get you started, then we may be able to figure out the best solution. Just throwing insulation around an uninsulated house can cause many serious issues with condensation, backdrafting and mold.
But I do like the idea of insulation with metal roofs, works great most of the time, we just need more detailed info.
Post back and ask anything you want....we are her for ya.

Thanks for all of the feedback. Correction... the rafters are 2x6.. 50 years old and straight as an arrow. See the underside of the roof (ceiling). I'm not exactly sure what would be required to protect against moisture, but if i'm understanding the issue, the moisture would build between the top of the foam board and the sheathing?.. given that it (condensation) occurs when the warm moist air hits the cold air. What changes if I leave the shingles on? yes, we intend for this to be a 4 season cottage.. Michigan where temps will hit zero or below a few days of the winter. A typical winter day is going to be high teens to low 20's. The heat is a propane wall furnace, but we will primarily burn wood in a fireplace insert stove. As for the fiberglass on the underside, the walls are only 7', I think, and covering the rafters may cause the perception of a smaller building and would certainly cover the pine ceiling. At this point, I am trying to understand the issues and find the best solution for us. I appreciate your continued feedback. Will figure out how to reduce the photos I have and send them later. To answer the other questions; slab, not crawl. Single pane now, but upgrading to thermo panes in the spring. We will use the cottage every weekend. thanks!

OK... here are two photos of the underside. Let me know if the detail is not clear enough.

Busyflyin, you have a beautiful cottage. If that was my place, I'd fill in the space between the rafters with blue styrofoam (5 1/2" should give you about R25-R30), install a vapour barrier across the rafters along the bottom edge, then nail on pine planks as the visible ceiling surface. This way, you get the wood look, and still have insulation in the ceiling. On the outside, I'd get rid of the shingles, install another two inches of blue styrofoam (giving you another R8-R10), vertical battens (2" high) then horizontal battens, then a vertical panel metal roof (with concealed fasteners). The vertical battens will allow vertical movement of air, while the horizontal battens will support the metal roof panels. You'd probably want to put a synthetic membrane like Triflex-30 over the vertical top styrofoam to catch any condensation moisture which may form on the underside of the metal roof and drip down. You would need to provide for venting at the top of the panels and along the lower edge. This way you would have a well-insulated, reasonably well ventilated roof assembly, that still retains a lot of the appeal of the wood interior finish. I like metal both for the looks and for its durability.

If you don't go with a metal roof, then putting on a plywood deck over the vertical battens will give you a good nailing surface for shingles.

That's what I would do. You still need advice regarding the walls, windows, floors. As I noted earlier, changing from a 3-season (essentially unheated) cottage to a 4-season, fully-heated cottage means that you have to re-engineer your dwelling for its new purpose.

On the other hand, as long as you don't mind heating the outdoors and having drafts, you could just use the cottage without making many changes. You'll have lot's of heat loss and you will have condensation on the inside walls and ceilings. But if the cottage is large and the number of people inside is small, then the amount of water vapour that will be added to the inside may not cause a huge condensation issue.

pgriz; I'm not sure I want a steel roof... we just build an adjacent garage and used asphalt shingles... so I want to match the look. Also, aslphalt is more economical. So if I understand you, on the top side... 2" blue foam followed by verticle battens to allow for ventilation, then osb, then standard roofing w/ridge vent. This would allow for sufficient ventilation? The walls are no issue for me. We intend to gut the building and wire, insulate, and such, just as a new home. Thanks for all of your advise. It's well taken and appreciated.

The code for minimum air channel is 2 inches. So by putting 2 inch vertical battens, you satisfy that requirement. However, don't forget that the purpose of ventilation is 1) to dissipate residual heat leaking past the insulation, and 2) to dry out any condensation that may be forming. If the vapour barrier is doing its job on the warm side of the insulation, then there should be little moisture or condensation to worrry about. Also don't forget the insulation BELOW the roof deck, as in your situation, you will need it.

How does a SIP work? Foam core with OSB on both sides. Do they shingle directly on top? What serves as the vapor barrier?

Busyflyin, we're not talking the same thing. First of all, a SIP panel does not give you anywhere near the R38-R40 of insulation you need in your roof system. So if you install a SIP panel over vertical battens, you've created a nailing surface, but you have neither a vapour barrier, nor enough insulation, and the inside surface of your roof deck (the part you see from the inside) will be at or below freezing in winter and any moisture you have in the inside air will condense there. Not a good scenario.

Let's look at this step by step. Your current situation is as illustrated in Image1.jpg.
E is your current deck, G are the 2x6 rafters and F is the space between them.

I am proposing that you fill in the space between the rafters with blue styrofoam, install a vapour barrier under this (across the rafters), and then install wood planks or boards across the rafters to give your the "wood" look to your ceiling. This is shown in the following image.

Note that the inside space between the rafters (F) is now filled completely. Then you install the vapour barrier below this (H), and nail your wood planks to the underside (I). So far, you've got the vapour barrier that prevents moisture from the cottage interior air from reaching the cold parts of the roof. However, note that the building code in some jurisdictions requires that if the space between rafters is filled with foam insulation, that a fire-proof material (like gyproc) is used as the covering layer. So you need to check the local codes.

Next, on the outside, you add the two inches of blue styrofoam insulation, place 2x2 inch purlins (oriented vertically) over these, place the OSB on top of the purlins. This is shown in the following image.

The two inches of external insulation (D) is laid on top of the stripped deck, then the 2x2 purlins (C) are laid on top, and then the plywood or OSB (A) is laid on top of these. You need to hold the whole assembly together with either very long nails or screws (J) that penetrate into the original rafters.

This assembly then gives you the vapour barrier (H) on the warm side of the insulation (F & D), a ventilated air channel (B), and a deck surface (A) on which you will need to install the underlayment and shingles. The combined R-value of the insulation should be about 36-38, which should be sufficient for your location and usage. The two inches of air space is the minimum required, but you therefore meet the minimum. In winter, the freezing point of the roof assembly will be located inside the insulation, and therefore you should not have problems with condensation (AS LONG AS YOU HAVE THE VAPOUR BARRIER ON THE WARM SIDE OF THE INSULATION, AND THE BARRIER IS INTACT).

In effect, you will be constructing a sealed, ventilated cathedral ceiling.

Anything less than this will give you problems, in my opinion.

Basically you have showed them a cold roof scenario. These are also in my opinion the way to go. The issue with the Structural insulated panels I did not understand ...you are saying you need to ventilate these panels?
I have run into delamination and ants with these panels failing so I do not recommend their installation, but they do not need ventilation in my opinion ,or the testing I have read through.
www.buildingscience.com has many solutions to these questions also. That was why I wanted to know the particulars or what you are trying to accomplish as an end result.
Another solution is to install a 2x6 below the existing rafter and install fiberglass with paper faced vapor barrier. With proper- vent Styrofoam channels under the roof deck. This is the way conventional cathedral systems are installed now.
And yet another is to install a closed cell spray foam insulation directly to the underside of the deck, with no ventilation. These are expensive products, and still new to residential construction. They have been used in commercial jobs for decades, but still not actually proven themselves in residential ..to me anyway.

I like Pgrizs solution. Continuous Insulation, no breaks.

Pgriz; what do you believe will happen if only address the underside as you described and I do not add the materials to the top of the roof? I'm concerned about the cost of the project. Thanks for the great graphics.

Busyflyin, you will then have an under-insulated and non-ventilated cathedral ceiling roof. Snow on top of the roof will melt due to the heat loss from the roof, run to the eaves and freeze, forming ice dams. The asphalt shingles will be persistently wet and in my experience wet shingles deteriorate faster. Without the ventilation, if you have any moisture in the roof system (whether due to condensation from incomplete vapour barrier or leakage from above), the moisture is trapped and will cause rot. A cathedral ceiling will work IF it is sealed, and if the amount of insulation is sufficient.

Got it. Thanks for all of the help with this. I don't know what I will do, but I know what I should do.. Have a great holiday!

Hi, After reading this topic with much interest I decided to just tack my question on the end.
I have an almost identical situation with an old cabin I just purchased in California in the Sierra's around 6300 feet.
After reading this post I have decided that it would be too expensive for me to make the drastic changes needed to insulate my roof. I don't intend to make it a 4 season cabin but just wanted to make it a bit warmer in the early spring and late fall. The only heat is a wood burning stove, I was also hoping that the insulation in the roof would keep it cooler in the summertime.

I would like to insulate the floor which is partly resting on a large granite boulder and partly on stilts. So I was wondering what is the best way to do this. I will attempt to add a photo of the underside of the floor so you can see what it looks like now.
Thanks

busyflyin said:

pgriz; I'm not sure I want a steel roof... we just build an adjacent garage and used asphalt shingles... so I want to match the look. Also, aslphalt is more economical. So if I understand you, on the top side... 2" blue foam followed by verticle battens to allow for ventilation, then osb, then standard roofing w/ridge vent. This would allow for sufficient ventilation? The walls are no issue for me. We intend to gut the building and wire, insulate, and such, just as a new home. Thanks for all of your advise. It's well taken and appreciated.

Click to expand...

Asphalt shingles are only economical in the short run. Our modular has shingles that were supposed to be guarenteed for 25 yrs..the have lasted less than 20 and this is a premium Palm Harbor and we are not the first owners.

To Pgriz a huge THANK YOU. I was planning a much cheaper solution and believe it would have cost me much more in the long run. Thanks to you I now know how to do it right the lst time.