Passive Porch Reno – Part 3

This third post focuses on the interior aspects of the project – specifically insulation and air sealing. Now that we’re in the coldest part of winter I can offer some preliminary feedback on how well my passive porch is performing.

Although keeping the heat in is top priority, I’ve incorporated some architectural elements to bring it all together – a space that is already becoming one of my favourite places to hang out on a sunny winter afternoon.

You can read Part 1 or Part 2 to see how this all started.

Interior work

Once I removed the wood panelling and acoustical ceiling tile I could see what I had to work with. Although the ceiling and walls were insulated to the standard of the time (1970’s) there was precious little space for insulation in the 2 x 4 walls – essentially the 30″ high space below the windows. Everything else was either windows or wood framing.

The illustration (below) shows where the exterior and interior insulation has been added.The previously existing insulation (shown in yellow) only filled a fraction of the wall space. To make matters worse, all those studs and window headers were a great thermal bridge – transferring the outside cold directly into the porch. Along with the single pane windows, and sagging aluminum door, the insulation was virtually useless.

Ceiling insulation

My first priority was to stuff as much insulation as I could into the attic space. I kept a few pieces of strapping in place so the existing batts wouldn’t fall out. Working one bay at a time, I pulled down the end of the existing batts at the front wall, stapled a stryofoam rafter vent to the roof sheathing and added a short batt of R-12 insulation which is all that would fit in that area.

Each ceiling bay had two 48″ long R-12 batts so I could pull down the ends where they met in the centre of the room, add two layers of R-20 batts above them (laid across the ceiling joists) and tuck the existing batts back in place. Once the insulation was added, I removed the remaining strapping and installed new vapour barrier on the ceiling.

Header insulation

In order to minimize heat loss at the window headers, especially along the front wall where there wasn’t any space to add more ceiling insulation, I added 2″ of rigid foam on the inside (the upside-down ‘L’ in the drawing). This band of rigid foam runs along all three exterior walls and insulates the top section of wall above the exterior foam which stopped at the soffit.

My plan was to incorporate this additional insulation into a coffered ceiling with recessed beadboard panelling in the centre. I did a lot of planning work to ensure I would have wood everywhere I needed it to attach all the components for the perimeter coffer structure, paneling, beadboard ceiling and mouldings.

The channel for the rigid foam was defined by installing 2″ x 1 1/2″ continuous cleats running along the wall and ceiling. I used 3 1/2″ screws to attach paneling over the 2” foam on the wall and ceiling – it’s a long way to reach solid wood behind that foam!

(Note: I chose to use wood and MDF for all the interior finishes for the look I want to achieve. You could use drywall for the interior finishing and reduce the cost for a project like this.)

Wall insulation

I figured there really wasn’t much I could do with the walls besides installing new vapour barrier below the windows and air sealing. The one thing I discovered by accident was that you should do air sealing when it’s windy.

As I was framing for the rigid insulation one very windy evening, I was amazed by how much cold air was blasting in through the cracks between the headers and exterior wall studs. I knew the walls were well sealed on the outside so it had to be coming in through the soffit/attic. I caulked every gap between the wood framing from the ceiling down to the window sills and it’s made a huge difference.

I also added 1/2” rigid foam to the framing around the windows since I needed to build out the window casings to match the 1/2” MDF panelling below the windows. Once the window jambs are installed I’ll fill the cavities with more low expansion foam.

Floor

The passive solar consultant suggested I should check under the old composite tile floor to make sure there was no moisture problem – especially since I was insulating the foundation wall and closing off any ventilation. I cut the tile and subfloor into 2 x 4 foot sections with a circular saw and pried them up to reveal the original floor boards. I pried one up and had a look underneath – thankfully the joists were dry and in good shape so I nailed it back in place. Since the floor is effectively insulated at the perimeter by the foam on the foundation exterior I laid a 3/8″ plywood subfloor and will add a pine board floor later. The floor warms up nicely with the rest of the room on sunny days.

Porch performance

Although I still have a lot of finish work to do, most of the insulation and air sealing is done and I’ve had a chance to find out how it’s performing in the cold winter weather. I’ve tracked the temperature ranges over a period of a couple of weeks during the coldest part of our winter including sunny, cloudy and partly cloudy days. I’ve included a few sample days below which seem to be consistent for days of similar weather.

Unfortunately I don’t have any comparison data from before the renovation.

Method

Just a quick note on how I gathered the data:

Outdoor temperature is from a thermometer located out of direct sunlight until very late in the afternoon.
I have two thermometers in the porch itself
– one located on the front wall out of direct sunlight near the floor which is likely the coldest part of the room
– the other hanging near the centre of the back wall shared with my living room, about 6 feet above the floor and also out of direct sunlight which is likely the warmest point.
*The porch temperature in the charts is the median temperature of the two porch thermometers which I figure is the effective temperature at the time.
The ‘differential’ temperature is simply the difference between the outside temperature and the mean porch temperature which is an indicator of how well it holds the heat.

Full Sun – 0 to 20% cloud cover
Variable Sun – 20 to 70% cloud cover
Overcast – 70 to 100% cloud

I’ve still got a ways to go to finish this project but I’m really very pleased with the way the porch is performing in the cold weather. Whenever the mean porch temperature reaches about 20C/68F – usually about noon on a full sun day – I open the door between the porch and living room and let the warm air into the house for the afternoon. It seems to do very well even on extremely cold nights with the indoor temperature hovering a few degrees above or below the 0C/32F mark.