09 December 2010 ~ How to Prioritize the Insulating of a Historic Home

Below is an email to someone who is the recent owner of an 1800s house located in a historic district. The house had a Home Energy Audit recently in which it had a blower door test and a lot of air sealing done (in the obvious and easy places around door, at electrical outlets and any exterior wall penetrations) and then the blower door test was repeated. The air sealing measures improved the blower door test by about 18%- not bad.

But the final blower door test resulted in an Air Changes per Hour number of 10.45 ACH @50Pa. (If you recall a recent post on this blog the LEED registered house in Rowayton achieved a 1.24 ACH @50Pa. Another recent home we did in Westport (a well done house but not LEED level) got a 3.52 ACH. For the record a Passive House would need to be 0.6 ACH and the average decent house is about a 5.0  or 6.0 ACH. SO 10 is not great but it is a very old house. To improve this number and get a better thermal envelope for the home, this was my advice:

Hi Rudy,

It was very interesting analyzing your house and talking about your blower door test last night. 

If it was my house I would prioritize things this way:

(of course as money comes available...perhaps when our lovely economy changes)

1. Insulate the attic as much as possible (with cellulose or sim. If you don't use the attic for storage then fill it up! Leave roof vents of course. Try for a minimum R50 which is about 11".

2. Insulate and seal the ceiling/floor between your basement and the first floor. Definitely use a closed cell foam for this. This involves 'filling' each joist bay (maybe 4"-6" thick continuous) with spray-on closed cell foam. Your house is balloon framed so you will have to block the tops of each joist sill box (the area between the top of the foundation wall /the sill plate and vertical exterior wall rising above) to keep the foam where you want it (in the joists bays at the top of the foundation wall..not up in the walls.) This will not only give you a great R value to keep house warmer but it will reduce to almost zero the air and moisture infiltration from the basement into the conditioned house. In the end it's really all about air infiltration - as I am sure they told you.

3. Since you do not have the original historic windows on the house- and instead have wood with triple track aluminum storm windows,  I would recommend - eventually - replacing windows. The Historic board would never let you put the windows that you currently have on the house today. Replacing the windows with a U.28 or lower double pane window with true historic lite patterns and muntin design will not only greatly improve your energy efficiency but will also bring the house back closer to its true historic aesthetic. It is actually getting easier every day to get some more highly energy efficient windows that actually mimic the original window designs and satisfy historic review boards.

Also, by getting windows with a U-value of at least U.28 you will achieve a  better thermal envelope overall and you will earn the federal tax credit (for as long as it lasts.) and be sales tax exempt.

There are windows out there that get U-value as low as .14 readily available now (awesome!) and I happen to know they are working hard to make these things look good too so if you wait a year or 2 you could actually get some fantastically insulative windows that look good too. Replacing windows either way double or triple or 'awesome' will run you probably $30,000 - $40,000 or more depending on a lot of things. (Yes if you go 2 pane vinyl you can get much cheaper but I don't think historic will let you.)

(Financially, you kill 2 birds with one stone by replacing windows - efficiency and aesthetics. Since it is easier to replace windows than to retrofit the walls with insulation, it is perhaps not a horrible idea to do the windows if you- financially - can only do one. Historic homes sell better if they look more accurately 'historic'. I also am a believer in aesthetics. I believe that houses that look and feel beautiful and 'quality' get treated better, resale better and are cared for longer. And a long lived house is a very green thing.)

4. As the ultimate move- take off your vinyl siding  and possibly your exterior sheathing and insulate the walls of the house from the outside. You can do this the easy way or the hard way. The hard way means taking off the sheathing (the boards behind the siding) and adding insulation to the wall cavities from the outside. This is nice because you do not disturb the interior plaster. However, because you have interior plaster walls you have to be careful of what you put in the wall cavity. There are dangers of moisture passing from inside to the outside and condensing in the wall cavity- causing all sort of moisture problems. Blown in cellulose truly depends on a vapor barrier to keep it from getting soggy inside the walls. Most old houses don't have vapor barriers. Unless you can really stop the air infiltration through the wall, this is not a good way to go. Foam insulation would provide a form of air barrier and also has the qualities of a  vapor barrier but many people feel it is too untested for use in old homes. This is a route I would like to look into more thoroughly.

Your house is balloon framed so blocks would have to be introduced to prevent the insulation from simply falling into the basement. If you have already foamed the floor between first floor and basement than this is taken care of at the bottom.

The easy way to insulate from the outside, would mean leaving the sheathing on and adding insulation that wraps the house. Consider putting one layer of 1/2" to 1" polyiso foam wrapping the house. Tape all the seams  and carefully tape or caulk every wall penetration- including window and door openings - to create an exterior air barrier.  This will greatly reduce the air flow thru the walls. Doing so will  greatly enhance the functional R-value by significantly reducing air infiltration, plus it adds a bit of thermal value. You can shoot the siding right through the new exterior insulation and only add 1/2" to1" thickness to the walls. It could work with the current detailing and add a lot of insulative value to the house (air infiltration and thermal bridging being greatly reduced....)

Insulating the outside walls- in the end is the hardest and most 'dangerous' part of making your house energy efficient. Moisture is much harder to control in older, existing houses than it is in new construction or more recent retrofits. The most bang for you buck will come, anyway, from taking care of attic and basement heat loss and stopping air infiltration wherever possible.

I think if you do the first 2 for now you would see a very good result. If you only ever do the first 3 I think you would be doing great. If we found a way to do all 4 your house would rock!