Pack insulation against a stone foundation

Hey there, time traveller!
This article was published 21/07/2012 (4819 days ago), so information in it may no longer be current.

Question: We have a 100-year old-house in Wolseley and I am planning to insulate my interior basement walls. I have lived here for 15 years and have never had any water or moisture enter the basement through the walls or floor. I know that the weeping tile is no longer functioning. Do you think I can get away without replacing the weeping tile.

— Adrian Challis

My 21/2-storey house was built in 1913. It has a stone foundation in very good shape. The basement is dry for the most part. Occasionally, water will seep in one corner of the basement in the spring and sometimes after a very heavy rain. It does get a musty smell in the summer, but I don’t see any water. I don’t remember having any water come in this year at all, even with all the rain we had. Some of the stone has become exposed, with the parging fallen off in some places. Some cracks in the walls have been repaired sometime in the past. I’ve replaced missing mortar in a few places on the exterior walls.

My question is regarding insulating the walls. I was looking for some good info on the Internet, and it appears the consensus is not to insulate the walls. This seems to be your opinion as stated in a previous column. I’m trying to save some money on energy, not trying to create living space, just a laundry room, storage and a workshop. I already studded, insulated and vapour-barriered about a third of the basement walls a few years ago. I used R12 batt insulation between 2x studs. The insulation is not touching the stone wall, as the studs are out from the wall about an inch. There is some air space between the bottom plate and the concrete floor in some places, due to the unevenness of the concrete floor.

If there is a chance for some air circulation between the interior wall and the stone foundation below the space between the floor joists, would this be sufficient to keep the stone wall warm enough so any moisture wouldn’t freeze? This would allow any moisture to run down the wall and drain out under the studded wall. I already bought five bags of insulation a few years ago, so I can’t return them. Styrofoam SM, as you know, is very expensive to do a whole basement, especially to R12. As I said earlier, I’m trying to save some money, not create an R2000 energy-efficient home. Also, I understand the need for access behind the drywall for possible future wall repairs. The drywall has been screwed on, but the joints and screw heads have not been taped and mudded.

Please share your thoughts on my idea.

— Mark Merrick

ANSWER: With the recent summer heat wave, it may seem like a strange time to be answering these two questions on insulating basements, but this is actually an ideal time to complete the work, while the weather is warm. While both of your questions deal with different issues, they do have similarities in a desire for answers about insulating old stone foundations, and will be addressed together.

You are correct in stating, in the second question, that I am not a big fan of insulating older stone foundations. The main reason for this viewpoint is the difficulty in accessing the interior of these foundations for mortar repairs after the walls are insulated. Almost all of these older foundations will require regular reointing of the mortar between the stones, due to deterioration. If this critical maintenance is not done, the structural integrity of the foundation could be compromised and moisture intrusion will be much more likely. If the foundation is left uncovered, it should be relatively easy to monitor the mortar for damage and repair it as required. I have also read various opinions that insulation can create problems with increased deterioration to the mortar due to freezing, but I’m not sure if that is a legitimate concern or not.

Weeping-tile replacement should not be a major concern if your foundation is dry, and lots of these older foundations that I have inspected never had weeping tiles installedin the first place. To properly replace or install weeping tile, the entire exterior of the foundation would have to be excavated, which should not be necessary unless the basement is leaky or badly deteriorated. If there is no visible seepage through the stone walls, your grading and water management must be quite effective and may be the only item to address on the exterior in the future.

If both of you are insistent on insulating the interior of your foundation walls, care must be taken to avoid a common error identified in the second inquiry. You must never leave a gap between the foundation walls and the insulation, no matter what type of insulation you are using. While this seems like a good idea, it is the worst possible thing you can do to prevent moisture issues. Any air gap left between the insulation and the foundation walls can cause frost to build up on the cold stone walls due to warm air intrusion from the basement in the winter. Because the insulation prevents heat from the home maintaining a relatively warm interior surface temperature on the foundation, condensation is almost guaranteed. Leaving a small gap at the bottom of the wall plate will be hard to prevent and will allow melted frost to leak out and dry, but ensuring the cavity behind the walls is completely filled with insulation will help prevent the frost in the first place.

While it is commendable to attempt to insulate an older foundation to prevent heat loss and achieve, lower utility bills, care must be taken with stone foundations for the reasons given. The main consideration is the condition of the foundation. If it is dry, you should be fine; if it is damp, you should reconsider until the moisture problem is solved.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and president of the Canadian Association of Home & Property Inspectors — Manitoba (www.cahpi.mb.ca). Questions can be emailed to the address below. Ari can be reached at (204) 291-5358, or check out his website at www.trainedeye.ca .

trainedeye@iname.com