Low R-value walls and high R-value windows: inviting mold?
Hi,
I have an old house with low r-value exterior walls, they are double wythe brick with an air space between the wythes and then plaster on the interrior. I reckon this wall is about R-2. I need to upgrade windows in several parts of the house and I had planned on using triple pane tilt and turn windows with an R-value of 5. I am in Climate Zone 4. My walls will be colder than the windows, am I inviting condensation and hence mold growth on the walls with this planned change?
By the way, I full understand that in new construction, the money should go towards a better wall and not the windows. But in retrofit, the cost of exterior insulation + new siding for the whole house is about 50K or more. The marginal cost of better windows if you have to replace the old ones anyways is a 1K or so. However, if joining bad walls with good windows is inviting mold, I would rather not do that. I appreciate your thoughts.
Best, Matt
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Replies
Matt,
Q. "My walls will be colder than the windows. Am I inviting condensation and hence mold growth on the walls with this planned change?"
A. No. Go ahead and install the new windows without worries.
To add to Martins response, if you have better insulating windows you’ll lose less heat and make your house a little more comfy. That’s an improvement.
Your walls will remain as they are after the new windows are installed. Maybe you can add some insulation to the walls as part of a future renovation project but you won’t have any problems upgrading the windows first.
Bill
Matt,
Almost your exact setup (double brick, no insulation installed triple pane tilt and turns). Works great, not issues with condensation or mold. The rooms are much more comfortable as you don't get the cold air coming off the old single panes.
One thing to watch, if you are replacing old double hungs, the existing wood frame is extremely leaky. I would recommend going removing them and going brick to brick with the new windows.
Thank you. I was planning on going brick to brick. Akos, any installation advice you can give?
Matt,
Install was pretty straight forward. The fixed glass and operable window unit is very easy to remove on these, makes install easy as you are only dealing with a light frame. Mine came with screws similar to these:
https://www.amazon.co.uk/70mm-CONCRETE-FRAME-FIXING-SCREW/dp/B00P55LWIY
Felt way sturdier than typical fasteners here.
They didn't come with jamb extensions, those had to be made onsite which is probably better as nothing is straight in old houses. I managed to reuse about 60% of the original window trim, in retrospect, this was way more work then getting new ones to match the originals.
The pockets for the counter weights left big holes in the inner row of brick, if this is the case for you, it might be best to get one of the smaller spray foam kits to fill the void (I glued the jamb extensions to the window frame then spray foamed around it to fill the voids).
Good luck, let us know how your project turns out.