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Our Victorian house has solid brick walls without any cavity and I would like to retrofit rigid PIR insulation boards to some upstairs rooms.

Traditionally one might build a stud wall against the brickwork, insulate between studs and affix plasterboard on top. But these days we know this typically leads to air gaps if not cut perfectly as well as cold bridging through the studs themselves.

Given an existing decorated room (plastered brickwork with paint/wallpaper), what is the modern preferred way? And is there any reason to remove existing decoration from the wall being insulated?

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    What do you mean by decorated? Plaster direct to brick finish, or something else? Are you restricted from insulating over the exterior? That has many advantages if you don't have historical appearance rules in play preventing that.
    – Ecnerwal
    Commented Aug 24, 2022 at 20:55
  • I think that adhesive is often used to attach foam insulation to masonry walls.
    – Hot Licks
    Commented Aug 24, 2022 at 21:32
  • @Ecnerwal edited... I looked into external insulation but it runs to tens of thousands of <currency units> and I couldn't find someone local in the timeframe.
    – Mr. Boy
    Commented Aug 29, 2022 at 19:03

3 Answers 3

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Sure, you may have small air gaps from inaccurate cutting and some thermal bridging from the studs, but overall, this method will significantly increase the R-value of the room over what it is now.

As noted in a comment, you could just glue the foam boards on. This would eliminate the thermal bridging concern of studs, but you could still end up with small gaps anywhere you need to cut the rigid foam. You would still need to apply something over the top of the foam boards to

  1. protect them from damage and
  2. give a solid surface for attaching anything to the walls.

Whatever you use for this would need to be anchored through the foam and into the masonry wall for stability, bringing back some amount of bridging and possibly leaving unintentional air gaps. This would lead me back to using studs, as they would be the perfect thing to hang that interior finish plasterboard from and they'd give you that solid mount for any wall-mounted furniture and/or decorations.

Any air gaps caused by inaccurate cutting of the foam board to fit between studs can be fixed with low-expansion spray foam to fill the gaps between the foam & studs.

A double-stud wall with offset studs (as is often used when looking for noise isolation) will also break the thermal gap if you're really looking to maximize the insulation. The bigger concern (in my mind) is the reduction in interior floor space, especially if you go with a double-stud wall.

Assuming that "decorated room" is British English for "there are interior finishes on the wall" (American English), well, yes, you'll have to remove those finishes and/or cover them up, depending on the type of finish. If it's just a layer of painted plaster over the masonry exterior wall, I can't see any added value to removing either the paint or plaster. If you've got wall paper, I don't know that attempting to remove it would buy you anything either.

If it's wood paneling/wainscotting or something like that, then you'd probably want to carefully remove it so you can reuse it on the newly build wall (or for some other purpose), instead of leaving it trapped inside the wall forever. I doubt that it would do any harm, though, if it were left in the wall. It would make a nice archeological find for someone in the future who may choose to do a full renovation.

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  • I've been advised that using a stud, you still need a (thin) layer of insulation on top to avoid cold spots which may show as visible lines. I've no idea if that's a small or large risk but seems to make more work, so I was curious about insulation directly on the wall either by adhesive (probably not my preferred option), mechanical fixing (e.g. insulated plasterboard like K18) or battening over the top and screwing through. I think you've largely covered all this
    – Mr. Boy
    Commented Aug 29, 2022 at 19:09
  • Do whatever is necessary to meet your local building codes. Sure, anything that goes through the insulation will be some sort of thermal bridge between exterior and interior but I doubt that there is anywhere in the UK (where you are?) where the temperature difference between outside & inside is so great that it'll cause any sort of long-term issue. You may have 1 extremely cold day every few years where you may get a touch of condensation on the inside, but I doubt it. I would consider the overall increase in insulation value a significant win vs the tiny cold spots.
    – FreeMan
    Commented Aug 30, 2022 at 15:04
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Is there a space between the brick and the decorated wall?

Have heard of blown in insulation, might be too many cons to do it, but they usually make small holes at top and bottom to blow it in.

If no space, then forget about it or will need to go with a stud wall. Prefer fibreglass insulation.

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  • The statement about fiberglass is just wrong. PIR is typical R7 per inch. Fiberglass is about R3.2 per inch. Cellose is about R3.5 per inch....
    – Ecnerwal
    Commented Aug 24, 2022 at 20:54
  • Will edit it out. Thanks for the information.
    – crip659
    Commented Aug 24, 2022 at 20:55
  • brick walls without any cavity
    – Traveler
    Commented Aug 24, 2022 at 22:50
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I'm just learning about all of this, but I believe there is a method called the "warm batten method" where you start with a layer of insulation directly against the internal brickwork (glued on, perhaps?). You then put up the battens on top of the insulation (thus giving you studs, and holding the insulation in place) - this reduces the cold bridge problem. More solid insulation goes between the studs as you describe, hopefully tightly fitted or foam-filled as necessary. Then you must have a vapour barrier and finally some plasterboard.

This approach means that you're unlikely to have an air path from the bricks to the vapour barrier, no matter how janky your insulation fitting is. It also means you've got some decent, solid studs to attach the plasterboard and any subsequent cupboards or shelves or whatever else.

My main sources for this: https://www.homebuilding.co.uk/advice/internal-wall-insulation-guide and https://forum.buildhub.org.uk/topic/8520-warm-batten-insulation/

Lastly, just off the top of my head, I'd suggest getting any existing plaster or woodwork off the wall before putting up insulation. It's doubtful any of that would ever help insulate you, and if anything will be a sponge for moisture or other problems. It'll also give you a chance to fix any problems with the brickwork or pointing.

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