I have a two storey partial fill block house, built in the mid-1980s in Ireland. It has a partial fill one storey block lean-to extension on its north side, built in the late 1990s. The house has always been cold so I am doing work to improve its heat retention. The north facing extension is especially cold and is truly hard to heat. The house had blown in cavity insulation, done perhaps around 2005. The blown in helped but there is clearly a lot of gaps. I am posting a picture to ask is the former external wall (which remains external higher up) a significant thermal bridge? I have a cheap thermal camera and when we have the heating on full blast the wall might be 15c at shoulder level and slightly less at the top where there is a possible thermal bridge.

I am not a building professional, so please excuse the amateur nature of my sketch.

I) is this a likely cold bridge?
II) For various reasons, we are reluctant to get external insulation, is internal insulation on the former external wall a viable solution?

Thanks for the feedback Tony. Inserting a steel beam sounds like a very expensive solution but I will get a quote and see. Given that it is invasive it suggests you do believe this may be a major thermal bridge worth worrying about?

The south side of the house has a conservatory, with the same lean-to design but as its an oval conservatory I don't think I can use a steel beam unless inverted V-shape beams are available. We don't want to lose space so would something like Spacetherm WL be an appropriate solution there?

Thanks djh for the feedback.

I will investigate both options. As the drawing shows, the extension attaches to a block built wall on the North. However, the extension's gable to the west attaches to an earlier cottage, single leaf stone rubble. As its single leaf is it ok to attach Spacetherm WL to this wall to stop this cold bridge as Spacetherm WL is vapour permeable?

The conservatory is a five-facet shape so it attaches to southern gable end of the house at the top at an inverted V shape. The conservatory heats up well. I guess removing that cold bridge is much more marginal.

Posted By: wholaaThe house had blown in cavity insulation, done perhaps around 2005. The blown in helped but there is clearly a lot of gaps.

Cavitys are notoriously draughty places and if the CWI was only a part fill (badly done?) is it possible that there is (still) a howling gale blowing through the cavity? If you are now heating both sides of the cavity wall there will be increased thermal flow in the air spaces within the cavity which could be sucking heat out, essentially making the cavity wall not much better than a single brick wall.

As a thought closing off the cavity completely in the area of the extension may help, and a possible way to do this would be to inject the ubiquitous squirty foam through drilled holes around the perimeter of the extension.

As a solution to the cold bridge perhaps raise the valley gutter about 40cm and externally insulate the cavity wall under the new much wider gutter giving a longer heat path. If the existing gutter really is 'V' shaped this will not be self cleaning and will be prone to blocking and overflowing both into the roof area and making the wall wet (don't ask how I know!). Making the gutter wide and flat bottomed will remove this potential problem.

Tony's suggestion of a steel beam sounds like major work with SEs needed and considerable expense and disruption - although it would get rid of both the cold bridge and and any air flow within the cavity ('cos no cavity!)

Plenty of room to EWI the house , easier, better and cheaper most likely especially on a DIY basis

Agree

Posted By: wholaaPeter_in_Hungary,
I didn't know you could do spot external wall insulation. How high would it have to be up the wall? If the real problem is just a gale blowing through the cavity shouldn't rooms apart the extension be also very cold?

It is pretty hard to inspect if there are any cavity closers in the extension or that north facing wall but it sounds very plausible there is a gale. All I can tell from crawling in the attic is the gable cavities appear to have cement on them but I can't be sure.

On the south side of the house is a conservatory. It has another cold bridge but it is less disruptive as it heats from the sun well and it is also well-sealed from the rest of the house. I am posting a picture to show the inverted V shape I mentioned that would be suitable for a steel girder.

Yes you can do partial wall external insulation (EWI), you just have to have suitable flashing to keep the weather from getting behind it. For your wall I wold have thought that the EWI could go up to the underside of the window ledge as a logical point.

Does the extension in question have 3 outside wall? typical with extensions, in which case it will be harder to heat than other parts of the house and so any deficiencies will be more noticeable.

If you get a thermometer with a probe (capillary type dial thermometer) then drilling a series of small holes will allow the bulb at the end of the capillary tube to be inserted into the cavity and the temperature can then be measured. This could be done in various places (in the house) as the hole would only need to be 6mm and easily repaired and this would give a good indication of the thermal state of the cavity.

a single brick wall (aka the outer skin or a cavity wall) on its own has a u value of about 3. If you had a brick wall 500mm thick e.g. the outer skin calculated vertically rather than horizontally covered with 100% efficient insulation you would have a u value of about 1. If you put 100mm of EWI on a single brick wall you get a u value of 0.33. So IMO a single sheet of EPS (1000mm x 500mm) laid sideways would reduce the cold bridge to acceptable levels and looking at the photo above this should be doable DIY without scaffold working from the valley gutter.

<blockquote><cite>Posted By: wookey</cite><blockquote>Do you think knocking out a course of the dense block and replacing with the Marmox thermal blocks that has been mentioned could be done reasonably affordably and quickly compared to inserting a steel beam</blockquote>

Yes. I did this for my extension where it abutted the house wall. I assumed this would be a big job, but it actually took the builder a day to do a 5m length of wall. (He really should have taken 2 days to let the cement set IMHO, but nothing fell down so I guess it was OK :-). I used Foamglas Perinsul blocks rather than Marmox, but they do the same job.<a href="https://uk.foamglas.com/en-gb/products/product-overview/foamglas-perinsul" rel="nofollow">https://uk.foamglas.com/en-gb/products/product-overview/foamglas-perinsul</a>
Insulating concrete blocks like quinn-lite are often used for this application too, and I presume are significantly cheaper. Not quite such a good thermal break but still adequate. Do some therm sums to see how much it matters.

<a href="http://wookware.org/pics/house/extension/html/117-image00242.jpg.html" rel="nofollow">http://wookware.org/pics/house/extension/html/117-image00242.jpg.html</a>shows the finished results before timber wallplate was bolted on.

The tricky bit is doing this where the extension has already been built - it may be difficult to get access to the row of bricks you need to remove, but doing one below that and adding a bit of insulation so the wall should give you reasonable de-bridgification.</blockquote>


That is very interesting. Thanks for the tips and pic. My builder seems a bit sceptical but I will see if I can convince him.

Hi Wholaa,

I'll tell you what I would advise my client with this issue...

1. Do exactly as P in H suggested above and stitch drill around the perimeter of the extension where butts onto the house and inject spray foam, to close off the cavity. It's very quick and very cheap. Likely the part cavity insul will be against the orig internal wall, not as you've shown it, hence much colder on extension side of this common wall. This will probably have the greatest impact, as I'll lay a small wager there's cold air blowing around that vented cavity.

2. Remove the bottow couple of rows of slate, and build a new wide gutter across to below the window cill. Form the gutter structure in timber, with say plywood as the gutter floor. Put a rubber membrane as the waterproof surface, cutting into the block to get a seal for the membrane, come down the blockwork wall, say 150mm, then across the gutter floor, and back up under the slates.

Fill the void below, and into the rafter voids with high density wool batts. That's effectively what was suggested in terms of a row of EWI, but probably looks a bit less like an after thought. Also the cheapest option.

Go for lead in lieu of rubber membrame if you prefer, and have the cash...more work in gutter steps required for lead.

3. Forget about adding some millimeters of insulating plaster. Waste of money.