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Author Topic: Air tightness - existing property  (Read 196 times)
MeatyFool
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« on: October 08, 2018, 03:54:01 PM »

I'm revisiting improving my airtightness again, hopefully followed up with action rather than procrastination this time!

I just need to get my head around something - I suspect I am getting hung up between loss of energy through air leakage and loss of energy through insufficient insulation.

Probably should point out right now that I am a DIYer who is capable of simple stuff - I do not feel competent enough for more invasive activity - I struggle to get my head around construction principles!

I see airtightness at two levels: at the inner heated envelope, and outside.

So, for the former, I could prevent leakage via ceiling roses, electrical sockets, under skirting boards, through the floor, through the joint between ceiling and wall(?), where pipes/ducts/wastes penetrate the cavity wall/ceiling/floor, where doors/windows are embedded into the outer leaf etc.  If it was possible to do it perfectly, then the only ventilation would be through opening doors and windows.

However there would still be all the air leakage going on inside the house but outside of the heated envelope.  For example, I have ground floor joists penetrating the inner leaf and ventilation bricks on the outer leaf but just a hole opposite on the inner leaf.  All that air getting under the ground floor can then flow into and out of the cavity at any height, getting into the loft, and from there under the floor of my loft extension (as an example). I do have cavity insulation, but because of the open aspect of the inner leaf, it stops short of the ground floor!

I suspect that the reduction in heating costs of doing a good airtightness job on only the inner envelope is far outweighed by the cold air passing around the house on the other side of the envelope!

So should I rip the house apart where necessary to stop these "hidden" air leaks, or should I accept them but take steps to ensure that I insulate where possible to mitigate their effects?  As an example, rather than try to prevent air getting into to the void between the ceiling and the floor of my loft conversion, I could seal up the box sections either end of the loft rooms with insulation so that the cold air cannot get under.

I suspect I will be told that that last suggestion is a bad idea as you need to allow the timbers to ventilate as well, but is this essential if you subsequently fit a mechanical ventilation to suck/push all the damp air out?  If I am to answer my own question, I believe I need to beware interstitial condensation? 

Hope I am making myself clear!

Meatyfool..
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A.L.
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« Reply #1 on: October 08, 2018, 04:18:19 PM »

hello,

Quote
Hope I am making myself clear!
- Unfortunately not very  Grin

Quote
For example, I have ground floor joists penetrating the inner leaf and ventilation bricks on the outer leaf but just a hole opposite on the inner leaf.  All that air getting under the ground floor can then flow into and out of the cavity at any height, getting into the loft, and from there under the floor of my loft extension (as an example). I do have cavity insulation, but because of the open aspect of the inner leaf, it stops short of the ground floor!

- how can it get in and out of the cavity at any height? and how does the inner leaf have an open aspect? surely the CWI blocks vertical movement of air up the cavity into the loft?


- In general if the air movement is on the cold side of the insulation then air movement is unimportant, unless it is a serious draft blowing at right angles onto open loft roll type insulation, this is referred to a wind-washing and can reduce insulation effectiveness significantly.
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kristen
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« Reply #2 on: October 09, 2018, 05:05:48 PM »

I'm no expert, but one thing I have discovered is that if building is not air tight the exhaust air (through MVHR) will be less than the incoming fresh air, and therefore the heat transfer/recovery will be compromised.

If you have cold rooms and warm rooms you might be able to design air system to "feed" the cold rooms, and extract from the hot rooms (or the other way round, no idea which is more efficient).  Normal approach is to extract from wet rooms, and kitchen (for smells) so would need to consider that too.
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