Mixing internal and external insulation

D

David_Dublin

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We're planning extension and refurb. Refurb will include complete re-wire, insulation upgrade, ground floor under floor heating and possibly MHRV.

We're not able to insulate the front of the house externally as it is period/redbrick so will need to do so internally, however we intend doing gable end and back of house externally.

Is there anything specific I should be watching out for with architect/engineer when mixing these two approaches? My understanding is that, as long as one overlaps the other, that this "should" result in reasonably good overall insulation.
 
You won't be able to avail of at least some of the SEAI Better Energy grants with that approach, though your mixed approach might be granted an exception. For EWI to qualify, it must fully cover all exposed walls. Further details [broken link removed].

How much overlap are you proposing? The mix will introduce thermal bridging issues.
 
Hi Leo,

Thanks for the response.

Regarding overlap, I'm hoping very little, I need to confirm with architect/engineer first.

See attached pic of the house. The garage is set back about 3 feet from the front of the house, i.e. it's flush with the upstairs. My hope is that I won't have to overlap at all. We'll knock the garage and build the extension up to it's footprint at the front, and wrap the extension and gable end, and the entire back of the house in external. Only the front of the house would be internally insulated.

I don't know at this time if there might be planning issues around bringing the external insulation all the way to the front on the gable end above the extension.

Off the top of your head, would that give rise to thermal bridging issues?

Thanks again,

David
 

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This Wikipedia article will give you a background. Without a full wrap, you will have some degree of thermal bridging, which will lower the overall effectiveness, hence the grants don't usually apply. It'll still be better than the present, but at the junction of the internally insulated front wall and the side wall that is externally insulated, there will be a temp difference that could lead to for formation of condensation / mould.

Re planning, again, from the SEAI FAQ:

 
Hi Leo,

Been reading up a fair bit on this and don't see an obvious solution to the issue that would be 100% guaranteed to work, other than to internally insulate throughout, which would be a shame given how much space a high spec insulation would take from the house, particularly on smaller rooms.

D
 
Yeah, I haven't seen much either. Might be best talk to a few installers who have carried out similar works (must be common enough with all the red brick in Dublin), and see what they did and perhaps see if you can talk to some previous customers in similar houses to yours to see how much of a problem it is in reality.
 

Thanks, yes, I must have a wander around the neighbourhood and see what others have done, good suggestion!
 
Out side insulation is a joke
You turn on your heating system rads heat up the air in the room ,
Heated air dissipates into the walls then through the brick/block until it meets with insulation

How much heat would you think you would need to generate before external insulation does its job
Versus interior insulation
and not foam backed plasterboard
Use full 8x4 sheets of kingspan or similar all joints foil taped and covered with plaster board
You will not get better bang for buck the only downside is taking a few inches off the room size
If anyone tells you different there selling you something have vested intrests or don't understand how insulation works
 


It doesn't disappear, your walls become a thermal store that help regulate the temperature within the house. When you turn the heating off, the energy stored in the walls gradually dissipates back into the room significantly slowing the rate at which the room cools. An internally insulation room will heat up more quickly as you say, but the converse is it will also cool more quickly as the only thermal stores are the objects within the room.

A certain amount of heat will be lost to the outside through the external insulation, but unless you're prepared to loose 100+mm from each internal wall, you'll lose more heat through the internal insulation.

Internal insulation can also lead to serious problems with mould as a result of moving the condensation point within the walls. Every electrical point is going to penetrate the vapour barrier. Take a read of this paper for more.
 
Thanks for the input all. how have people effectively avoided mould issues from internally insulated?
 
Room temperature is somewhere around 20 degrees give or take so the heat that your block walls absorb will carry on to the outside where its colder

Good point on the mould , But providing you have a double leaf exterior wall
Ie. Block on edge -cavity- skin of brick or similar build up with timber frame ect...
You would have nothing to worry about if your room is properly vented .

All this air tightness stuff that has been introduced to construction in the last few years is going to cause a huge amount of mould problems in the coming years .
There is a lot of cheating going on to achieve numbers that are not achievable .
In my opinion the double skin build up with internally insulated board with joints foil taped to achieve air tightness
Even at 25mm ,is alot better than 200mm or 500mm externally insulated .


Im basing my opinion on 22 years of construction.
it is only my opinion from how these various products and techniques FELT after the install and im sure there are architects, designers and insulation sales people with letters after their names that can back up there arguement with numbers and figures to debunk what ive just said.
 
aprentice said:
Room temperature is somewhere around 20 degrees give or take so the heat that your block walls absorb will carry on to the outside where its colder
Click to expand...

Yes, your wall will gradually heat up to the point where it reaches the same temp as the room, a certain amount of that thermal energy will be lost through the insulation, but you can easily calculate that, so I'm sure you know this is a small percentage. When the heating turns off, and the room starts to cool, the thermal energy stored in the wall will slowly release back into the room, much lower thermal resistance there compared to outside. Yes, you will continue to have some escape through the insulation to the exterior so long as the outside temp is lower than that of the wall, but this will be very low.

With internal insulation, given the lower thermal resistance (unless you're prepared to lose more than 4 inches off every wall), you will have a significantly higher rate of heat loss through the insulation to the outside, where it is lost, with negligible recovery once the heating shuts off.


aprentice said:
Good point on the mould , But providing you have a double leaf exterior wall
Ie. Block on edge -cavity- skin of brick or similar build up with timber frame ect...
You would have nothing to worry about if your room is properly vented .
Click to expand...

And by properly vented you mean a big hole in the wall to let all the heat out, right? And even then having to regularly open windows to reduce humidity. Most of Europe stopped using the internally insulated cavity a long time ago due to mould problems.

aprentice said:
All this air tightness stuff that has been introduced to construction in the last few years is going to cause a huge amount of mould problems in the coming years .
Click to expand...

Last few years! It's not even the last few decades, even the Passivhaus-Institut was formed 20 years ago. But that's just how far behind Irish construction methods are.


aprentice said:
Even at 25mm ,is alot better than 200mm or 500mm externally insulated .
Click to expand...

Forget 200 or 500, what U value are you achieving there versus 100mm external insulation? Kingspan's technical guidance docs states you can only achieve 0.79 W/m2K for their 25mm Kooltherm K5 product. They give 0.5 W/m2K for 100mm external.



The the OP, if any of the builders you approach can only recommend a solution based on 'feel', and can't back it up with real calculations, run away!!
 
I'm not out to put anyone in their place, just making sure the OP knows of the potential problems and doesn't end up starting another mould problem thread.
 
aprentice said:
Out side insulation is a joke
You turn on your heating system rads heat up the air in the room ,
Heated air dissipates into the walls then through the brick/block until it meets with insulation
Click to expand...

We installed external insulation about 2 years ago and it's been fantastic. We have an old house and before we got this done the house would be cold shortly after turning off the heating. It was -5 in Cork this morning and the house was warm when we got up without any heating on. I actually thought the forecast was wrong until I opened the front door, felt the chill and saw my windscreen frozen solid.

On a typical cold day we might only put on the heating for 25-30 mins (until the rads heat up) as the heat is retained for so long.

Our oil consumption has fallen dramatically, but we accept it will be years before it breaks even. Our rationale for getting it done was (1) We wanted a warm house - I hate having the heating on, not from a cost perspective per se, but that drowsy feeling that comes with putting the heating on and (2) We felt that although it will take years to pay for itself, we had the cash and it's one reduced cost we have going forward.

We do have an issue with mould however but as we are getting the rest of out house done up in the coming months we will be addressing this then. In the meantime, Mrs Firefly bought a spay in Woodies and it killed off most of the mould straight away. Another blast this weekend should see us right.

I'm no expert in building or insulation and have no contacts in the trade whatsoever, but from an end-user perspective I couldn't recommend it highly enough.
 
David_Dublin said:
Hi Firefly. Have you guys identified the cause and remedy for the mould?
Click to expand...

I think so. I insulated a year or 2 before the external insulation was completed and we never had any mould. But now the hot air has nowhere to go and gets trapped in the corners. I read about rolling back the insulation in the attic a bit as it was rolled out down to the eves, to create room for the hot air to escape, however we still have some mould. I read that the mould is a living organism itself and therefore needs to be killed. That spray I mentioned zapped most of it within minutes, however our landing is quite high so it's hard to get into the corners and so a little of the mould remains. I have an empty house this weekend so I'll attack the rest of it then - the spray is pungent and leaves the place smelling like a swimming pool! I'll post the name of it when I find it too if anyone's interested. As we have young kids, God knows where Mrs. Firefly has hidden it!

If anyone else can offer advice that would be great!
 
When u say you insulated 2 years before adding the external...do you mean you added internal insulation or attic etc back then, and then followed up with external 2 years later?
 
David_Dublin said:
When u say you insulated 2 years before adding the external...do you mean you added internal insulation or attic etc back then, and then followed up with external 2 years later?
Click to expand...

Sorry, typo - I insulated the attic 2 years before the external insulation.
 
I'm kind of going round and round in circles a bit, maybe someone can help.

I'm interested in MHRV. So that means the refurb needs to include air tightness considerations.

That would be separate to the decision on insulation, right? We'd effectively be externally insulating, but internally would have to add an airtight membrane, and then cover it up with boards.

Would we be better to look at taking the hit regarding loss of space, and going for internal insulation? This would make re-wiring easier (no need to trace), satisfy air-tightness, and insulation all in one.

Thoughts welcome. Thanks.
 
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